Showing papers by "Gordon P. Garmire published in 2002"

An overview of the performance and scientific results from the Chandra X-ray Observatory (CXO)

Abstract: The Chandra X-Ray Observatory (CXO), the x-ray component of NASA's Great Observatories, was launched on 1999, July 23 by the Space Shuttle Columbia. After satellite systems activation, the first x-rays focused by the telescope were observed on 1999, August 12. Beginning with the initial observation it was clear that the telescope had survived the launch environment and was operating as expected. Despite an initial surprise due to the discovery that the telescope was far more efficient for concentrating CCD-damaging low-energy protons than had been anticipated, the observatory is performing well and is returning superb scientific data. Together with other space observatories, most notably XMM-Newton, it is clear that we have entered a new era of discovery in high-energy astrophysics.

Chandra Detects Relativistic Broad Absorption Lines from APM 08279+5255

Abstract: We report the discovery of X-ray broad absorption lines (BALs) from the BAL quasar APM 08279+5255 originating from material moving at relativistic velocities with respect to the central source. The large flux magnification by a factor of ~100 provided by the gravitational lens effect combined with the large redshift (z = 3.91) of the quasar have facilitated the acquisition of the first high signal-to-noise X-ray spectrum of a quasar containing X-ray BALs. Our analysis of the X-ray spectrum of APM 08279+5255 places the rest-frame energies of the two observed absorption lines at 8.1 and 9.8 keV. The detection of each of these lines is significant at a greater than 99.9% confidence level based on the F-test. Assuming that the absorption lines are from Fe XXV Kα, the implied bulk velocities of the X-ray BALs are ~0.2c and ~0.4c, respectively. The observed high bulk velocities of the X-ray BALs combined with the relatively short recombination timescales of the X-ray-absorbing gas imply that the absorbers responsible for the X-ray BALs are located at radii of 2 × 1017 cm, within the expected location of the UV absorber. With this implied geometry, the X-ray gas could provide the necessary shielding to prevent the UV absorber from being completely ionized by the central X-ray source, consistent with hydrodynamical simulations of line-driven disk winds. Estimated mass-outflow rates for the gas creating the X-ray BALs are typically less than a solar mass per year. Our spectral analysis also indicates that the continuum X-ray emission of APM 08279+5255 is consistent with that of a typical radio-quiet quasar with a spectral slope of Γ = 1.72.

X-Ray-emitting Young Stars in the Orion Nebula

Abstract: The Orion Nebula Cluster and the molecular cloud in its vicinity have been observed with the ACIS-I detector on board the Chandra X-ray Observatory with 23 hr exposure in two observations. We detect 1075 X-ray sources, most with subarcsecond positional accuracy. Ninety-one percent of the sources are spatially associated with known stellar members of the cluster, and an additional 7% are newly identified deeply embedded cloud members. This provides the largest X-ray study of a pre-main-sequence stellar population and covers the initial mass function from brown dwarfs up to a 45 M☉ O star. Source luminosities span 5 orders of magnitude from log L_X ≃ 28.0 to 33.3 ergs s^(-1) in the 0.5-8 keV band, plasma energies range from 0.2 to >10 keV, and absorption ranges from log N_H < 20.0 to ~23.5 cm^(-2). Comprehensive tables providing X-ray and stellar characteristics are provided electronically. We examine here the X-ray properties of Orion young stars as a function of mass; other studies of astrophysical interest will appear in companion papers. Results include: (a) the discovery of rapid variability in the O9.5 31 M☉ star θ^2A Ori, and several early B stars, inconsistent with the standard model of X-ray production in small shocks distributed throughout the radiatively accelerated wind; (b) support for the hypothesis that intermediate-mass mid-B through A type stars do not themselves produce significant X-ray emission; (c) confirmation that low-mass G through M type T Tauri stars exhibit powerful flaring but typically at luminosities considerably below the "saturation" level; (d) confirmation that the presence or absence of a circumstellar disk has no discernable effect on X-ray emission; (e) evidence that T Tauri plasma temperatures are often very high with T ≥ 100 MK, even when luminosities are modest and flaring is not evident; and (f) detection of the largest sample of pre-main-sequence very low-mass objects showing flaring levels similar to those seen in more massive T Tauri stars and a decline in magnetic activity as they evolve into L and T type brown dwarfs.

X-Ray, Optical, and Infrared Imaging and Spectral Properties of the 1 Ms Chandra Deep Field North Sources

Abstract: We present the optical, near-infrared, submillimeter, and radio follow-up catalog of the X-ray–selected sources from an ≈1 Ms Chandra observation of the Hubble Deep Field North region. We have B, V, R, I, and z' magnitudes for the 370 X-ray point sources, HK' magnitudes for 276, and spectroscopic redshifts for 182. We present high-quality spectra for 175 of these. The redshift distribution shows indications of structures at z = 0.843 and 1.0175 (also detected in optical surveys), which could account for a part of the field-to-field variation seen in the X-ray number counts; however, these structures do not dominate the number of X-ray sources in the sample and hence should not strongly affect the redshift distribution. All of the X-ray sources with z > 1.6 are either broad-line active galactic nuclei (AGNs) or have narrow Lyα and/or C III] 1909 A emission; none of the known z > 1.6 absorption-line galaxies in the field are detected individually in X-rays. We estimate photometric redshifts for the sources with (B-I) > 1.5 (bluer than this and it is hard to distinguish between low-redshift irregulars and luminous high-redshift AGNs) and find agreement (most are within 25%) with the available spectroscopic redshifts. The majority of the galaxies in both the 2–8 keV (hard) and 0.5–2 keV (soft) samples have absolute magnitudes comparable to or more luminous than M = -22. The flux contributions separated into unit bins of redshift show that the z < 1 spectroscopically identified sources already contribute about one-third of the total flux in both the hard and soft bands. Thus, major accretion onto supermassive black holes has occurred since the universe was half its present age. We find from ratios of the X-ray counts that the X-ray spectra are well described by absorption of an intrinsic Γ = 1.8 power law, with NH values ranging from about 1021 cm-2 to 5 × 1023 cm-2. We find very little evolution in the maximum rest-frame opacity-corrected and K-corrected 2–8 keV X-ray luminosities with decreasing redshift until z 0.5, where the volume becomes too small to probe effectively very high luminosity sources. We estimate that the Chandra sources that produce 87% of the HEAO A X-ray background (XRB) at 3 keV produce 57% at 20 keV, provided that at high energies the spectral shape of the sources continues to be well described by a Γ = 1.8 power law. However, when the Chandra contributions are renormalized to the BeppoSAX XRB at 3 keV, the shape matches fairly well the observed XRB at both energies. Thus, whether a substantial population of as yet undetected Compton-thick sources or some change in the spectral shape of the current sources from the simple power-law dependence is required to completely resolve the XRB above 10 keV depends critically on how the currently discrepant XRB measurements in the 1-10 keV energy range tie together with the higher energy XRB.

X-ray emitting young stars in the Orion Nebula

Abstract: The Orion Nebula Cluster and the molecular cloud in its vicinity have been observed with the ACIS-I detector on board the Chandra X-ray Observatory with 23 hours exposure. We detect 1075 X-ray sources: 91% are spatially associated with known stellar members of the cluster, and 7% are newly identified deeply embedded cloud members. This provides the largest X-ray study of a pre-main sequence stellar population. We examine here the X-ray properties of Orion young stars as a function of mass. Results include: (a) the discovery of rapid variability in the O9.5 31 M_o star \theta^2A Ori, and several early B stars, inconsistent with the standard model of X-ray production in small wind shocks; (b) support for the hypothesis that intermediate-mass mid-B through A type stars do not themselves produce significant X-ray emission; (c) confirmation that low-mass G- through M-type T Tauri stars exhibit powerful flaring but typically at luminosities considerably below the `saturation' level; (d) confirmation that the presence or absence of a circumstellar disk has no discernable effect on X-ray emission; (e) evidence that T Tauri plasma temperatures are often very high with T >= 100 MK, even when luminosities are modest and flaring is not evident; and (f) detection of the largest sample of pre-main sequence very low mass objects showing high flaring levels and a decline in magnetic activity as they evolve into L- and T-type brown dwarfs.

X-Ray Spectroscopy of Quasi-Stellar Objects with Broad Ultraviolet Absorption Lines

Abstract: For the population of quasi-stellar objects (QSOs) with broad ultraviolet absorption lines, we are just beginning to accumulate X-ray observations with enough counts for spectral analysis at CCD resolution. From a sample of eight QSOs (including four broad absorption line [BAL] QSOs and three mini-BAL QSOs) with ASCA or Chandra spectra with more than 200 counts, general patterns are emerging. Their power-law X-ray continua are typical of normal QSOs with Γ ≈ 2.0, and the signatures of a significant column density [NH ≈ (0.1-4) × 1023 cm-2] of intrinsic, absorbing gas are clear. Correcting the X-ray spectra for intrinsic absorption recovers a normal ultraviolet-to-X-ray flux ratio, indicating that the spectral energy distributions of this population are not inherently anomalous. In addition, a large fraction of our sample shows significant evidence for complexity in the absorption. The subset of BAL QSOs with broad Mg II absorption apparently suffers from Compton-thick absorption completely obscuring the direct continuum in the 2-10 keV X-ray band, complicating any measurement of their intrinsic X-ray spectral shapes.

X-ray, Optical, and Infrared Imaging and Spectral Properties of the 1 Ms Chandra Deep Field North Sources

Abstract: We present the optical, near-infrared, submillimeter, and radio follow-up catalog of the X-ray selected sources from the 1 Ms Chandra observation of the Hubble Deep Field North region. We have B, V, R, I, and z' magnitudes for the 370 X-ray point sources, HK' magnitudes for 276, and spectroscopic redshifts for 182. We present high-quality spectra for 175 of these. The redshift distribution shows indications of structures at z=0.843 and z=1.0175 (also detected in optical surveys) which could account for a part of the field-to-field variation seen in the X-ray number counts. The flux contributions separated into unit bins of redshift show that the z<1 spectroscopically identified sources already contribute about one-third of the total flux in both the hard and soft bands. We find from ratios of the X-ray counts that the X-ray spectra are well-described by absorption of an intrinsic Gamma=1.8 power-law, with log NH values ranging from 21 to 23.7. We estimate that the Chandra sources that produce 87% of the HEAO-A X-ray background (XRB) at 3 keV produce 57% at 20 keV, provided that at high energies the spectral shape of the sources continues to be well-described by a Gamma=1.8 power-law. However, when the Chandra contributions are renormalized to the BeppoSAX XRB at 3 keV, the shape matches fairly well the observed XRB at both energies. Thus, whether a substantial population of as-yet undetected Compton-thick sources is required to completely resolve the XRB above 10 keV depends critically on how the currently discrepant XRB measurements in the 1-10 keV energy range tie together with the higher energy XRB. (Abridged)

A Chandra Study of Sagittarius A East: A Supernova Remnant Regulating the Activity of Our Galactic Center?

Abstract: We report on the X-ray emission from the shell-like, nonthermal radio source Sgr A East (SNR 000.0+00.0), located in the inner few parsecs of the Galaxy based on observations made with the ACIS detector on board the Chandra X-Ray Observatory. This is the first time Sgr A East has been clearly resolved from other complex structures in the region. The X-ray-emitting region is concentrated within the central 2 pc of the larger radio shell. The spectrum shows strong Kα lines from highly ionized ions of S, Ar, Ca, and Fe. A simple isothermal plasma model gives electron temperature ~2 keV, absorption column ~1 × 1023 H cm-2, luminosity ~8 × 1034 ergs s-1 in the 2-10 keV band, and gas mass ~2η1/2 M☉ with a filling factor η. The plasma appears to be rich in heavy elements, overabundant by roughly a factor of 4 with respect to solar abundances, and shows a spatial gradient of elemental abundance; the spatial distribution of iron is more compact than that of the lighter elements. The gas mass and elemental abundance of the X-ray emission support the long-standing hypothesis that Sgr A East is a supernova remnant (SNR), perhaps produced by the Type II supernova explosion of a massive star with a main-sequence mass of 13-20 M☉. The combination of the radio and X-ray morphologies classifies Sgr A East as a new metal-rich "mixed morphology" (MM) SNR. The size of the Sgr A East radio shell is the smallest of the known MM SNRs, which strongly suggests that the ejecta have expanded into a very dense interstellar medium. The ejecta-dominated chemical compositions of the plasma indicate that the ambient materials should be highly homogeneous. We thus evaluate a simplified dynamical evolution model where an SNR was formed about 10,000 yr ago and expanded into an ambient medium with a homogeneous density of 103 cm-3. The model roughly reproduces most of the observed properties in the X-ray and radio wavelengths. A comparison with the radio observations requires the dense ambient medium to be ionized, but a luminous X-ray irradiator with an expected X-ray luminosity of ~1040 ergs s-1 is not currently present. The presence of the ionized gas may be explained if the massive black hole (MBH) associated with the compact, nonthermal radio source Sgr A* was bright in X-rays about 300 yr ago but is presently dim. It is possible that the dust/molecular ridge compressed by the forward shock of Sgr A East hit Sgr A* in the past, and the passage of the ridge may have supplied material to accrete onto the black hole in the past and may have removed material from the black hole vicinity, leading to its present quiescent state. This may be a specific example of the intimate relationship between an SNR and MBH accretion activity in galactic nuclei.

Magnetic flaring in the pre-main-sequence Sun and implications for the early solar system

Abstract: To address the role of energetic processes in the solar nebula, we provide a detailed characterization of magnetic flaring in stellar analogs of the pre-main-sequence Sun based on two 0.5 day observations of the Orion Nebula cluster obtained with the Chandra X-Ray Observatory. The sample consists of 43 stars with masses between 0.7 and 1.4 M☉ and ages from less than 0.3 to 10 Myr. We find that the X-ray luminosities measured in the 0.5-8 keV band are strongly elevated over main-sequence levels with an average = 30.3 ergs s-1 and = -3.9. The X-ray emission is strongly variable within our exposures in nearly all solar analogs; about 30 flares with 29.0 ergs s-1 < log LX(peak) < 31.5 ergs s-1 on timescales from 0.5 to more than 12 hr are seen during the Chandra observations. Analogs of the ≤1 Myr old pre-main-sequence Sun exhibited X-ray flares that are 101.5 times more powerful and 102.5 times more frequent than the most powerful flares seen on the contemporary Sun. Radio observations indicate that acceleration of particles to relativistic energies is efficient in young stellar flares. Extrapolating the solar relationship between X-ray luminosity and proton fluence, we infer that the young Sun exhibited a 105-fold enhancement in energetic protons compared to contemporary levels. Unless the flare geometries are unfavorable, this inferred proton flux on the disk is sufficient to produce the observed meteoritic abundances of several important short-lived radioactive isotopes. Our study thus strengthens the astronomical foundation for local proton spallation models of isotopic anomalies in carbonaceous chondritic meteorites. The radiation, particles, and shocks produced by the magnetic reconnection flares seen with Chandra may also have flash-melted meteoritic chondrules and produce excess 21Ne seen in meteoritic grains.

The Chandra Deep Field-North Survey. XIV. X-ray detected Obscured AGNs and Starburst Galaxies in the Bright Submm Source Population

Abstract: We provide X-ray constraints and perform the first X-ray spectral analyses for bright (f_850>=5mJy; S/N>=4) SCUBA sources in an 8.4'x8.4' area of the 2 Ms Chandra Deep Field-North survey containing the Hubble Deep Field-North. X-ray emission is detected from 7 of the 10 bright submm sources in this region, corresponding to an X-ray detected submm source density of ~360 deg^-2 (>~36% of the bright submm source population). Two of the X-ray detected sources have nearby (within 3") X-ray companions, suggesting merging/interacting sources or gravitational lensing effects, and 3 lie within the approximate extent of a proto-cluster candidate. Five of the X-ray detected sources have flat X-ray spectral slopes, suggesting obscured AGN activity. X-ray spectral analyses suggest that one of these AGNs may be a Compton-thick source; of the other 4 AGNs, 3 appear to be Compton-thin sources and one has poor constraints. The rest-frame unabsorbed X-ray luminosities of these AGNs are more consistent with those of Seyfert galaxies than QSOs. Thus, the low X-ray detection rate of bright submm sources by moderately deep X-ray surveys appears to be due to the relatively low luminosities of the AGNs rather than Compton-thick absorption. A comparison of these sources to the well-studied heavily obscured AGN NGC6240 shows that the average AGN contribution is negligible at submm wavelengths. The X-ray properties of the other 2 X-ray detected sources are consistent with those expected from luminous star formation; however, we cannot rule out the possibility that low-luminosity AGNs are present. The 3 X-ray undetected sources appear to lie at high redshift (z>4) and could be either AGNs or starbust galaxies.

Chandra Evidence of a Flattened, Triaxial Dark Matter Halo in the Elliptical Galaxy NGC 720

Abstract: We present an analysis of a Chandra ACIS-S observation of the elliptical galaxy NGC 720, to verify the existence of a dark matter halo and to measure its ellipticity. The ACIS-S3 image reveals over 60 point sources distributed throughout the field, most of which were undetected and therefore unaccounted for in previous X-ray studies. For semimajor axes a 150'' (18.2 h kpc), the ellipticity of the diffuse X-ray emission is consistent with a constant value, X ≈ 0.15, which is systematically less than the values of 0.2-0.3 obtained from previous ROSAT PSPC and HRI observations because of the unresolved point sources contaminating the ROSAT values. The Chandra data confirm the magnitude of the ~20° position angle (P.A.) twist discovered by ROSAT over this region. However, the twist in the Chandra data is more gradual and occurs at smaller a, also because of the point sources contaminating the ROSAT values. For a 150'' out to a = 185'' (22.4 h kpc), which is near the edge of the S3 CCD, X and P.A. diverge from their values at smaller a. Possible origins of this behavior at the largest a are discussed. Overall, the ellipticities and P.A. twist for a 150'' can be explained by the triaxial mass model of NGC 720 published by Romanowsky & Kochanek (which could not produce the abrupt P.A. twist in the ROSAT HRI data). Since the optical image displays no substantial isophote twisting, the X-ray P.A. twist requires a massive dark matter halo if the hot gas is in hydrostatic equilibrium. Furthermore, the values of X obtained by Chandra are too large to be explained if the gravitating mass follows the optical light (M L*), irrespective of the P.A. twist. The M L* hypothesis is inconsistent with the Chandra ellipticities at the 96% confidence level, assuming oblate symmetry, and at the 98% confidence level for prolate symmetry. Thus, both the P.A. twist and the ellipticities of the Chandra image imply the existence of dark matter, independent of the temperature profile of the gas. This geometric evidence for dark matter cannot be explained by alternative gravity theories, such as the modification of Newtonian dynamics (MOND). To constrain the ellipticity of the dark matter halo, we considered both oblate and prolate spheroidal mass models to bracket the full range of (projected) ellipticities of a triaxial ellipsoid. The dark matter density model, ρ (a + a2)-1, provides the best fit to the data and gives ellipticities and 1 σ errors of = 0.37 ± 0.03 for oblate and = 0.36 ± 0.02 for prolate models. Navarro-Frenk-White (NFW) and Hernquist models give similar ellipticities for the dark matter. These moderate ellipticities for the dark halo are inconsistent with both the nearly spherical halos predicted if the dark matter is self-interacting and the highly flattened halos predicted if the dark matter is cold molecular gas. These ellipticities may also be too large to be explained by warm dark matter, but they are consistent with galaxy-sized halos formed in the currently popular ΛCDM paradigm.

Magnetic flaring in the pre-main sequence Sun and implications for the early solar system

Abstract: To address the role of energetic processes in the solar nebula, we provide a detailed characterization of magnetic flaring in stellar analogs of the pre-main sequence Sun based on 23 hours observations of 43 analogs of the young Sun in the Orion Nebula Cluster obtained with the Chandra X-ray Observatory. We find the X-ray luminosities are strongly elevated over main sequence levels with average = 30.3 erg/s and = -3.9 (0.5-8 keV), and dozens of flares are present. Analogs of the <= 1 My old pre-main sequence Sun exhibit X-ray flares 10^{1.5} times more powerful and 10^{2.5} times more frequent than the most powerful flares seen on the contemporary Sun. Extrapolating the solar relationship between X-ray luminosity and proton fluence, we infer that the young Sun exhibited a 10^5-fold enhancement in energetic protons compared to contemporary levels. Unless the flare geometries are unfavorable, this inferred proton flux on the disk is sufficient to produce the observed meteoritic abundances of several important short-lived radioactive isotopes. Our study thus strengthens the astronomical foundation for local proton spallation models of isotopic anomalies in carbonaceous chondritic meteorites. The radiation, particles and shocks produced by the magnetic reconnection flares seen with Chandra may also have flash melted meteoritic chondrules and produced excess 21-Ne seen in meteoritic grains.

Chandra Observations of the Lensing Cluster EMSS 1358+6245: Implications for Self-interacting Dark Matter

Abstract: We present Chandra observations of EMSS 1358+6245, a relaxed cooling flow cluster of galaxies at z = 0.328. We employ a new deprojection technique to construct temperature, gas, and dark matter profiles. We confirm the presence of cool gas in the cluster core, and our deprojected temperature profile for the hot component is isothermal over 30 kpc < r < 0.8 Mpc. Fitting the mass profile to a Navarro-Frenk-White model yields rs = 153 kpc and c = 8.4. We find good agreement between our dark matter profile and weak gravitational lensing measurements. We place an upper limit of 42 kpc (90% confidence limit) on the size of any constant-density core. We compare this result to recent simulations and place a conservative upper limit on the dark matter particle-scattering cross section of 0.1 cm2 g-1. This limit implies that the cross section must be velocity-dependent if the relatively shallow core mass profiles of dwarf galaxies are a direct result of dark matter self-interaction.

Chandra Evidence for a Flattened, Triaxial Dark Matter Halo in the Elliptical Galaxy NGC 720

Abstract: (Abridged) We present an analysis of a Chandra ACIS-S observation of the elliptical galaxy NGC 720 to verify the existence of a dark matter (DM) halo and to measure its ellipticity. The ACIS-S3 image reveals over 60 point sources. For semi-major axes a<~150" (18.2h_{70}^{-1} kpc) the ellipticity of the diffuse emission is ex ~0.15, which is less than the values 0.2-0.3 obtained from ROSAT because the point sources contaminated the ROSAT values. The Chandra data confirm the ~20 deg position angle (PA) twist discovered by ROSAT, but the Chandra twist is more gradual also because of the point sources contaminating the ROSAT values. Overall the ex and PA values for a<~150" can be explained by the triaxial model of NGC 720 published by Romanowsky & Kochanek. Since the optical image displays no substantial isophote twisting, the X-ray PA twist requires a massive DM halo if the hot gas is in hydrostatic equilibrium. The mass-follows-light hypothesis is also inconsistent with the Chandra ellipticities at the 96% (98%) level for oblate (prolate) symmetry. Thus, both the PA twist and the ellipticities of the Chandra image imply a DM halo independent of the gas T profile -- evidence that cannot be obviated by alternative gravity theories such as MOND. The DM density model, rho ~a^{-2}, provides the best fit and gives ellipticities of 0.37 +/- 0.03 (0.36 +/- 0.02) for oblate (prolate) models. These moderate ellipticities for the DM halo are inconsistent with both the nearly spherical halos predicted if the DM is self-interacting and with the highly flattened halos predicted if the DM is cold molecular gas. These ellipticities may also be too large to be explained by warm DM, but are consistent with galaxy-sized halos formed in the Lambda-CDM paradigm.

The Bright Gamma-Ray Burst of 2000 February 10: A Case Study of an Optically Dark Gamma-Ray Burst

Abstract: The gamma-ray burst GRB 000210 had the highest gamma-ray peak flux of any event localized by BeppoSAX as yet, but it did not have a detected optical afterglow, despite prompt and deep searches down to Rlim � 23:5. It is therefore one of the events recently classified as dark GRBs, whose origin is still unclear. Chandra observations allowed us to localize the X-ray afterglow of GRB 000210 to within � 1 00 , and a radio transient was detected with the Very Large Array. The precise X-ray and radio positions allowed us to identify the likely host galaxy of this burst and to measure its redshift, z ¼ 0:846. The probability that this galaxy is a field object is � 1:6 � 10 � 2 . The X-ray spectrum of the afterglow shows significant absorption in excess of the Galactic one corresponding, at the redshift of the galaxy, to NH ¼ð 5 � 1 Þ� 10 21 cm � 2 . The amount of dust needed to absorb the optical flux of this object is consistent with the above H i column density, given a dust-to-gas ratio similar to that of our Galaxy. We do not find evidence for a partially ionized absorber expected if the absorption takes place in a giant molecular cloud. We therefore conclude that either the gas is local to the GRB but is condensed in small-scale high-density (ne10 9 cm � 3 ) clouds, or the GRB is located in a dusty, gas-rich region of the Galaxy. Finally, we examine the hypothesis that GRB 000210 lies at ze5 (and

Modeling charge transfer inefficiency in the Chandra Advanced CCD Imaging Spectrometer

Abstract: The front-illuminated (FI) CCDs in the Advanced CCD Imaging Spectrometer (ACIS) on the Chandra X-ray Observatory (Chandra) were damaged in the extreme environment of the Earth's radiation belts, causing charge traps that result in enhanced charge transfer inefficiency (CTI) during parallel readout. This causes row-dependent gain, event grade ‘morphing’ (spatial redistribution of charge) and energy resolution degradation. The ACIS back-illuminated (BI) CCDs also exhibit pronounced CTI due to their manufacturing. It is mild enough that position-dependent energy resolution is not seen, but it is present in both parallel and serial registers. This CTI also changes the gain and event grades, in a spatially complicated way as parallel and serial CTI interact. Given these realities, we have developed and tuned a phenomenological model of CTI for both FI and BI CCDs and incorporated it into our Monte Carlo simulations of the ACIS CCDs. It models charge loss and the spatial redistribution of charge (trailing), thus reproducing the spatially dependent gain and grade distribution seen in all ACIS CCDs and the row-dependent energy resolution seen in the FI devices. Here we explore the evidence for CTI, compare our simulations to data, and present a technique for CTI correction based on forward modeling.

The Chandra Deep Field North Survey. XII. The Link between Faint X-Ray and Radio Source Populations

Abstract: We investigate the relationship between faint X-ray and 1.4 GHz radio source populations detected within 3' of the Hubble Deep Field (North) using the 1 Ms Chandra and 40 μJy VLA surveys. Within this region, we find that ≈42% of the 62 X-ray sources have radio counterparts and ≈71% of the 28 radio sources have X-ray counterparts; thus, a 40 μJy VLA survey at 1.4 GHz appears to be well matched to a 1 Ms Chandra observation. Among the different source populations sampled, we find that the majority of the 18 X-ray–detected emission-line galaxies (ELGs) have radio and mid-infrared ISOCAM counterparts and appear to be luminous star-forming galaxies at z = 0.3–1.3. Importantly, the radio-detected ELGs make up ≈35% of the X-ray source population at 0.5–8.0 keV X-ray fluxes between ≈1 × 10-16 and ≈5 × 10-16 ergs cm-2 s-1 and signal the emergence of the luminous, high-z starburst galaxy population in the X-ray band. We find that the locally determined correlation between X-ray luminosities and 1.4 GHz radio luminosity densities of the late-type galaxies can easily be extended to include the luminous intermediate-redshift ELGs, suggesting that the X-ray and radio emission processes are generally associated in star-forming galaxies. This result implies that the X-ray emission can be used as an indicator of star formation rate for star-forming galaxies. Finally, we show that there appear to be two statistically distinct types of ISOCAM-detected star-forming galaxies: those with detectable radio and X-ray emission and those without. The latter type may have stronger mid-infrared emission-line features that increase their detectability at mid-infrared wavelengths.

The bright Gamma-Ray Burst of February 10, 2000: a case study of an optically dark GRB

Abstract: The gamma-ray burst GRB000210 had the highest gamma-ray peak flux of any event localized by BeppoSAX as yet but it did not have a detected optical afterglow. It is therefore one of the events recently classified as dark GRBs or GHOST (GRB Hiding Optical Source Transient), whose origin is still unclear. Chandra observations allowed us to localize this GRB within ~1" and a radio transient was detected with the VLA. We identify the likely (P=0.01) host galaxy of this burst at z=0.846. The X-ray spectrum of the afterglow shows intrinsic absorption N_H=5x10**21 cm-2. The amount of dust needed to absorb the optical flux of this object is consistent with the above HI column density, given a dust-to-gas ratio similar to that of our Galaxy. We do not find evidence for a partially ionized absorber expected if the absorption takes place in a Giant Molecular Cloud. We therefore conclude that either the gas is local to the GRB, but is condensed in small-scale high-density (n>~10**9 cm-3) clouds, or that the GRB is located in a dusty, gas-rich region of the galaxy. Finally, if GRB000210 lies at z>5, its X-ray absorbing medium would have to be substantially different from that observed in GRBs with optical afterglows.

The 2-8 keV X-Ray Number Counts Determined from Chandra Blank Field Observations

Abstract: We present the 2-8 keV number counts from the 1 Ms Chandra observation of the Chandra Deep Field-North. We combine these with the number counts from a 78 ks exposure of the Hawaii Survey field SSA22 and with the number counts obtained in independent analyses of the Chandra Deep Field-South and the Hawaii Survey field SSA13 to determine the number counts from 2 × 10-16 to 10-13 ergs cm-2 s-1. Over this flux range, the contribution to the X-ray background is 1.1 × 10-11 ergs cm-2 s-1 deg-2. When the contributions above 10-13 ergs cm-2 s-1 from BeppoSAX or ASCA observations are included, the total rises to 1.4 × 10-11 ergs cm-2 s-1 deg-2. However, there appears to be substantial field-to-field variation in the counts in excess of the statistical uncertainties. When the statistical and flux-calibration uncertainties (in both the background and source measurements) are taken into account, as much as 0.5 × 10-11 ergs cm-2 s-1 deg-2 could still be present in an unresolved component.

The structure of the oxygen-rich supernova remnant G292.0 + 1.8 from Chandra X-ray images: Shocked ejecta and circumstellar medium

Abstract: We present results from the observation of the young Galactic supernova remnant (SNR) G292.0+1.8 with the Advanced CCD Imaging Spectrometer (ACIS) on board the Chandra X-Ray Observatory. In the 0.3-8 keV band, the high-resolution ACIS images reveal a complex morphology consisting of knots and filaments as well as the blast wave around the periphery of the SNR. We present equivalent width (EW) maps for the elemental species O, Ne, Mg, and Si, which allow us to identify regions of enhanced metallicity in the SNR. G292.0+1.8 is bright in O, Ne, and Si and weaker in S and Ar, with little Fe. The EW and broadband images indicate that the metal-rich ejecta are distributed primarily around the periphery of the SNR. The central beltlike structure has a normal solar-type composition, strongly suggesting that it is primarily emission from a shocked circumstellar medium rather than metal-rich ejecta. We propose that the belt traces its origin to enhanced mass loss in the star's equatorial plane during the slow, red supergiant phase. We also identify thin filaments with normal composition, centered on and extending nearly continuously around the outer boundary of the SNR. These may originate in a shell caused by the stellar winds from the massive progenitor in the red/blue supergiant phases, overrun by the blast wave.

X-Raying the Ultraluminous Infrared Starburst Galaxy and Broad Absorption Line QSO Markarian 231 with Chandra

Abstract: With 40 ks of Clzandra ACIS-S3 exposure, new information on both the starburst and QSO components of the X-ray emission of Markarian 231, an ultraluminous infrared galaxy and broad absorption line QSO, has been obtained. The bulk of the X-ray luminosity is emitted from an unresolved nuclear point source, and the spectrum is remarkably hard, with the majority of the flux emitted above 2 keV. Most notably, significant nuclear variability (a decrease of -45% in approximately 6 hr) at energies above 2 keV indicates that Chuizdra has probed within light-hours of the central black hole. Although we concur with Maloney & Reynolds that the direct continuum is not observed, this variability coupled with the 188 eV upper limit on the equivalent width of the Fe K o emission line argues against the reflection-dominated model put forth by these authors based on their ASCA data. Instead, we favor a model in which a small, Compton-thick absorber blocks the direct X-rays, and only indirect, scattered X-rays from multiple lines of sight can reach the observer. Extended soft, thermal emission encompasses the optical extent of the galaxy and exhibits resolved structure. An off-nuclear X-ray source with a 0.35-8.0 keV luminosity of Lx = 7 x 10 sup39 ergs s sup -1 , consistent with the ultraluminous X-ray sources in other nearby starbursts, is detected. We also present an unpublished Faint Object Spectrograph spectrum from the Hirhhle Spuce Telescope archive showing the broad C IV absorption.

The Chandra Deep Field North Survey. XII. The Link Between Faint X-ray and Radio Source Populations

Abstract: We investigate the relationship between faint X-ray and 1.4 GHz radio source populations detected within 3' of the Hubble Deep Field North using the 1 Ms Chandra and 40 uJy VLA surveys. Within this region, we find that ~42% of the 62 X-ray sources have radio counterparts and ~71% of the 28 radio sources have X-ray counterparts; thus a 40 uJy VLA survey at 1.4 GHz appears to be well-matched to a 1 Ms Chandra observation. Among the different source populations sampled, we find that the majority of the 18 X-ray detected emission-line galaxies (ELGs) have radio and mid-infrared ISOCAM counterparts and appear to be luminous star-forming galaxies at z=0.3-1.3. Importantly, the radio-detected ELGs make up ~35% of the X-ray source population at 0.5-8.0 keV X-ray fluxes between ~(1-5)x10e-16 erg/cm2/s and signal the emergence of the luminous, high-z starburst galaxy population in the X-ray band. We find that the locally-determined correlation between X-ray luminosities and 1.4 GHz radio luminosity densities of the late-type galaxies can easily be extended to include the luminous intermediate-redshift ELGs, suggesting that the X-ray and radio emission processes are generally associated in star-forming galaxies. This result implies that the X-ray emission can be used as an indicator of star formation rate for star-forming galaxies. Finally, we show that there appear to be two statistically distinct types of ISOCAM-detected star-forming galaxies: those with detectable radio and X-ray emission and those without. The latter type may have stronger mid-infrared emission-line features that increase their detectability at mid-infrared wavelengths.

2-8 keV X-ray number counts determined from Chandra blank field observations

Abstract: We present the 2-8 keV number counts from the 1Ms Chandra observation of the Chandra Deep Field North (CDF-N). We combine these with the number counts from a 78ks exposure of the Hawaii Survey Field SSA22 and with the number counts obtained in independent analyses of the CDF-S and the Hawaii Survey Field SSA13 to determine the number counts from 2.10^(-16) to 10^(-13) ergs/cm2/s. Over this flux range the contribution to the X-ray background is 1.1 10^(-11) ergs/cm2/s/square degree. When the contributions above 10^(-13) ergs/cm2/s from BeppoSAX or ASCA observations are included, the total rises to 1.4 10^(-11) ergs/cm2/s/square degree. However, there appears to be substantial field-to-field variation in the counts in excess of the statistical uncertainties. When the statistical and flux calibration uncertainties (in both the background and source measurements) are taken into account, as much as 0.5 10^(-11) ergs/cm2/s/square degree could still be present in an unresolved component.

Caught in the Act: Chandra Observations of Microlensing of the Radio-loud Quasar MG J0414+0534

Abstract: We present results from monitoring of the distant (z = 2.64), gravitationally lensed quasar MG J0414+0534 with the Chandra X-Ray Observatory. An Fe K? line at 6.49 ? 0.09 keV (rest frame) with an equivalent width of ~190 eV, consistent with fluorescence from a cold medium, is detected at the 99% confidence level in the spectrum of the brightest image A. During the last two observations of our monitoring program, we detected a fivefold increase of the equivalent width of a narrow Fe K? line in the spectrum of image B but not in the brighter image A, whereas image C is too faint to resolve the line. The continuum emission component of image B did not follow the sudden enhancement of the iron line in the last two observations. We propose that the sudden increase in the iron-line strength from ~190 to 900 eV only in image B can be explained with a caustic crossing due to microlensing that selectively enhances a strip of the line emission region of the accretion disk. The nonenhancement of the continuum emission in the spectrum of image B suggests that the continuum emission region is concentrated closer to the center of the accretion disk than the iron-line emission region and that the magnification caustic has not reached close enough to the former region to amplify it. A model of a caustic crossing event predicts discontinuities in the light curve of the magnification and provides an upper limit of ~5 ? 10-4 pc on the outer radius of the Fe K? emission region. The nondetection of any relativistic or Doppler shifts of the iron line in the spectrum of image B implies that the magnification caustic for the last two observations was located at a radius greater than ~100 gravitational radii. Each observation of the quadruply lensed quasar MG J0414+0534 provides a view of the quasar at four different epochs spaced by the time delays between the lensed images. We produced a light curve of the quasar X-ray flux by normalizing the flux of each image to the mean flux of that image over all observations. We find significant deviations of the normalized light curve from unity especially for the faintest image C. A plausible mechanism to explain the flux variability of image C is a microlensing event. Finally, spectral analysis of MG J0414+0534 indicates the presence of significant absorption in excess of the Galactic value. For absorption at the redshift of the lensed quasar, we find an intrinsic column density of NH ~ 5 ? 1022 cm-2, consistent with the reddening observed in the optical band.

The Chandra Deep Field North Survey. IX. Extended X-Ray Sources*

Abstract: The ≈1 Ms Chandra Deep Field North observation is used to study the extended X-ray sources in the region surrounding the Hubble Deep Field North (HDF-N), yielding the most sensitive probe of extended X-ray emission at cosmological distances to date. A total of six such sources are detected, the majority of which align with small numbers of optically bright galaxies. Their angular sizes, band ratios, and X-ray luminosities—assuming they lie at the same distances as the galaxies coincident with the X-ray emission—are generally consistent with the properties found for nearby groups of galaxies. One source is notably different and is likely to be a poor-to-moderate X-ray cluster at high redshift (i.e., z 0.7). This source has a large angular extent, a double-peaked X-ray morphology, and an overdensity of unusual objects [very red objects, optically faint (I ≥ 24) radio and X-ray sources]. Another of the six sources is coincident with several z ≈ 1.01 galaxies located within the HDF-N itself, including the FR I radio galaxy VLA J123644+621133, and is likely to be a group or poor cluster of galaxies at that redshift. We are also able to place strong constraints on the optically detected cluster of galaxies ClG 1236+6215 at z = 0.85 and the wide-angle–tailed radio galaxy VLA J123725+621128 at z ~ 1–2; both sources are expected to have considerable associated diffuse X-ray emission, and yet they have rest-frame 0.5–2.0 keV X-ray luminosities of 3 × 1042 and (3–15) × 1042 ergs s-1, respectively. The environments of both sources are either likely to have a significant deficit of hot intracluster gas compared with local clusters of galaxies, or they are X-ray groups. We find the surface density of extended X-ray sources in this observation to be 167 deg-2 at a limiting soft-band flux of ≈3 × 10-16 ergs cm-2 s-1. No evolution in the X-ray luminosity function of clusters is needed to explain this value.

The Chandra Deep Field-North Survey. XI. X-Ray Emission from Luminous Infrared Starburst Galaxies

Abstract: Using the 1 Ms Chandra Deep Field-North and 15 μm ISOCAM Hubble Deep Field-North surveys, we find a tight correlation between the population of strongly evolving starburst galaxiess discovered in faint 15 μm ISOCAM surveys and the apparently normal galaxy population detected in deep X-ray surveys. Up to 100% of the X-ray-detected emission-line galaxies (ELGs) have 15 μm counterparts, in contrast to 10%-20% of the X-ray-detected absorption-line galaxies and AGN-dominated sources. None of the X-ray-detected ELGs are detected in the hard band (2-8 keV), and their stacked-average X-ray spectral slope of Γ ≈ 2.0 suggests a low fraction of obscured AGN activity within the X-ray-detected ELG population. The characteristics of the z = 0.4-1.3 X-ray-detected ELGs are consistent with those expected for M82- and NGC 3256-type starburst galaxies; these X-ray-detected ELGs contribute ≈2% of the 0.5-8.0 keV extragalactic X-ray background. The only statistical difference between the X-ray-detected and X-ray-undetected 15 μm-selected ELGs is that a much larger fraction of the former have radio emission.

The Radial Structure of SNR N103B

Abstract: We report on the results from a Chandra ACIS observation of the young, compact, supernova remnant N103B. The unprecedented spatial resolution of Chandra reveals sub-arcsecond structure, both in the brightness and in spectral variations. Underlying these small-scale variations is a surprisingly simple radial structure in the equivalent widths of the strong Si and S emission lines. We investigate these radial variations through spatially resolved spectroscopy using a plane-parallel, non-equilibrium ionization model with multiple components. The majority of the emission arises from components with a temperature of 1 keV: a fully ionized hydrogen component; a high ionization timescale (n_e*t > 10^12 s cm^-3) component containing Si, S, Ar, Ca, and Fe; and a low ionization timescale (n_e*t ~ 10^{11} s cm^-3) O, Ne, and Mg component. To reproduce the strong Fe Kalpha line, it is necessary to include additional Fe in a hot (> 2 keV), low ionization (n_e*t ~ 10^10.8 s cm^-3) component. This hot Fe may be in the form of hot Fe bubbles, formed in the radioactive decay of clumps of 56Ni. We find no radial variation in the ionization timescales or temperatures of the various components. Rather, the Si and S equivalent widths increase at large radii because these lines, as well as those of Ar and Ca, are formed in a shell occupying the outer half of the remnant. A shell of hot Fe is located interior to this, but there is a large region of overlap between these two shells. In the inner 30% of the remnant, there is a core of cooler, 1 keV Fe. We find that the distribution of the ejecta and the yields of the intermediate mass species are consistent with model prediction for Type Ia events.

Monitoring the Evolution of the X-Ray Remnant of SN 1987A

Abstract: We report on the results of our monitoring program of the remnant of SN 1987A with the Advanced CCD Imaging Spectrometer (ACIS) on board the Chandra X-Ray Observatory. Two new observations have been performed in AO2, bringing the total to four monitoring observations over the past 2 years. Over this time period, new techniques for correction of "charge transfer inefficiency" (CTI) and for use of charge spreading to provide angular resolution somewhat better than the pixel size of the CCD detector have become available at Pennsylvania State University. We have processed all four observations using subpixel resolution to obtain the highest possible angular resolution and using our CTI correction software to provide more reliable spectral analysis and flux estimations. The high angular resolution images indicate that the X-ray-bright knots are convincingly correlated with the optical spots, primarily at 1 keV, while higher energy photons are very well correlated with radio images. Our data also provide marginal evidence for radial expansion of the X-ray remnant at a rate of 5200 ± 2100 km s-1. The X-ray flux appears to linearly increase by ~60% over the 18 month period of these observations. The spectrum is dominated by broad complexes of atomic emission lines and can be fitted with a simple model of a plane-parallel shock with electron temperatures of kT ~ 2-4 keV and a postshock electron density of ne ~ 210-420 cm-3. The implied 0.5-10 keV band luminosity in 2001 April is ~1.3 × 1035 ergs s-1; as of that date, we still observe no direct evidence for the central point source, with an upper limit on the observed luminosity of LX ~ 5.5 × 1033 ergs s-1 in the 2-10 keV band.

The Chandra Deep Field North Survey. XI. X-ray emission from luminous infrared starburst galaxies

Abstract: Using the 1 Ms Chandra Deep Field North and 15 micron ISOCAM HDF-N surveys we find a tight correlation between the population of strongly evolving starbursts discovered in faint 15 micron ISOCAM surveys and the apparently normal galaxy population detected in deep X-ray surveys Up to 100% of the X-ray detected emission-line galaxies (ELGs) have 15 micron counterparts, in contrast to 10-20% of the X-ray detected absorption-line galaxies and AGN-dominated sources None of the X-ray detected ELGs is detected in the hard band (2-8 keV), and their stacked-average X-ray spectral slope of Gamma=20 suggests a low fraction of obscured AGN activity within the X-ray detected ELG population The characteristics of the z=04-13 X-ray detected ELGs are consistent with those expected for M82 and NGC 3256-type starbursts; these X-ray detected ELGs contribute ~2% of the 05-80 keV X-ray background The only statistical difference between the X-ray detected and X-ray undetected 15 micron selected ELGs is that a much larger fraction of the former have radio emission

The Chandra Deep Field North Survey. X. X-Ray Emission from Very Red Objects

Abstract: The multiwavelength properties of very red objects [VROs; (I-K) ≥ 4] are largely unknown since many of these sources are optically faint (I ≥ 24) and undetected at most wavelengths. Here we provide constraints on the X-ray (0.5–8.0 keV) properties of VROs using the 1 Ms Chandra exposure of an 84 × 84 region within the Hawaii flanking-field area containing the Hubble Deep Field North (HDF-N). We find that VROs detected in the hard band (2.0–8.0 keV) have flat X-ray spectral slopes (Γ ≈ 0.9) and X-ray properties consistent with those expected from luminous obscured AGNs. The fraction of such sources in the K ≤ 20.1 VRO population is 14%. Conversely, the average X-ray spectral slope of VROs detected in the soft band (0.5–2.0 keV) but not in the hard band is comparatively steep (Γ > 1.4), and the X-ray emission from these sources is consistent with that expected from less energetic processes (i.e., star formation, low-luminosity AGN activity, normal elliptical galaxy emission); star formation and low-luminosity AGN activity scenarios are favored in those sources with irregular optical morphologies. Stacking analyses of the X-ray emission from VROs not individually detected at X-ray energies yield significant detections (≥99% confidence level) in the soft band and in the full band (0.5–8.0 keV). We find this X-ray emission is produced predominantly by the optically brightest VROs. The simplest explanation of this result is that we have detected the average X-ray emission from nonactive VROs with low X-ray–to–optical flux ratios [log (fx/fI) ≈ -2]; this is consistent with that expected if the majority of these VROs are ≈M elliptical galaxies. A number of VROs are also detected with mid-infrared (15 μm) and radio emission, and we provide constraints on the nature of this emission.