Showing papers by "Gordon P. Garmire published in 2004"

The Swift Gamma-Ray Burst Mission

Abstract: The Swift mission, scheduled for launch in 2004, is a multiwavelength observatory for gamma-ray burst (GRB) astronomy. It is a first-of-its-kind autonomous rapid-slewing satellite for transient astronomy and pioneers the way for future rapid-reaction and multiwavelength missions. It will be far more powerful than any previous GRB mission, observing more than 100 bursts yr � 1 and performing detailed X-ray and UV/optical afterglow observations spanning timescales from 1 minute to several days after the burst. The objectives are to (1) determine the origin of GRBs, (2) classify GRBs and search for new types, (3) study the interaction of the ultrarelativistic outflows of GRBs with their surrounding medium, and (4) use GRBs to study the early universe out to z >10. The mission is being developed by a NASA-led international collaboration. It will carry three instruments: a newgeneration wide-field gamma-ray (15‐150 keV) detector that will detect bursts, calculate 1 0 ‐4 0 positions, and trigger autonomous spacecraft slews; a narrow-field X-ray telescope that will give 5 00 positions and perform spectroscopy in the 0.2‐10 keV band; and a narrow-field UV/optical telescope that will operate in the 170‐ 600 nm band and provide 0B3 positions and optical finding charts. Redshift determinations will be made for most bursts. In addition to the primary GRB science, the mission will perform a hard X-ray survey to a sensitivity of � 1m crab (� 2;10 � 11 ergs cm � 2 s � 1 in the 15‐150 keV band), more than an order of magnitude better than HEAO 1 A-4. A flexible data and operations system will allow rapid follow-up observations of all types of

The fall of active galactic nuclei and the rise of star-forming galaxies: A close look at the Chandra Deep Field X-ray number counts

Abstract: We investigate the X-ray number counts in the 1?2 Ms Chandra Deep Fields (CDFs) to determine the contributions of faint X-ray source populations to the extragalactic X-ray background (XRB). X-ray sources were separated into active galactic nuclei (AGNs), star-forming galaxies, and Galactic stars primarily on the basis of their X-ray?to?optical flux ratios, optical spectral classifications, X-ray spectra, and intrinsic X-ray luminosities. Number count slopes and normalizations below 2 ? 10-15 ergs cm-2 s-1 were calculated in each band for all source types assuming a single power-law model. We find that AGNs continue to dominate the number counts in the 0.5?2.0 and 2?8 keV bands. At flux limits of ?2.5 ? 10-17 ergs cm-2 s-1 (0.5?2.0 keV) and ?2.0 ? 10-16 ergs cm-2 s-1 (2?8 keV), the overall AGN source densities are 7166 and 4558 sources deg-2, respectively; these are factors of ~10?20 higher than those found in the deepest optical spectroscopic surveys. Although still a minority, the number counts of star-forming galaxies climb steeply, such that they eventually achieve source densities of 1727 sources deg-2 (0.5?2.0 keV) and 711 sources deg-2 (2?8 keV) at the CDF flux limits. The number of star-forming galaxies will likely overtake the number of AGNs at ~1 ? 10-17 ergs cm-2 s-1 (0.5?2.0 keV) and dominate the overall number counts thereafter. Adopting XRB flux densities of (7.52 ? 0.35) ? 10-12 ergs cm-2 s-1 deg-2 for 0.5?2.0 keV and (1.79 ? 0.11) ? 10-11 ergs cm-2 s-1 deg-2 for 2?8 keV, the CDFs resolve a total of 89.5 percent and 92.6 percent of the extragalactic 0.5?2.0 and 2?8 keV XRBs, respectively. AGNs as a whole contribute ?83% and ?95% to these resolved XRB fractions, respectively, whereas star-forming galaxies comprise only ?3% and ?2%, respectively, and Galactic stars comprise the remainder. Extrapolation of the number count slopes can easily account for the entire 0.5?2.0 and 2?8 keV XRBs to within statistical errors. We also examine the X-ray number counts as functions of intrinsic X-ray luminosity and absorption, finding that sources with L0.5?8 keV > 1043.5 ergs s-1 and NH < 1022 cm-2 are the dominant contributors to the 0.5?2.0 keV XRB flux density, whereas sources with L0.5?8 keV = 1042.5?1044.5 ergs s-1 and a broad range of absorption column densities primarily contribute to the 2?8 keV XRB flux density. This trend suggests that even less intrinsically luminous, more highly obscured AGNs may dominate the number counts at higher energies, where the XRB intensity peaks. Finally, we revisit the reported differences between the CDF-North and CDF-South number counts, finding that the two fields are consistent with each other except for sources detected at 2?8 keV below F2?8 keV ? 1 ? 10-15 ergs cm-2 s-1, for which deviations gradually increase to ?3.9 ?.

The Fall of AGN and the Rise of Star-Forming Galaxies: A Close Look at the Chandra Deep Field X-ray Number Counts

Abstract: We investigate the X-ray number counts in the 1-2 Ms Chandra Deep Fields (CDFs) to determine the contributions of faint X-ray source populations to the extragalactic X-ray background (XRB). X-ray sources were separated into Active Galactic Nuclei (AGN), star-forming galaxies, and Galactic stars based on several criteria. We find that AGN continue to dominate the number counts in the 0.5-2.0 keV and 2-8 keV bands. At flux limits of ~2.5e-17 erg cm-2 s-1 (0.5-2.0 keV) and ~1.4e-16 erg cm-2 s-1 (2-8 keV), the overall AGN source densities are 7166 (+304/-292) and 4558 (+216/-207) sources deg-2, respectively; these are factors of ~10-20 higher than found in the deepest optical spectroscopic surveys. While still a minority, the number counts of star-forming galaxies climb steeply such that they eventually achieve source densities of 1727 (+187/-169) and 711 (+270/-202) sources deg-2 at the CDF 0.5-2.0 keV and 2-8 keV flux limits, respectively. Adopting recent XRB flux densities measurements, the CDFs resolve a total of 89.5% (+5.9%/-5.7%) and 86.9% (+6.6%/-6.3%) of the extragalactic 0.5-2.0 keV and 2-8 keV XRBs, respectively. Extrapolation of the number-count slopes can easily account for the entire 0.5-2.0 keV and 2-8 keV XRBs to within statistical errors. We also revisit the reported differences between the CDF-North and CDF-South number counts, finding that the two fields are consistent except for sources in the 2-8 keV band below F(2-8 keV)~1e-15 erg cm-2 s-1, where deviations gradually increase to ~3.9 sigma.

Diffuse X-Ray Emission in a Deep Chandra Image of the Galactic Center

Abstract: We examine the spectrum of diffuse emission detected in the 17' ? 17' field around Sgr A* during 625 ks of Chandra observations. The spectrum exhibits He-like and H-like lines from Si, S, Ar, Ca, and Fe that are consistent with an origin in a two-temperature plasma, as well as a prominent low-ionization Fe K? line. The cooler, kT ? 0.8 keV plasma differs in surface brightness across the image in the range (0.2-1.8) ? 10-13 ergs cm-2 s-1 arcmin-2 (observed, 2-8 keV). This soft plasma is probably heated by supernovae, along with a small contribution from the winds of massive Wolf-Rayet (W-R) and O stars. The radiative cooling rate of the soft plasma within the inner 20 pc of the Galaxy could be balanced by 1% of the kinetic energy of one supernova every 3 ? 105 yr. The hotter, kT ? 8 keV component is more spatially uniform, with a surface brightness of (1.5-2.6) ? 10-13 ergs cm-2 s-1 arcmin-2 (observed, 2-8 keV). The intensity of the hard plasma is correlated with that of the soft, but they are probably only indirectly related, because neither supernova remnants nor W-R/O stars are observed to produce thermal plasma hotter than kT ? 3 keV. Moreover, a kT ? 8 keV plasma would be too hot to be bound to the Galactic center and therefore would form a slow wind or fountain of plasma. The energy required to sustain such a freely expanding plasma within the inner 20 pc of the Galaxy is ~1040 ergs s-1. This corresponds to the entire kinetic energy of one supernova every 3000 yr, which is unreasonably high. However, alternative explanations for the kT ? 8 keV diffuse emission are equally unsatisfying. The hard X-rays are unlikely to result from undetected point sources, because no known population of stellar objects is numerous enough to account for the observed surface brightness. Neither is there evidence that nonthermal mechanisms for producing the hard emission are operating, as the expected shifts in the line energies and ratios from their collisional-equilibrium values are not observed. We are left to conclude either that there is a significant shortcoming in our understanding of the mechanisms that heat the interstellar medium or that a population of faint (<1031 ergs s-1) hard X-ray sources that is a factor of 10 more numerous than cataclysmic variables remains to be discovered.

First Simultaneous NIR/X-ray Detection of a Flare from SgrA*

Abstract: We report on the first simultaneous near-infrared/X-ray detection of the Sgr A* counterpart which is associated with the massive black hole at the center of the Milky Way. The observations have been carried out using the NACO adaptive optics (AO) instrument at the European Southern Observatory's Very Large Telescope and the ACIS-I instrument aboard the Chandra X-ray Observatory. We also report on quasi-simultaneous observations at a wavelength of 3.4 mm using the Berkeley-Illinois-Maryland Association (BIMA) array. A flare was detected in the X-domain with an excess 2-8 keV luminosity of about 6$\times10^{33}$ erg/s. A fading flare of Sgr A* with $>$2 times the interim-quiescent flux was also detected at the beginning of the NIR observations, that overlapped with the fading part of the X-ray flare. Compared to 8-9 hours before the NIR/X-ray flare we detected a marginally significant increase in the millimeter flux density of Sgr A* during measurements about 7-9 hours afterwards. We find that the flaring state can be conveniently explained with a synchrotron self-Compton model involving up-scattered sub-millimeter photons from a compact source component, possibly with modest bulk relativistic motion. The size of that component is assumed to be of the order of a few times the Schwarzschild radius. The overall spectral indices $\alpha_{NIR/X-ray}$ ($S_{ u}$$\propto$$ u^{-\alpha}$) of both states are quite comparable with a value of $\sim$1.3. Since the interim-quiescent X-ray emission is spatially extended, the spectral index for the interim-quiescent state is probably only a lower limit for the compact source Sgr A*. A conservative estimate of the upper limit of the time lag between the ends of the NIR and X-ray flare is of the order of 15 minutes.

First simultaneous NIR/X-ray detection of a flare from Sgr A

Abstract: We report on the first simultaneous near-infrared/X-ray detection of the Sgr A* counterpart associated with the massive 3-4 × 10 6 Mblack hole at the center of the Milky Way. The observations have been carried out using the NACO adaptive optics (AO) instrument at the European Southern Observatory's Very Large Telescopeand the ACIS-I instrument aboard the Chandra X-ray Observatory. We also report on quasi-simultaneous observations at a wavelength of 3.4 mm using the Berkeley-Illinois-Maryland Association (BIMA) array. A flare was detected in the X-domain with an excess 2-8 keV luminosity of about 6 × 10 33 erg/s. A fading flare of Sgr A* with >2 times the interim-quiescent flux was also detected at the beginning of the NIR observations, that overlapped with the fading part of the X-ray flare. Compared to 8-9 h before the NIR/X-ray flare we detected a marginally significant increase in the millimeter flux density of Sgr A* during measurements about 7-9 h afterwards. We find that the flaring state can be conveniently explained with a synchrotron self-Compton model involving up-scattered sub-millimeter photons from a compact source component, possibly with modest bulk relativistic motion. The size of that component is assumed to be of the order of a few times the Schwarzschild radius. The overall spectral indices αNIR/X−ray (S ν ∝ ν −α ) of both states are quite comparable with a value of ∼1.3. Since the interim-quiescent X-ray emission is spatially extended, the spectral index for the interim-quiescent state is probably only a lower limit for the compact source Sgr A*. A conservative estimate of the upper limit of the time lag between the ends of the NIR and X-ray flare is of the order of 15 min.

The Spectra and Variability of X-Ray Sources in a Deep Chandra Observation of the Galactic Center

Abstract: We examine the X-ray spectra and variability of the sample of X-ray sources with LX ≈ 1031-1033 ergs s-1 identified within the inner 9' of the Galaxy by Muno et al. Very few of the sources exhibit intraday or intermonth variations. We find that the spectra of the point sources near the Galactic center are very hard between 2-8 keV, even after accounting for absorption. When modeled as power laws, the median photon index is Γ = 0.7, while when modeled as thermal plasma we can only obtain lower limits to the temperature of kT > 8 keV. The combined spectra of the point sources is similarly hard, with a photon index of Γ = 0.8. Strong line emission is observed from low-ionization, He-like, and H-like Fe, both in the average spectra and in the brightest individual sources. The line ratios of the highly ionized Fe in the average spectra are consistent with emission from a plasma in thermal equilibrium. This line emission is observed whether average spectra are examined as a function of the count rate from the source or as a function of the hardness ratios of individual sources. This suggests that the hardness of the spectra may in fact be to due local absorption that partially covers the X-ray-emitting regions in the Galactic center systems. We suggest that most of these sources are intermediate polars, which (1) often exhibit hard spectra with prominent Fe lines, (2) rarely exhibit either flares on short timescales or changes in their mean X-ray flux on long timescales, and (3) are the most numerous hard X-ray sources with comparable luminosities in the Galaxy.

Characteristics of diffuse X-ray line emission within 20 parsecs of the Galactic center

Abstract: Over the last three years, the Galactic center region has been monitored with the Chandra X-Ray Observatory. Besides the X-ray emission from the target object, Sgr A*, diffuse X-ray emission was detected throughout most of the 17' × 17' field of view. With 11 Chandra observations through 2002 June, the total effective exposure reaches ~590 ks, providing significant photon statistics on much of the detailed structure within the faint, often filamentary, diffuse X-ray emission. The true-color X-ray image and the equivalent-width images for the detected elemental species of the Galactic center region demonstrate that the diffuse X-ray features have a broad range of spatiospectral properties. Enhancements of the low ionization state Fe line emission (E ~ 6.4 keV) to the northeast of Sgr A* can be interpreted as fluorescence within the dense interstellar medium resulting from irradiation by hard, external X-ray sources. They may also be explained by emission induced by the bombardments by high-energy particles on the ambient medium, such as may accompany unresolved supernova ejecta intruding into dense interstellar medium. The detection of molecular cloud counterparts to the 6.4 keV Fe line-emitting features indicates that these Fe line features are associated with dense Galactic center clouds and/or active star-forming regions, which strongly supports the proposed origins of the X-ray reflection and/or supernova ejecta for the Fe line emission. We detect highly ionized S and Si lines that are generally coincident with the low ionization state Fe line emission and the dense molecular clouds in the northeast of Sgr A*. These hot plasmas are then most likely produced by massive star-forming activities and/or supernova remnants in the Galactic center. In contrast, we find that highly ionized He-like Fe line emission (E ~ 6.7 keV) is primarily distributed along the plane instead of being concentrated in the northeast of Sgr A*. The implied high temperature and the relatively uniform, but strong, alignment along the plane are consistent with the magnetic-confinement model, suggesting that this hot gas component has been reheated by the strong interstellar magnetic fields in the Galactic center to produce the observed He-like Fe line emission.

A Study of QSO Evolution in the X-Ray Band with the Aid of Gravitational Lensing

Abstract: We present results from a mini-survey of relatively high redshift (1.7 1.5, especially in the low luminosity range between 10^{43}-10^{45.5} erg s^{-1}. This result is surprising considering that such a correlation is not found for quasars with redshifts below 1.5. We suggest that this correlation can be understood in the context of the hot-corona model for X-ray emission from quasar accretion disks, under the hypothesis that the quasars in our sample accrete very close to their Eddington limits and the observed luminosity range is set by the range of black hole masses (this hypothesis is consistent with recent predictions of semi-analytic models for quasar evolution). The upper limits of X-ray variability of our relatively high redshift sample of lensed quasars are consistent with the known correlation between variability and luminosity observed in Seyfert 1s when this correlation is extrapolated to the larger luminosities of our sample.

Nucleosynthesis in the Oxygen-rich Supernova Remnant G292.0+1.8 from Chandra X-Ray Spectroscopy

Abstract: We continue our analysis of the Galactic oxygen-rich supernova remnant (SNR) G292.0+1.8, which was observed with the Chandra X-Ray Observatory. The high angular resolution Chandra data resolve metal-rich ejecta knots as well as the shocked circumstellar medium. X-ray emission from the ejecta material in G292.0+1.8 is dominated by highly ionized O, Ne, and Mg. Measured abundance ratios suggest that this material was produced during the hydrostatic evolution of the massive progenitor star. In contrast to Cassiopeia A, there is little evidence for X-ray-emitting ejecta from explosive nucleosynthesis, i.e., material enriched in Si, S, and particularly, Fe. This limits the amount of mixing or overturning of deep ejecta material in G292.0+1.8 and suggests that the ejecta are strongly stratified by composition and that the reverse shock has not propagated to the Si/S- or Fe-rich zones. On the other hand, the bright equatorial belt is dominated by X-ray emission with normal chemical composition, which supports shocked dense circumstellar material for its origin. We find that the thermal pressure in the SNR is much higher than the pressure in the pulsar wind nebula (PWN), indicating that the reverse shock has not yet begun to interact with the PWN.

A Study of Quasar Evolution in the X-Ray Band with the Aid of Gravitational Lensing

Abstract: We present results from a minisurvey of relatively high redshift (1.7 1.5, especially in the low-luminosity range between 1043 and 1045.5 ergs s-1. This result is surprising, considering that such a correlation is not found for quasars with redshifts below 1.5. We suggest that this correlation can be understood in the context of the hot-corona model for X-ray emission from quasar accretion disks, under the hypothesis that the quasars in our sample accrete very close to their Eddington limits and that the observed luminosity range is set by the range of black hole masses (this hypothesis is consistent with recent predictions of semianalytic models for quasar evolution). The upper limits of X-ray variability for our relatively high redshift sample of lensed quasars are consistent with the known correlation between variability and luminosity observed in Seyfert 1 galaxies when this correlation is extrapolated to the larger luminosities of our sample.

The Chandra Deep Field-North Survey. XVII. Evolution of Magnetic Activity in Old Late-Type Stars*

Abstract: The extremely sensitive Chandra Deep Field–North (CDF-N) pencil-beam X-ray survey is used to identify and characterize the X-ray emission from old high-latitude main-sequence Galactic stars Our principal goal is to investigate the expected long-term decay of magnetic activity of late-type stars due to the gradual spin-down of stellar rotation from a magnetized stellar wind Thirteen X-ray sources are associated with late-type stars; 11 of these constitute a well-defined sample for statistical analysis This sample consists of two G, two K0–K4, and seven M2–M5 stars with median V-band magnitude around 19 and median distance around 300 pc X-ray luminosities are typically logLX ’ 27 ergs s � 1 but are substantially higher in two cases The combination of large-amplitude variations on timescales of hours and plasma temperatures around 5–30 MK indicates that the observed X-ray emission is dominated by magnetic reconnection flares rather than quiescent coronal emission These X-ray properties are quantitatively similar to those seen in the active contemporary Sun The CDF-N stellar sample is compared to simulations based on convolution of X-ray luminosity functions (XLFs) with the known spatial distribution of old-disk stars The model indicates that the CDF-N stars are the most magnetically active old-disk stars A substantial decline in X-ray luminosities over the 1 Gyr < t < 11 Gyr age interval is required: 39 rather than 11 stars should have been detected if the XLF does not evolve over this time interval This is a clear demonstration that the coronal and flaring components of stellar magnetic activity—and presumably the interior magnetic dynamos responsible for the reconnecting fields at the stellar surface—exhibit long-term decay over the age of the Galactic disk The model that best fits the magnitudes, spectral types, and X-ray luminosities of the sample has LX / t � 2 ergs s � 1 , which is faster than the t � 1 decay rate predicted from widely accepted rotational spin-down rates and X-ray–activity relations Subject headingg stars: activity — stars: coronae — stars: late-type — stars: magnetic fields — X-rays: stars

A Chandra View of the Morphological and Spectral Evolution of Supernova Remnant 1987A

Abstract: We present an update on the results of our monitoring observations of the X-ray remnant of supernova (SN) 1987A with Chandra. As of 2002 December, we have performed a total of seven observations of SN 1987A, which allows us to monitor the details of the earliest stage of the supernova remnant evolution in X-rays. The high angular resolution images from the latest data reveal developments of new X-ray-bright spots in the northwestern and the southwestern portions of the remnant, as well as changes on the eastern side. The observed soft X-ray flux is increasing more rapidly than ever, and the latest 0.5-2 keV band flux (fX ~ 6 × 10-13 ergs cm-2 s-1) is 4 times brighter than 3 yr earlier, when this monitoring began. The overall X-ray emission is primarily from the blast wave shock with kT ~ 2.4 keV. As the blast wave approaches the dense circumstellar material, the contribution from the decelerated slow shock (kT ~ 0.22 keV) to the observed X-ray emission is becoming significant. The increase of this slow shock contribution over the last 2 yr is particularly noticeable in the western half of the remnant. These results indicate that the shock front is now reaching the main body of the inner circumstellar ring and that SN 1987A will be a complete ring with dramatic brightening in coming years. Based on the best-fit two-shock spectral model, we derive approximate densities of the X-ray-emitting regions (ne ~ 235 cm-3 for the fast shock and ne ~ 7500 cm-3 for the slow shock). There is no direct observational evidence to date for a neutron star associated with supernova remnant 1987A. We obtain an upper limit on the observed X-ray luminosity of any embedded point source (LX ≤ 1.5 × 1034 ergs s-1) in the 2-10 keV band. The X-ray remnant continues to expand linearly at a rate of 4167 km s-1.

The Chandra Deep Field-North Survey: XVII. Evolution of magnetic activity in old late-type stars

Abstract: The extremely sensitive Chandra Deep Field-North (CDF-N) pencil-beam X-ray survey is used to identify and characterize the X-ray emission from old high-latitude main sequence Galactic stars. Our principal goal is to investigate the expected long-term decay of magnetic activity of late-type stars due to the gradual spindown of stellar rotation from a magnetized stellar wind. Eleven X-ray sources constitute a well-defined sample of 2 G, 2 K0-K4, and 7 M2-M5 stars with median distance around 300 pc. X-ray luminosities are typically log Lx ~ 27 erg/s and is dominated by flares rather than quiescent coronal emission. Models of the population indicates that the CDF-N stars are the most magnetically active old disk stars. A substantial decline in X-ray luminosities over the 1

Chandra Observations of Radio-Loud Quasars at z > 4: X-Rays from the Radio Beacons of the Early Universe

Abstract: We present the results of Chandra observations of six radio-loud quasars (RLQs) and one optically bright radioquiet quasar (RQQ) at z 4.1‐4.4. These observations cover a representative sample of RLQs with moderate radio-loudness (R 40‐400), filling the X-ray observational gap between optically selected radio-quiet quasars (predominantly R 2‐10) and the five known blazars at z 4 (R 800‐27000), where R= f5 GHz f4400 ˚ A (rest frame). We study the relationship between X-ray luminosity and radio-loudness for quasars at high redshift and constrain RLQ X-ray continuum emission and absorption. From a joint spectral fit of nine moderate-R RLQs observed by Chandra, we find tentative evidence for absorption above the Galactic NH , with a best-fit neutral intrinsic column density of NH 2 4 2 0 1 8 10 22 cm 2 , consistent with earlier claims of increased absorption toward high-redshift RLQs. We also search for evidence of an enhanced jet-linked component in the X-ray emission due to the increased energy density of the cosmic microwave background (CMB) at high redshift, but we find neither spatial detections of X-ray jets nor a significant enhancement in the X-ray emission relative to comparable RLQs at low-to-moderate redshifts. Overall, the z 4‐5 RLQs have basic X-ray properties consistent with comparable RLQs in the local universe, suggesting that the accretion/jet mechanisms of these objects are similar as well. Subject headings: galaxies: active — galaxies: high-redshift — galaxies: jets — galaxies: nuclei — quasars: general

Detection of X-Rays from Galaxy Groups Associated with the Gravitationally Lensed Systems PG 1115+080 and B1422+231

Abstract: Gravitational lenses that produce multiple images of background quasars can be an invaluable cosmological tool. Deriving cosmological parameters, however, requires modeling the potential of the lens itself. It has been estimated that up to a quarter of lensing galaxies are associated with a group or cluster that perturbs the gravitational potential. Detection of X-ray emission from the group or cluster can be used to better model the lens. We report on the first detection in X-rays of the group associated with the lensing system PG 1115+080 and the first X-ray image of the group associated with the system B1422+231. We find a temperature and rest-frame luminosity of 0.8 keV and 7 × 1042 ergs s-1, respectively, for PG 1115+080 and 1.0 keV and 8 × 1042 ergs s-1, respectively, for B1422+231. We compare the spatial and spectral characteristics of the X-ray emission with the properties of the group galaxies, with lens models, and with the general properties of groups at lower redshift.

AC 114: A Cluster with a Soft X-Ray Tail

Abstract: We present Chandra observations of the galaxy cluster AC 114. The cluster shows a strongly irregular morphology, with signs of multiple merging activity. We report the discovery of a soft X-ray filament originating close to the core of the cluster, curving to the southeast for approximately 15. We confirm that X-ray emission is associated with two of three mass concentrations identified in previous gravitational lensing studies of this object. These two mass concentrations are located at opposite ends of the soft filament, and we interpret this as evidence for interaction between them. The northern part of the cluster reveals signs of further dynamical activity. Two sharp discontinuities are detected in both the surface brightness and temperature profiles, evincing another more recent merger event that took place in or close to the cluster core. A preliminary combined mass and lensing analysis shows, in spite of the merger activity, remarkably good agreement between lensing and X-ray masses computed with the assumption of hydrostatic equilibrium.

An evaluation of a bake-out of the ACIS instrument on the Chandra X-Ray Observatory

Abstract: The sensitivity of the Advanced CCD Imaging Spectrometer (ACIS) instrument on the Chandra X-ray Observatory (CXO) to low-energy X-rays (0.3 - 2.0 keV) has been declining throughout the mission. The most likely cause of this degradation is the growth of a contamination layer on the cold (-60 C) filter which attenuates visible and near-visible light incident on the CCDs. The contamination layer is still increasing 4 years after launch, but at a significantly lower rate than initially. We have determined that the contaminant is composed mostly of C with small amounts of O and F. We have conducted ground experiments to determine the thermal desorption properties of candidate materials for the contaminant. We have conducted experiments to determine the robustness of the thin filter to the thermal cycling necessary to remove the contaminant. We have modeled the migration of the contaminant during this bake-out process to ensure that the end result will be a reduction in the thickness of the contamination layer. We have considered various profiles for the bake-out consisting of different temperatures for the ACIS focal plane and detector housing and different dwell times at these temperatures. The largest uncertainty which affects our conclusions is the volatility of the unknown contaminants. We conclude that bakeout scenarios in which the focal plane temperature and the detector housing temperature are raised to +20~C are the most likely to produce a positive outcome.

The Fall of AGN and the Rise of Star-Forming Galaxies: A Close Look at the Chandra Deep Field X-ray Number

Abstract: We investigate the X-ray number counts in the 1-2 Ms Chandra Deep Fields (CDFs) to determine the contributions of faint X-ray source populations to the extragalactic X-ray background (XRB). X-ray sources were separated into Active Galactic Nuclei (AGN), star-forming galaxies, and Galactic stars based on several criteria. We find that AGN continue to dominate the number counts in the 0.5-2.0 keV and 2-8 keV bands. At flux limits of ~2.5e-17 erg cm-2 s-1 (0.5-2.0 keV) and ~1.4e-16 erg cm-2 s-1 (2-8 keV), the overall AGN source densities are 7166 (+304/-292) and 4558 (+216/-207) sources deg-2, respectively; these are factors of ~10-20 higher than found in the deepest optical spectroscopic surveys. While still a minority, the number counts of star-forming galaxies climb steeply such that they eventually achieve source densities of 1727 (+187/-169) and 711 (+270/-202) sources deg-2 at the CDF 0.5-2.0 keV and 2-8 keV flux limits, respectively. Adopting recent XRB flux densities measurements, the CDFs resolve a total of 89.5% (+5.9%/-5.7%) and 86.9% (+6.6%/-6.3%) of the extragalactic 0.5-2.0 keV and 2-8 keV XRBs, respectively. Extrapolation of the number-count slopes can easily account for the entire 0.5-2.0 keV and 2-8 keV XRBs to within statistical errors. We also revisit the reported differences between the CDF-North and CDF-South number counts, finding that the two fields are consistent except for sources in the 2-8 keV band below F(2-8 keV)~1e-15 erg cm-2 s-1, where deviations gradually increase to ~3.9 sigma.

An X-Ray Measurement of Titan’s Atmospheric Extent from Its Transit of the Crab Nebula

Abstract: Saturn's largest satellite, Titan, transited the Crab Nebula on 2003 January 5. We observed this astronomical event with the Chandra X-Ray Observatory. An "occultation shadow" has clearly been detected and is found to be larger than the diameter of Titan's solid surface. The difference gives a thickness for Titan's atmosphere of 880 ± 60 km. This is the first measurement of Titan's atmospheric extent at X-ray wavelengths. The value measured is consistent with or slightly larger than those estimated from earlier Voyager observations at other wavelengths. We discuss the possibility of temporal variations in the thickness of Titan's atmosphere.

The Spectra and Variability of X-ray Sources in a Deep Chandra Observation of the Galactic Center

Abstract: We examine the X-ray spectra and variability of the sample of X-ray sources with L_X = 10^{31}-10^{33} erg s^{-1} identified within the inner 9' of the Galaxy. Very few of the sources exhibit intra-day or inter-month variations. We find that the spectra of the point sources near the Galactic center are very hard between 2--8 keV, even after accounting for absorption. When modeled as power laws the median photon index is Gamma=0.7, while when modeled as thermal plasma we can only obtain lower limits to the temperature of kT>8 keV. The combined spectra of the point sources is similarly hard, with a photon index of Gamma=0.8. Strong line emission is observed from low-ionization, He-like, and H-like Fe, both in the average spectra and in the brightest individual sources. The line ratios of the highly-ionized Fe in the average spectra are consistent with emission from a plasma in thermal equilibrium. This line emission is observed whether average spectra are examined as a function of the count rate from the source, or as a function of the hardness ratios of individual sources. This suggests that the hardness of the spectra may in fact to due local absorption that partially-covers the X-ray emitting regions in the Galactic center systems. We suggest that most of these sources are intermediate polars, which (1) often exhibit hard spectra with prominent Fe lines, (2) rarely exhibit either flares on short time scales or changes in their mean X-ray flux on long time scales, and (3) are the most numerous hard X-ray sources with comparable luminosities in the Galaxy.

An X-ray measurement of Titan's atmospheric extent from its transit of the Crab Nebula

Abstract: Saturn's largest satellite, Titan, transited the Crab Nebula on 5 January 2003. We observed this astronomical event with the {\it Chandra} X-ray Observatory. An ``occultation shadow'' has clearly been detected and is found to be larger than the diameter of Titan's solid surface. The difference gives a thickness for Titan's atmosphere of 880 $\pm$ 60 km. This is the first measurement of Titan's atmospheric extent at X-ray wavelengths. The value measured is consistent with or slightly larger than those estimated from earlier Voyager observations at other wavelengths. We discuss the possibility of temporal variations in the thickness of Titan's atmosphere.

Monitoring the Evolution of SNR 1987A with Chandra

Abstract: We report on the results of our monitoring program of SNR 1987A with the Chandra X-Ray Observatory. The high resolution images and the spectra from the latest Chandra data suggest that the blast wave has reached the dense inner ring on the western side of the SNR, as well as in the east. The observed soft X-ray flux is increasing more rapidly than ever, and the latest flux is four times brighter than three years ago.

Diffuse X-ray Emission in a Deep Chandra Image of the Galactic Center

Abstract: We examine the spectrum of diffuse emission detected in the 17' by 17' field around Sgr A* during 625 ks of Chandra observations. The spectrum exhibits He-like and H-like lines from Si, S, Ar, Ca, and Fe, that are consistent with originating in a two-temperature plasma, as well as a prominent low-ionization Fe line. The cooler, kT=0.8 keV plasma differs in surface brightness across the image by a factor of 9. This soft plasma is probably heated by supernovae. The radiative cooling rate of the plasma within the inner 20 pc of the Galaxy could be balanced by 1% of the kinetic energy of one supernova every 300,000 y. The hotter, kT=8 keV component is more spatially uniform, ranging over a factor of 2 in surface brightness. The intensity of the hard plasma is correlated with that of the soft, but they are probably only indirectly related, because supernova remnants are not observed to produce thermal plasma hotter than kT=3 keV. Moreover, a kT=8 keV plasma is too hot to be bound to the Galactic center, and therefore would form a slow wind or fountain of plasma. The energy required to sustain such a freely-expanding plasma within the inner 20 pc of the Galaxy is ~10^40 erg/s, which corresponds to the entire kinetic energy of one supernova every 3000 y. This rate is unreasonably high. However, alternative explanations for the kT=8 keV diffuse emission are equally unsatisfying. We are left to conclude that either the diffuse emission is heated by an unanticipated source of energy, or that a population of faint (< 10^31 erg/s), hard X-ray sources that are a factor of 10 more numerous than CVs remains to be discovered. (Abridged)