Showing papers by "Luigi Stella 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

SN 2003lw and GRB 031203: A Bright Supernova for a Faint Gamma-Ray Burst

Abstract: Optical and near-infrared observations of the gamma-ray burst GRB 031203, at z = 0.1055, are reported. A very faint afterglow is detected superposed onto the host galaxy in our first infrared JHK observations, carried out ~9 hr after the burst. Subsequently, a rebrightening is detected in all bands, peaking in the R band about 18 rest-frame days after the burst. The rebrightening closely resembles the light curve of a supernova like SN 1998bw, assuming that the GRB and the SN went off almost simultaneously, but with a somewhat slower evolution. Spectra taken close to the maximum of the rebrightening show extremely broad features as in SN 1998bw. The determination of the absolute magnitude of this SN (SN 2003lw) is difficult owing to the large and uncertain extinction, but likely this event was brighter than SN 1998bw by 0.5 mag in the VRI bands, reaching an absolute magnitude MV = -19.75 ± 0.15.

SN 2003lw and GRB 031203: a bright supernova for a faint gamma-ray burst

Abstract: Optical and near-infrared observations of the gamma-ray burst GRB 031203, at , are reported. A z p 0.1055 very faint afterglow is detected superposed onto the host galaxy in our first infrared JHK observations, carried out ∼9 hr after the burst. Subsequently, a rebrightening is detected in all bands, peaking in the R band about 18 rest-frame days after the burst. The rebrightening closely resembles the light curve of a supernova like SN 1998bw, assuming that the GRB and the SN went off almost simultaneously, but with a somewhat slower evolution. Spectra taken close to the maximum of the rebrightening show extremely broad features as in SN 1998bw. The determination of the absolute magnitude of this SN (SN 2003lw) is difficult owing to the large and uncertain extinction, but likely this event was brighter than SN 1998bw by 0.5 mag in the VRI bands, reaching an absolute magnitude . M p 19.75 0.15 V

A study of the low-frequency quasi-periodic oscillations in the X-ray light curves of the black hole candidate XTE J1859+226

Abstract: We present the results of an extensive timing analysis of the 1999 outburst of the soft X-ray transient and black hole candidate XTE J1859+226 as observed with the Rossi X-Ray Timing Explorer . Three main different types of low frequency (1-9 Hz) quasi-periodic oscillations (QPOs) were observed and classified, strengthening the general picture that is emerging for the variability properties of black hole X-ray binaires. Rapid transitions between different power spectral shapes were observed and their link with the count rate was studied. Furthermore, we show that a frequency of ~6 Hz seems to hold a particular place: one of the three QPO types we found was very stable when at this frequency, as it happens in Z sources as well. The coherence of its subharmonic peak was higher when the fundamental was close to 6 Hz, thus suggesting the presence of some resonance at this frequency.

Indirect Evidence of an Active Radio Pulsar in the Quiescent State of the Transient Millisecond Pulsar SAX J1808.4-3658

Abstract: Millisecond radio pulsars are neutron stars that have been spun up by the transfer of angular momentum during the low-mass X-ray binary phase. The transition from an accretion-powered pulsar to a rotation-powered pulsar takes place on evolutionary timescales at the end of the accretion process; however, it may also occur sporadically in systems undergoing transient X-ray activity. We have obtained the first optical spectrum of the low-mass transient X-ray pulsar SAX J1808.4-3658 in quiescence. Similar to the black widow millisecond pulsar B1957+20, this X-ray pulsar shows a large optical modulation at the orbital period due to an irradiated companion star. Using the brightness of the companion star as a bolometer, we conclude that a very high irradiating luminosity, a factor of ~100 larger than directly observed, must be present in the system. This most likely derives from a rotation-powered neutron star that resumes activity during quiescence.

A flash in the dark: UVES/VLT high resolution spectroscopy of GRB afterglows

Abstract: We present the first high resolution (R=20000--45000, corresponding to 14 km/s at 4200A to 6.6 km/s at 9000A) observations of the optical afterglow of Gamma Ray Bursts. GRB020813 and GRB021004 were observed by UVES@VLT 22.19 hours and 13.52 hours after the trigger, respectively. These spectra show that the inter--stellar matter of the GRB host galaxies is complex, with many components contributing to each main absorption system, and spanning a total velocity range of up to about 3000 km/s. Several narrow components are resolved down to a width of a few tens of km/s. In the case of GRB021004 we detected both low and high ionization lines. Combined with photoionization results obtained with CLOUDY, the ionization parameters of the various systems are consistent with a remarkably narrow range with no clear trend with system velocity. This can be interpreted as due to density fluctuations on top of a regular R^-2 wind density profile.

The variable quiescence of Centaurus X-4

Abstract: Centaurus X-4 is one of the best-studied low-mass neutron star transients in quiescence. Thanks to XMM-Newton's large throughput, Centaurus X-4 was observed at the highest signal-to-noise ratio ever. This allowed us to discern rapid (>100 s), large (45% ± 7% rms in the 10-4 to 1 Hz range) intensity variability, especially at low energies. In order to highlight the cause of this variability, we divided the data into intensity intervals and fit the resulting spectra with the canonical model for neutron star transients in quiescence, i.e., an absorbed power law plus a neutron star atmosphere. The fit is consistent with a variable column density plus variability in (at least) one of the spectral models. Variations in the neutron star atmosphere might suggest that accretion onto the active millisecond radio pulsar emitting X-rays at the shock between a radio pulsar wind and inflowing matter from the companion star.

Indirect evidence for an active radio pulsar in the quiescent state of the transient ms pulsar SAX J1808.4-3658

Abstract: Millisecond radio pulsars are neutron stars that have been spun-up by the transfer of angular momentum during the low-mass X-ray binary phase. The transition from an accretion-powered to a rotation-powered pulsar takes place on evolutionary timescales at the end of the accretion process, however it may also occur sporadically in systems undergoing transient X-ray activity. We have obtained the first optical spectrum of the low mass transient X-ray pulsar SAX J1808.4-3658 in quiescence. Similar to the black widow millisecond pulsar B1957+20, this X-ray pulsar shows a large optical modulation at the orbital period due to an irradiated companion star. Using the brightness of the companion star as a bolometer, we conclude that a very high irradiating luminosity, a factor of ~100 larger than directly observed, must be present in the system. This most likely derives from a rotation-powered neutron star that resumes activity during quiescence.

Infrared and X-ray variability of the transient Anomalous X-ray Pulsar XTE J1810-197 ⋆

Abstract: We report on observations aimed at searching for flux variations from the proposed IR counterpart of the Anomalous X-ray Pulsar (AXP) XTE J1810 197. These data, obtained in March 2004 with the adaptive optics camera NAOS-CONICA at the ESO VLT, show that the candidate proposed by Israel et al. (2004) was fainter by �H = 0.7 ± 0.2 andKs = 0.5 ± 0.1 with respect to October 2003, confirming it as the IR counterpart of XTE J1810 197. We also report on an XMM-Newton observation carried out the day before the VLT observations. The 0.5-10keV absorbed flux of the source was 2.2 × 10 11 erg cm 2 s 1 , which is less by a factor of about two compared to the previous XMM-Newton observation on September 2003. Therefore, we conclude that a similar flux decrease took place in the X-ray and IR bands. We briefly discuss these results in the framework of the proposed mechanism(s) responsible for the IR variable emission of AXPs.

On the jet structure and magnetic field configuration of GRB 020813

Abstract: The polarization curve of GRB 020813 is discussed and compared to different models for the structure, evolution and magnetisation properties of the jet and the interstellar medium onto which the fireball impacts. GRB 020813 is best suited for this kind of analysis for the smoothness of its afterglow light curve, ensuring the applicability of current models. The polarization dataset allows us to rule out the standard GRB jet, in which the energy and Lorentz factor have a well defined value inside the jet opening angle and the magnetic field is generated at the shock front. We explore alternative models finding that a structured jet or a jet with a toroidal component of the magnetic field can fit equally well the polarization curve. Stronger conclusions cannot be drawn due to the incomplete sampling of the polarization curve. A more dense sampling, especially at early times, is required to pin down the structure of the jet and the geometry of its magnetic field.Based on observations collected at the European Southern Observatory, Cerro Paranal (Chile), ESO programmes 69.D-0461(A) and 69.D-0701(A).

Pronounced Long-Term Flux Variability of the Anomalous X-Ray Pulsar 1E 1048.1-5937

Abstract: We present XMM-Newton and Chandra observations of 1E 1048.1-5937, being the first to show evidence for a significant variation in the X-ray luminosity of this anomalous X-ray pulsar (AXP). While during the first XMM-Newton (2000 December) and Chandra (2001 July) observations the source had a flux consistent with that measured on previous occasions (~5 × 10-12 ergs cm-2 s-1), two more recent observations found it at a considerably higher flux level of 2 × 10-11 ergs cm-2 s-1 (2002 August; Chandra) and 10-11 ergs cm-2 s-1 (2003 June; XMM-Newton). All the spectra are fit by the sum of a blackbody with kT ~ 0.6 keV and a power law with photon index ~3. No significant changes were seen in the spectral parameters, while the pulsed fraction in the 0.6-10 keV energy range decreased from ~90% in 2000 to ~53% in 2003. The spectral invariance does not support the presence of two physically distinct components in the AXP emission. The sparse coverage of the data does not permit us to unambiguously relate the observed variations to the two bursts seen from this source in the fall of 2001.

Accurate X-ray position of the Anomalous X-ray Pulsar XTE J1810-197 and identification of its likely IR counterpart

Abstract: We report the accurate sub-arcsec X-ray position of the new Anomalous X-ray Pulsar (AXP) XTE J1810-197, derived with a Chndra-HRC Target of Opportunity observation carried out in November 2003. We also report the discovery of a likely IR counterpart based on a VLT (IR band) Target of Opportunity observation carried out in October 2003. Our proposed counterpart is the only IR source (Ks=20.8) in the X-ray error circle. Its IR colors as well as the X-ray/IR flux ratio, are consistent with those of the counterparts of all other AXPs (at variance with field star colors). Deep Gunn-i band images obtained at the 3.6m ESO telescope detected no sources down to a limiting magnitude of 24.3. Moreover, we find that the pulsed fraction and count rates of XTE J1810-197 remained nearly unchanged since the previous Chandra and XMM-Newton observations (2003 August 27th and September 8th, respectively). We briefly discuss the implications of these results. In particular, we note that the transient (or at least highly variable) nature of this AXP might imply a relatively large number of hidden members of this class.

Accurate X-Ray Position of the Anomalous X-Ray Pulsar XTE J1810-197 and Identification of Its Likely Infrared Counterpart

Abstract: We report the accurate subarcsecond X-ray position of the new anomalous X-ray pulsar (AXP) XTE J1810-197, derived with a Chandra High Resolution Camera Target of Opportunity observation carried out in 2003 November. We also report the discovery of a likely IR counterpart based on a Very Large Telescope (IR band) Target of Opportunity observation carried out in 2003 October. Our proposed counterpart is the only IR source (Ks = 20.8) in the X-ray error circle. Its IR colors as well as the X-ray/IR flux ratio are consistent with those of the counterparts of all other AXPs (at variance with field star colors). Deep Gunn i-band images obtained at the 3.6 m ESO telescope detected no sources down to a limiting magnitude of 24.3. Moreover, we find that the pulsed fraction and count rates of XTE J1810-197 remained nearly unchanged since the previous Chandra and XMM-Newton observations (2003 August 27 and September 8, respectively). We briefly discuss the implications of these results. In particular, we note that the transient (or at least highly variable) nature of this AXP might imply a relatively large number of hidden members of this class.

Correlated Infrared and X-ray variability of the transient Anomalous X-ray Pulsar XTE J1810-197

Abstract: We report on observations aimed at searching for flux variations from the proposed IR counterpart of the Anomalous X-ray Pulsar (AXP) XTE J1810−197. These data, obtained in March 2004 with the adaptive optics camera NAOS-CONICA at the ESO VLT, show that the candidate proposed by Israel et al. (2004a, ApJ, 603, L97) was fainter by ∆H = 0.7 ± 0.2 and ∆Ks = 0.5 ± 0.1 with respect to October 2003, confirming it as the IR counterpart of XTE J1810−197. We also report on an XMM-Newton observation carried out the day before the VLT observations. The 0.5−10 keV absorbed flux of the source was 2.2 × 10 −11 erg cm −2 s −1 , which is less by a factor of about two compared to the previous XMM-Newton observation on September 2003. Therefore, we conclude that a similar flux decrease took place in the X-ray and IR bands. We briefly discuss these results in the framework of the proposed mechanism(s) responsible for the IR variable emission of AXPs.

Pronounced long term flux variability of the Anomalous X-ray Pulsar 1E1048.1-5937

Abstract: We present XMM-Newton and Chandra observations of 1E1048.1-5937, being the first to show evidence for a significant variation in the X-ray luminosity of this Anomalous X-ray Pulsar (AXP). While during the first XMM-Newton (2000 December) and Chandra (2001 July) observations the source had a flux consistent with that measured on previous occasions (~5x10e-12 ergs/cmq/s), two more recent observations found it at a considerably higher flux level of 2x10e-11 ergs/cmq/s (2002 August; Chandra) and 10e-11 ergs/cmq/s (2003 June; XMM-Newton). All the spectra are fit by the sum of a blackbody with kT~0.6 keV and a power law with photon index ~3. No significant changes were seen in the spectral parameters, while the pulsed fraction in the 0.6-10 keV energy range decreased from ~90% in 2000 to ~53% in 2003. The spectral invariance does not support the presence of two physically distinct components in the AXP emission. The sparse coverage of the data does not permit us to unambiguously relate the observed variations to the two bursts seen from this source in the fall of 2001.

Are All Gamma-Ray Bursts Like GRB 980425, GRB 030329, and GRB 031203?

Abstract: We study the probability that three GRBs (980425, 030329, 031203) are found within z = 0.17, given the luminosity functions consistent with the log N-log S relationship for classical cosmological bursts (i.e., those observed by BATSE). We show that, in order for the probability of these three low-z events to be nonnegligible (thus making it more likely that they belong to the same class of the classical cosmological bursts), the bursts' luminosity function must be a broken power law. By reasoning in analogy with beamed active galactic nuclei, we show that observations are consistent with the expectations if GRB 980425 and GRB 031203 are indeed normal bursts seen sideways. Within this model, no bright burst within z = 0.17 should be observed by a High Energy Transient Explorer-like instrument within the next ~20 yr.

X-ray/optical observations of A0535+26/HDE 245770 in quiescence

Abstract: We present the result of three BeppoSAX observations of the X-ray binary pulsar A0535+26 in its quiescent state. The source is quite well detected up to 200 keV (6σ detection in the stronger observation). No Iron line is detected in the MECS data (3σ upper limit on its equivalent width of 150 eV). There is evidence of a soft excess below 2 keV. Pulsation is detected in all energy bands up to 10 keV, with a pulsed fraction of ∼0.5, not varying with energy. There is a marginal detection (4σ) of a cyclotron resonance feature (CRF) at 118±20 keV in the PDS spectrum. During the BeppoSAX observations HDE 245770, the optical counterpart to A0535+26, was monitored spectroscopically and photometrically. These observations show that the H α and H β lines previously observed in absorption returned in emission, indicating that the Be disk formed again. The presence of pulsation and a CRF is a clear indication that accretion onto the polar caps is occurring, and that the propeller mechanism is leaking. The presence of a soft excess could be explained either in terms of thermal emission ( kT ∼1.2 keV) from the neutron star surface, as observed in other accreting X–ray pulsars, or in terms of an overabundance of Mg in the circumstellar matter.

Disappearance of Hard X-Ray Emission in the Last BeppoSAX Observation of the Z Source GX 349+2

Abstract: We report on the results from two BeppoSAX observations of the Z source GX 349+2 performed in 2001 February and covering the broad energy range 0.12-200 keV. The light curve obtained from these observations shows a large flaring activity, the count rate varying from ~130 to ~260 counts s-1, indicating that the source was in the flaring branch during these observations. The average spectrum is well described by a soft blackbody (kTBB ~ 0.5 keV) and a Comptonized component having a seed photon temperature of kT0 ~ 1 keV, an electron temperature of kTe ~ 2.7 keV, and optical depth of τ ~ 11. To well fit the energy spectrum, three Gaussian lines are needed at 1.2, 2.6, and 6.7 keV with corresponding equivalent widths of 13, 10, and 39 eV, probably associated to L-shell emission of Fe XXIV, Lyα S XVI, and Fe XXV, respectively. These lines may be produced at different distances from the neutron star, which increase when the count rate of the source increases. An absorption edge is also needed at 9 keV with an optical depth of ~3 × 10-2. From the color-color diagram (CD), we selected five zones from which we extracted the corresponding energy spectra. The temperatures of the blackbody and of the Comptonized component tend to increase when the intensity of the source increases. We discuss our results comparing them to those obtained from a previous BeppoSAX observation, performed in 2000 March, during which the source was in a similar position on its Z-track. In particular, we find that, although the source showed similar spectral states in the 2000 and the 2001 observations, a hard tail that was significantly detected in 2000 March is not observed in these recent observations.

SN 2003lw and GRB 031203: A Bright Supernova for a Faint Gamma-ray Burst

Abstract: Optical and near-infrared observations of the gamma-ray burst GRB 031203, at z = 0.1055, are reported. A very faint afterglow is detected superimposed to the host galaxy in our first infrared JHK observations, carried out ~9 hours after the burst. Subsequently, a rebrightening is detected in all bands, peaking in the R band about 18 rest-frame days after the burst. The rebrightening closely resembles the light curve of a supernova like SN 1998bw, assuming that the GRB and the SN went off almost simultaneously, but with a somewhat slower evolution. Spectra taken close to the maximum of the rebrightening show extremely broad features as in SN 1998bw. The determination of the absolute magnitude of this SN (SN 2003lw) is difficult owing to the large and uncertain extinction, but likely this event was brighter than SN 1998bw by 0.5 mag in the VRI bands, reaching an absolute magnitude M_V = -19.75+-0.15.

GRB 020813: Polarization in the case of a smooth optical decay

Abstract: We present the results of a VLT polarimetric monitoring campaign of the GRB 020813 optical afterglow carried out in three nights, from 0.88 to 4.20 days after the gamma-ray event. The mean values of the degree of linear polarization (P) and its position angle (θ) resulting from our campaign areP� = 1.18 ± 0.10% and � θ� = 148.7 ◦ ± 2.3 ◦ , after correcting for Galactic interstellar polarization. Our VLT data set is most naturally described by a constant degree of linear polarization and position angle, nonetheless a slow θ evolution cannot be entirely ruled out by our data. The VLT monitoring campaign did not reveal either a significant θ rotation respect to the Keck spectropolarimetric observations performed ∼0.25 days after the GRB (Barth et al. 2003). However, � Pis a factor of two lower than the polarization degree measured from Keck. Additionally, the VLT polarization data allowed us to construct an accurate V-band light curve. The V-band photometric data revealed a smooth light curve with a break located between the last Keck and our first VLT polarimetric measurement, 0.33 < tbreak,V < 0.88 days after the GRB. The typical magnitude fluctuations of the VLT V-band lightcurve are 0.003 mag, 0.010 mag and 0.016 mag for our three observing nights, respectively. We speculate that the stability of θ might be related to the smoothness of the light curve.

Young crab-like pulsars and luminous x-ray sources in starbursts and optically dull galaxies

Abstract: Recent Chandra observations of nearby galaxies have revealed a number of ultraluminous X-ray sources (ULXs) with super-Eddington luminosities, away from the central regions of nonactive galaxies. The nature of these sources is still debated. We argue that a fraction of them could be young, Crab-like pulsars, the X-ray luminosity of which is powered by rotation. We use the pulsar birth parameters estimated from radio pulsar data to compute the steady state pulsar X-ray luminosity distribution as a function of the star formation rate (SFR) in the galaxy. We find that ~10% of optically dull galaxies are expected to have a source with LX 1039 ergs s-1, while starburst galaxies should each have several of these sources. We estimate that the X-ray luminosity of a few percent of galaxies is dominated by a single bright pulsar with LX 1039 ergs s-1, roughly independently of its SFR. We discuss observational diagnostics that can help distinguish the young pulsar population in ULXs.

A BeppoSAX study of the Galactic Z-source GX 340+0

Abstract: We present the results of a BeppoSAX broad band (0.1–200 keV) observation of the Z-source GX 340+0. The 1.8–30 keV continuum is well described by a blackbody ( kT BB ∼0.5 keV) plus a Comptonized component with seed photons temperature ∼ 1 keV and electron temperature ∼ 3 keV. A hard tail dominates the spectrum above 30 keV. It can be fitted using a bremsstrahlung component or, equivalently, a powerlaw (with a low-energy cutoff). We detect also a Gaussian line at ∼6.8 keV and an absorption edge at ∼9.2 keV. A low-energy (∼1 keV) unresolved feature needs further investigations.

A study of the low-frequency quasi-periodic oscillations in the X-ray light curves of the black hole candidate XTE J1859+226

Abstract: We present the results of an extensive timing analysis of the 1999 outburst of the soft X-ray transient and black hole candidate XTE J1859+226 as observed with the Rossi X-Ray Timing Explorer. Three main different types of low frequency (1-9 Hz) quasi-periodic oscillations (QPOs) were observed and classified, strenghtening the general picture that is emerging for the variability properties of black hole X-ray binaires. Rapid transitions between different power spectral shapes were observed and their link with the count rate was studied. Furthermore, we show that a frequency of ~6 Hz seems to hold a particular place: one of the three QPO types we found was very stable when at this frequency, as it happens in Z sources as well. The coherence of its subharmonic peak was higher when the fundamental was close to 6 Hz, thus suggesting the presence of some resonance at this frequency.

New results on transient neutron-star low-mass X-ray binaries in quiescence

Abstract: We review the main observational properties of neutron star low mass transients in quiescence. We first survey the discoveries of BeppoSAX. We then focus on recent discoveries by Chandra and XMM-Newton, with special emphasis on the detection of the quiescent counterpart of SAX J1808.4–3658 and the study of variability in the quiescent state.

A possible new dipping X-ray source in the field of M 31

Abstract: We report the discovery of the new dipping X-ray source, XMMU J004308.6+411247, in M 31, during a systematic search for periodicities in XMM- Newton archival observations. During the 2002 January 6 observation, the dips recur with a 107 min period and the source count rate is consistent with zero at the dip minimum. Dips with the same modulation and period are also observed during the XMM- Newton observations carried out on 2000 June 25, 2001 June 29 and the Chandra observation of 2001 October 5. The dips of XMMU J004308.6+411247 show no evidence of energy dependence. The average X-ray flux of XMMU J004308.6+411247 is nearly constant across different observations (${\sim} 10^{37}$ erg s -1 for an assumed M 31 distance of 780 kpc in the 0.3-10 keV band); the spectrum is well fitted by an absorbed power law with a photon index ∼ 0.8 or an absorbed Comptonization model. The photo-electric absorption is consistent with the Galactic value in the source direction. If XMMU J004308.6+411247 is located in M 31 its properties are consistent with those of dipping low mass X-ray binaries in the Galaxy. Present observations do not make it possible to distinguish between dips and eclipses. The possibility that XMMU J004308.6+411247 is a foreground X-ray source cannot be ruled out at present; in this case the source might be a magnetic cataclysmic variable.

The discovery of hard X-ray emission in the persistent flux of the Rapid Burster

Abstract: We report the first detection with INTEGRAL of persistent hard X-ray emission (20 to 100 keV) from the Rapid Burster (MXB 1730-335), and describe its full spectrum from 3 to 100 keV. The source was detected on February/March 2003 during one of its recurrent outbursts. The source was clearly detected with a high signal to noise ratio during the single pointings and is well distinguished from the neighboring source GX 354-0. The 3-100 keV X-ray spectrum of the persistent emission is well described by a two-component model consisting of a blackbody plus a power-law with photon index ∼2.4. The estimated luminosity was ∼8.5 × 10 36 erg s −1 in the 3-20 keV energy band and ∼1.3 × 10 36 erg s −1 in the 20-100 keV energy range, for a

A possible new dipping X-ray source in the field of M31

Abstract: We report the discovery of the new dipping X-ray source, XMMU J004308.6+411247, in M31, during a systematic search for periodicities in XMM-Newton archival observations. During the 2002 January 6 observation, the dips recur with a 107 min period and the source count rate is consistent with zero at the dip minimum. Dips with the same modulation and period are also observed during the XMM-Newton observations carried out on 2000 June 25, 2001 June 29 and the Chandra observation of 2001 October 5. The dips of XMMU J004308.6+411247 show no evidence of energy dependence. The average X-ray flux of XMMU J004308.6+411247 is nearly constant across different observations (~1.e37 erg/s for an assumed M31 distance of 780 kpc in the 0.3-10 keV band); the spectrum is well fit by an absorbed power law with a photon index ~0.8 or an absorbed Comptonization model. The photo-electric absorption is consistent with the Galactic value in the source direction. If XMMU J004308.6+411247 is located in M31 its properties are consistent with those of dipping low mass X-ray binaries in the Galaxy. Present observations do not allow to distinguish between dips and eclipses. The possibility that XMMU J004308.6+411247 is a foreground X-ray source cannot be ruled out at present; in this case the source might be a magnetic cataclysmic variable.

Unveiling the Multi-wavelength Phenomenology of Anomalous X-ray Pulsars

Abstract: During 2002-2003 the number of IR-identified counterparts to the Anomalous X-ray Pulsars (AXPs) has grown to four (4U0142+614, 2E2259+584, 1E 1048-59 and 1RXS J170849-400910) out of the six assessed objects of this class, plus two candidates. More importantly, some new common observational characteristics have been identified, such as the IR variability, the IR flattening in the broad band energy spectrum, the X-ray spectral variability as a function of pulse phase (which are not predicted by the magnetar model), and the SGR-like bursts (which can not be explained in terms of standard accretion models). We present the results obtained from an extensive multi-wavelength observational campaign carried out collecting data from the NTT, CFHT for the optical/IR bands, and XMM, Chandra (plus BeppoSAX archival data) in the X-rays. Based on these results and those reported in the literature, the IR-to-X-ray band emission of AXPs has been compared and studied.

The Multi-frequency Robotic facility REM: first results

Abstract: The REM Observatory, recently installed and commissioned at la Silla Observatory Chile, is the first moderate aperture robotic telescope able to cover simultaneously the visible-NIR (0.45-2.3 microns) wavelength range. Due to its very fast pointing and its full robotization REM is conceived for fast transients observation. The high throughput Infrared Camera (REMIR) and the Visible imaging spectrograph (ROSS), simultaneously fed by a dichroic, allow to collect high S/N data in an unprecedented large spectral range on a telescope of this size. The REMobservatory is an example of a versatile and agile facility necessary complement to large telescopes in fields in which rapid response and/or target pre-screening are necessary. This paper describes the principal Characteristics and operation modes of the REM observatory and gives on overview of the preliminary results obtained during the commissioning phase. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

XMM-Newton observations of the x-ray pulsating companion of HD49798

Abstract: RX J0648.0–4418 is a pulsating (Pspin=13.2 s) soft X-ray source in a single–component spectroscopic binary (Porb=1.55 days) containing a hydrogen depleted subdwarf O6 star, HD49798. It is currently unclear whether this compact X-ray source is a neutron star or a white dwarf. In either case HD49798/RX J0648.0–4418 corresponds to a previously unobserved evolutionary stage of a massive binary system, after common envelope and spiral–in. We present the analysis of XMM-Newton observations taken at different phases of the binary orbit.