P. Romano, C. Guidorzi, L. Sidoli, E. Montanari, F. Capitanio, L. Amati, A. Cucchiara, F. Frontera, N. Masetti, S. Mereghetti, F. Rossi
During our Swift/XRT program to obtain X-ray positions at arcsecond level for a sample of Galactic X-ray binaries, we discovered that SAX J0840.7+2248 is not a binary, but rather BeppoSAX/WFC+GRBM X-ray Rich GRB 980429. Here we report on this discovery and on the properties of this long, X-ray rich gamma-ray burst, from prompt to (very) late followup.
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We report on the serendipitous discovery of a new X-ray source, SAX J1802.7-2017, ~22' away from the bright X-ray source GX 9+1, during a BeppoSAX observation of the latter source on 2001 September 16-20. SAX J1802.7-2017 remained undetected in the first 50 ks of observation; the source count rate in the following ~300 ks ranged between 0.04 c/s and 0.28 c/s, corresponding to an averaged 0.1-10 keV flux of 3.6 10^{-11} ergs cm^{-2} s^{-1}. We performed a timing analysis and found that SAX J1802.7-2017 has a pulse period of 139.612 s, a projected semimajor axis of a_x sin i ~ 70 lt-s, an orbita…
We report the results of a 1998 July BeppoSAX observation of a field in the SMC which led to the discovery of 345s pulsations in the X-ray flux of SAXJ0103.2-7209. The BeppoSAX X-ray spectrum is well fit by an absorbed power-law with photon index 1.0 plus a black body component with kT=0.1keV. The unabsorbed luminosity in the 2-10 keV energy range is 1.2x10^{36} erg/s. In a very recent Chandra observation the 345s pulsations are also detected. The available period measurements provide a constant period derivative of -1.7s/yr over the last three years making SAXJ0103.2-7209 one of the most rapi…
We have discovered three certain (SAX J1324.5-6313, 2S 1711-339 and SAX J1828.5-1037) and two likely (SAX J1818.7+1424 and SAX J2224.9+5421) new thermonuclear X-ray burst sources with the BeppoSAX Wide Field Cameras, and observed a second burst ever from a sixth one (2S 0918-549). Four of them (excluding 2S 1711-339 and 2S 0918-549) are newly detected X-ray sources from which we observed single bursts, but no persistent emission. We observe the first 11 bursts ever from 2S 1711-339; persistent flux was detected during the first ten bursts, but not around the last burst. A single burst was rece…
SAX J1819.3-2525 is a nearby X-ray transient which exhibited a fast and large X-ray outburst on Sep. 15, 1999 (Smith et al. 1999). The Wide Field Cameras and the Narrow Field Instruments (NFI) on board BeppoSAX observed SAX J1819.3-2525 at various stages of its activity before that, in the spring and fall of 1999. The fluxes range between 0.012 and 0.3 Crab units (2-10 keV). The NFI observation is unique because it is the longest semi-continuous observation of SAX J1819.3-2525 so far, and it offers a study of the spectrum at a relatively high resolution of 8% full width at half maximum at 6 ke…
We have observed the millisecond X-ray pulsar SAX J1808.4-3658 on three occasions during its 2000 outburst with the BeppoSAX satellite. The source was highly variable and erratic during this outburst, and by coincidence we obtained data only during times when the source had very low luminosities. During our observations, we detected four faint sources. The source closest to the position of SAX J1808.4-3658 is still ~1.6' away. This source can only be identified with SAX J1808.4-3658 if we assume that the BeppoSAX positional reconstruction is not completely understood. We also reanalyzed a Bepp…
We report on a 20 ks BeppoSAX observation of the transient 2.5 ms X-ray pulsar SAX J1808.4-3658 in its quiescent state. A source at a level of ~ 3 x 10^-3 c/s was detected with the BeppoSAX/MECS at a position consistent with that of SAX J1808.4-3658. The inferred 0.5-10 keV luminosity was to that measured from other quiescent low mass X-ray transients hosting an old neutron star. This result is discussed in the light of the propeller and radio pulsar shock emission models for the quiescent emission of these systems.
The transient accreting millisecond pulsar SAX J1808.4-3658 has shown several outbursts to date but the transition from outburst to quiescence has never been investigated in detail. Thanks to the Swift observing flexibility, we monitored for the first time the decay to quiescence during the 2005 outburst. At variance with other transients, wide luminosity variations are observed. In addition, close to quiescence, SAX J1808.4-3658 seems to switch between two different states. We interpret them in terms of the accretion states accessible to a magnetized, fast rotating neutron star.
According to the latest versions of synthesis modeling of the Cosmic X-ray Background, Compton Thick AGN should represent ~50% of the total absorbed AGN population. However, despite their importance in the cosmological context, only a few dozens of Compton Thick AGN have been found and studied so far. We will briefly review this topic and discuss the improvement in this field offered by the Simbol-X mission with its leap in sensitivity (E>10 keV) of more than a factor 500 with respect to previous X-ray missions.
During observation campaigns of the Galactic Bulge region, BeppoSAX detected a total of 21 new X-ray bursters in about 6 years of operation. These sources are mostly transient and often feature a hard X-ray spectrum, extending up to ~ 200 keV. A hard spectrum is generally found in weak, relatively short outbursts. On the other hand two sources, SAX J1747.0-2853 and SAX J1750.8-2900, have been seen with soft X-ray transient behaviour. In both low/hard and high/soft state, a thermal component is observed at energies below \~10 keV and the spectra are compatible with the same model consisting of …
Long Gamma Ray Bursts hold the promise of probing star-formation and metal enrichment up to very high redshifts. The present GRB samples with redshift determination are largely incomplete and therefore a careful analysis of selection effects is mandatory before any conclusion can be drawn from the observed GRB redshift distribution. We study and compare three well defined samples of long GRBs detected by Swift, HETE2 and BeppoSAX. We find that Swift GRBs are slighly fainter and harder than BeppoSAX and HETE2 GRBs, as expected due to the higher energy range in which Swift GRBs are detected and …
We report on the results of an EPIC XMM-Newton observation of the faint source SAXJ1748.2-2808 and the surrounding field. This source was discovered during the BeppoSAX Galactic center survey performed in 1997-1998. A spatial analysis resulted in the detection of 31 sources within the EPIC field of view. SAXJ1748.2-2808 is clearly resolved into 2 sources in EPIC images with the brighter contributing almost 80% of the 2-10keV flux. Spectral fits to this main source are consistent with an absorbed power-law with a photon index of 1.4+/-0.5 and absorption equivalent to 14E22cm-2 together with an …
The accretion-powered millisecond X-ray pulsar, SAX J1808.4-3658, was observed in quiescence with ASCA in September 1999. We detected a dim X-ray source in the SIS data at the position consistent to SAX J1808.4-3658. The source count rate was (1.1+-0.4)x10E-3 cts/s (0.5-5 keV) for a single SIS, which corresponds to (3+-1)x10E-14 ergs/s/cmE2 if a power law energy spectrum of photon index 2 with low-energy absorption corresponding to a hydrogen column density of 1.3x10E21 cmE-2 is assumed. The statistical quality of the data was insufficient to constrain the energy spectrum or to detect the 401 …
We report on the BeppoSAX/WFC observation of a strong type-I X-ray burst from the recently discovered transient source SAX J1810.8-2609. The observed event showed evidence of photospheric radius expansion due to super-Eddington burst luminosity. Burst detection identifies the source as a probable low-mass X-ray binary harbouring a neutron star.
We report BeppoSAX NFI observations of the X-ray source SAX~J1747.0--2853 recently discovered in the region of the Galactic Center. The presence of type I X-ray bursts indicates that this source, positionally coincident with the transient GX~0.2--0.2 observed in 1976, is a neutron star accreting from a low mass companion.
We revisited the BeppoSAX observation of SAX J0635+0533, suggested as a possible counterpart to the gamma-ray source 2EG J0635+0521. We have discovered a 33.8 ms pulsation from the source and derived an improved position, consistent with the location of the Be star proposed as a binary companion. We interpret the periodicity as the spin period of a neutron star in a binary system with a Be companion.
SAX J1818.6-1703 has been characterised as a Supergiant Fast X-ray Transient system on the basis of several INTEGRAL/IBIS detections since the original BeppoSAX Wide Field Camera detection. Using IBIS/ISGRI, Swift/BAT and archival observations, we show that in fact SAX J1818.6-1703 exhibits emission on a period of 30 +/- 0.1 days, with a high degree of recurrence. SAX J1818.6-1703 is therefore the second SFXT shown to exhibit periodic outbursts, but with a considerably shorter period than the other known system, IGR J11215-5952.
Using a sample of 14 BeppoSAX and 74 Swift GRBs with measured redshift we tested the correlation between the intrinsic peak energy of the time-integrated spectrum, E_p,i, the isotropic-equivalent peak luminosity, L_p,iso, and the duration of the most intense parts of the GRB computed as T_0.45 ("Firmani correlation"). For 41 out of 88 GRBs we could estimate all of the three required properties. Apart from 980425, which appears to be a definite outlier and notoriously peculiar in many respects, we used 40 GRBs to fit the correlation with the maximum likelihood method discussed by D'Agostini, su…
A search for pulsations from the Be X-ray binary SAX J2239.3+6116, through observations with the Narrow Field Instruments on BeppoSAX and the Proportional Counter Array on RXTE, yielded the clear presence of a 1247 s coherent oscillation. Given the fairly high X-ray luminosity on previous occasions (up to a few times 10^36 erg/s in 2 to 28 keV), the oscillation must be due to the spin of a neutron star. Assuming that the 262 day recurrence time is the orbital period, SAX J2239.3+6116 has both the longest orbital period and the longest pulse period of 24 Be X-ray binaries with measured orbital …
We present some statistical considerations on the BeppoSAX hunt for subsecond GRBs at the Scientific Operation Center. Archive analysis of a BATSE/SAX sub-sample of bursts indicates that the GRB Monitor is sensitive to short (< 2 sec) events, that are in fact about 22% of the total. The non-detection of corresponding prompt X-ray counterparts to short bursts in the Wide Field Cameras, in about 3 years of operations, is discussed: with present data no implications on the X-to-gamma-ray spectra of short vs long GRBs may be inferred. Finally, the status of searching procedures at SOC is reviewed.
We report the discovery with BeppoSAX-WFC of two new X-ray sources that were only seen during bursts: SAX J1753.5-2349 and SAX J1806.5-2215. For both sources, no steady emission was detected above an upper limit of 5 mCrab (2 to 8 keV) for 3 10**5 s around the burst events. The single burst detected from SAX J1753.5-2349 shows spectral softening and a black body color temperature of 2.0 keV. Following the analogy with bursts in other sources the burst very likely originates in a thermonuclear flash on a neutron star. The first of two burst detected from SAX J1806.5-2215 does not show spectral …
We report on our discovery and follow-up observations of the X-ray source SAX J1747.0-2853 detected in outburst on 1998, March 10 with the BeppoSAX Wide Field Cameras in the energy range 2-28 keV. The source is located about half degree off the Galactic Nucleus. A total of 14 type-I X-ray bursts were detected in Spring 1998, thus identifying the object as a likely low-mass X-ray binary harboring a weakly magnetized neutron star. Evidence for photospheric radius expansion is present in at least one of the observed bursts, leading to an estimate of the source distance of about 9 kpc. We performe…
We report the BeppoSAX observations of 6 Flat Spectrum Radio Quasars. Three of them have a clear detection up to 100 keV with the PDS instrument. For 4 objects the X-ray spectrum is satisfactorily fitted by a power-law continuum with Galactic absorption. 2251+158 show the presence of absorption higher than the galactic value, while the spectrum of the source 0208-512 shows a complex structure, with evidence of absorption at low energy. We construct the Spectral Energy Distributions adding historical data to the broad band X-ray spectra obtained with $Beppo$SAX and reproduce them with a one-zon…
We present the complete on-line catalogue of gamma-ray bursts observed by the two Wide Field Cameras on board \sax in the period 1996-2002. Our aim is to provide the community with the largest published data set of GRB's prompt emission X-ray light curves and other useful data. This catalogue (BS-GRBWFCcat) contains data on 77 bursts and a collection of the X-ray light curves of 56 GRB discovered or noticed shortly after the event and of other additional bursts detected in subsequent searches. Light curves are given in the three X-ray energy bands (2-5, 5-10, 10-26 keV). The catalogue can be a…
We report on the highly variable and erratic long-term X-ray luminosity behavior of the only known accretion-driven millisecond X-ray pulsar SAX J1808.4-3658 during its 2000 outburst, as observed with RXTE. The maximum observed luminosity is ~2.5 x 10^35 erg s^-1 (3-25 keV; for a distance of 2.5 kpc), which is a factor of ~10 lower than that observed during the 1996 and 1998 outbursts. Due to solar constraints, the source could not be observed for several months with RXTE before 21 Jan. 2000. Therefore, the exact moment of the outburst onset is unknown and the peak luminosity could have been s…
SWIFT bursts appear to be more distant than previous bursts. We present the Boer & Gendre relation that link redshift and afterglow luminosities. Taking advantage of the XMM-Newton, Chandra and BeppoSAX catalogs, and using this relation, we have investigated the redshift distribution of GRBs. We find that XMM burst sources with unknown redshift appear to be more distant than those with a known redshift. We propose that this effect may be due to a selection effect of pre-SWIFT optical observations.
PKS 2155-304 was observed with BeppoSAX in November 1997 for 64 ksec (total elapsed time 33.5 hours) and, for the first time, simultaneously in gamma-rays with EGRET on board the Compton Gamma Ray Observatory and with the ground based TeV telescope CANGAROO, during a phase of high brightness in the X-ray band. The LECS and MECS light curves show a pronounced flare (with an excursion of a factor 3.5 between min and max), with evidence of spectral hardening at maximum intensity. The source is weakly detected by the PDS in the 12-100 keV band with no significant evidence of variability. The broad…
In the redshift range z = 0-1, the gamma ray burst (GRB) redshift distribution should increase rapidly because of increasing differential volume sizes and strong evolution in the star formation rate. This feature is not observed in the Swift redshift distribution and to account for this discrepancy, a dominant bias, independent of the Swift sensitivity, is required. Furthermore, despite rapid localization, about 40-50% of Swift and pre-Swift GRBs do not have a measured redshift. We employ a heuristic technique to extract this redshift bias using 66 GRBs localized by Swift with redshifts determ…
We report the discovery of a fairly bright transient during observations with the Wide Field Cameras on board the BeppoSAX satellite in September 1996. It was detected at a peak intensity of 0.1 Crab (2 to 10 keV) and lasted between 6 and 40 days above a detection threshold of 2 mCrab. Two very bright type I X-ray bursts were detected from this transient in the same observations. These almost certainly identify this X-ray transient as a low-mass X-ray binary with a neutron star as compact component. The double-peaked time history of both bursts at high energies suggests a peak luminosity close…
Many of the extragalactic sources detected in $γ$ rays at TeV energies are BL Lac objects. In particular, they belong to the subclass of ``high frequency peaked BL Lacs" (HBLs), as their spectral energy distributions exhibit a first peak in the X-ray band. At a closer look, their X-ray spectra appear to be generally curved into a log-parabolic shape. In a previous investigation of Mrk 421, two correlations were found between the spectral parameters. One involves the height $S_p$ increasing with the position $E_p$ of the first peak; this was interpreted as a signature of synchrotron emission fr…
Using X-ray data from the Rossi X-Ray Timing Explorer (RXTE), we carried out pulse timing analysis of the transient X-ray pulsar SAX J2103.5+4545. An outburst was detected by All Sky Monitor (ASM) October 25 1999 and reached a peak X-ray brightness of 27 mCrab October 28. Between November 19 and December 27, the RXTE/PCA carried out pointed observations which provided us with pulse arrival times. These yield an eccentric orbit (e= 0.4 \pm 0.2) with an orbital period of 12.68 \pm 0.25 days and light travel time across the projected semimajor axis of 72 \pm 6 sec. The pulse period was measured t…
We have analysed in detail the discovery measurements of the X-ray burster SAX J1750.8-2900 by the Wide Field Cameras on board BeppoSAX in spring 1997, at a position ~1.2 degrees off the Galactic Centre. The source was in outburst on March 13th when the first observation started and showed X-ray emission for ~ 2 weeks. A total of 9 bursts were detected, with peak intensities varying from ~ 0.4 to 1.0 Crab in the 2-10 keV range. Most bursts showed a fast rise time (~ 1s), an exponential decay profile with e-folding time of ~ 5s, spectral softening during decay, and a spectrum which is consisten…
We present recent observations of the X-ray pulsar SAX J1324-6200 obtained in December 2007 with the Swift satellite yielding a significant improvement in the source localization with respect to previous data and a new measurement of the spin period P=172.84s. A single object consistent in colors with a highly reddened early type star is visible in the X-ray error box. The period is significantly longer than that obtained in 1997, indicating that SAX J1324-6200 has been spinning down at an average rate of ~6x10^-9 s s^-1. We discuss the possible nature of the source showing that it most likely…
We report timing and spectral properties of the transient Be X-ray pulsar EXO 053109--6609.2 studied using observations made with the ASCA and BeppoSAX observatories. Though there must have been at least one spin-down episode of the pulsar since its discovery, the new pulse period measurements show a monotonic spin-up trend since 1996. The pulse profile is found to have marginal energy dependence. There is also evidence for strong luminosity dependence of the pulse profile, a single peaked profile at low luminosity that changes to a double peaked profile at high luminosity. This suggests a cha…
Excess soft X-ray emission in clusters of galaxies has so far been detected for sources that lie along lines-of-sight to very low Galactic HI column density (such as Coma, A1795, A2199 and Virgo, N_H 0.9-2.0 10^{20} cm-2). We show that the cluster soft excess emission can be investigated even at higher N_H, which provides an opportunity for investigating soft X-ray emission characteristics among a large number of clusters. The ROSAT PSPC analysis of some members of the Shapley concentration (A3571, A3558, A3560 and A3562, at N_H 4-4.5 10^{20} cm-2) bears evidence for excess emission in the 1/4…
SAX J1808.4-3658 is a unique source being the first Low Mass X-ray Binary showing coherent pulsations at a spin period comparable to that of millisecond radio pulsars. Here we present an XMM-Newton observation of SAX J1808.4-3658 in quiescence, the first which assessed its quiescent luminosity and spectrum with good signal to noise. XMM-Newton did not reveal other sources in the vicinity of SAX J1808.4-3658 likely indicating that the source was also detected by previous BeppoSAX and ASCA observations, even if with large positional and flux uncertainties. We derive a 0.5-10 keV unabsorbed lumin…
For the second time in 27 years a bright transient X-ray source has been detected coincident with the globular cluster NGC 6440. It was found to be active in August, 1998, with the Wide Field Camera and the narrow field instruments on the BeppoSAX spacecraft, and with the All-Sky Monitor and the Proportional Counter Array on the RossiXTE spacecraft. Four X-ray bursts were detected, at least one of which shows the characteristics of a thermonuclear flash on a neutron star, in analogy with some ~20 optically identified low-mass X-ray binaries. The broad-band spectrum is hard as is common among l…
During a 50 ks monitoring observation of the Galactic bulge performed in September 1999 by the Wide Field Cameras on board the BeppoSAX satellite, an X-ray burst was detected from a sky position ~3 degrees off the Galactic centre. No previously known X-ray sources are located within the position error circle of the observed burst. The new burster, SAX J1752.3-3138, did not show any persistent emission during the whole observation. No other bursting events, as well as steady emission, were reported so far by other instruments or detected in the WFC archive, which covers ~6 Ms and ~4 Ms for burs…
SAX J1711.6-3808 is an X-ray transient in the Galactic bulge that was active from January through May of 2001 and whose maximum 1-200 keV luminosity was measured to be 5X10-9 erg/s/cm2 which is less than ~25% of the Eddington limit, if placed at a distance equal to that of the galactic center. We study the X-ray data that were taken of this moderately bright transient with instruments on BeppoSAX and RXTE. The spectrum shows two interesting features on top of a Comptonized continuum commonly observed in low-state X-ray binaries: a broad emission feature peaking at 7 keV and extending from 4 to…
In the introduction we give the main characteristics of the Swift mission outlining how the design was driven by the science goals and by the heritage we got from the Italian - Dutch satellite Beppo SAX. We show some of the new characteristics of the X - ray light curves that became evident soon after we obtained the first set of data. In addition to the early phase steep slope afterglow discovered by Swift, we discuss the frequently observed GRB flares and the first localization of a short burst.
We report the results of a survey of the Galactic Center region (|l| < 2deg, |b| < 0.5deg) performed with the BeppoSAX satellite. The flux from the center of our Galaxy corresponds to a luminosity of ~3 10^{35} erg/s in the 2-10 keV range. Due to the limited angular resolution ($\gsim 1'$) only part of it is supposed to come from Sagittarius A*, the non-thermal radio source which is believed to mark the dynamical center of the Galaxy. In addition to the diffuse emission, several bright (L$_X\gsim10^{36} ergs/s) point sources have been observed, both persistent (A 1742-294, SLX 1744-299, SLX 17…
SAX J1818.6-1703 is a flaring transient X-ray source serendipitously discovered by BeppoSAX in 1998 during an observation of the Galactic centre. The source was identified as a High-Mass X-ray Binary with an OB SuperGiant companion. Displaying short and bright flares and an unusually very-low quiescent level implying intensity dynamical range as large as 1e3-4, the source was classified as a Supergiant Fast X-ray Transient. The mechanism triggering the different temporal behaviour observed between the classical SGXBs and the recently discovered class of SFXTs is still debated. The discovery of…
We have discovered with the Wide Field Cameras on board BeppoSAX the weak transient X-ray source SAXJ2239.3+6116 whose position coincides with that of 4U2238+60/3A2237+608 and is close to that of the fast transient AT2238+60 and the unidentified EGRET source 3EG2227+6122. The data suggest that the source exhibits outbursts that last for a few weeks and peak to a flux of 4E-10 erg/s/cm2 (2-10 keV) at maximum. During the peak the X-ray spectrum is hard with a photon index of -1.1+/-0.1. Follow-up observations with the Narrow-Field Instruments on the same platform revealed a quiescent emission le…
We present X-ray observations of two high redshift clusters of galaxies carried out with the BeppoSAX satellite. One cluster, RXJ0152.7-1357 at z = 0.83, was selected from the ROSAT Deep Cluster Survey sample, as one of the most X-ray luminous systems known at z>0.5. The optical and ROSAT-PSPC data show a complex morphology with at least two cores. Our SAX observations yield a gas temperature kT = 6.46^{+1.74}_{-1.19} keV and a metallicity A = 0.53^{+0.29}_{-0.24}, with a prominent iron K(alpha) line. The second cluster, MS2053.7-0449 at z = 0.58, was selected from the EMSS sample. Given the p…
The transient X-ray source SAX J1810.8-2609 was discovered on 1998, March 10 with the Wide Field Cameras on board the BeppoSAX satellite, while observing the Galactic Bulge in the 2-28 keV energy range. On March 11, a strong type-I X-ray burst was detected with evidence of photospheric radius expansion. A follow-up target of opportunity observation with the Narrow-Field Instruments (NFI) was performed on March 11 and 12, for a total elapsed time of 8.51 x 10^{4} s. The wide band spectral data (0.1-200 keV) obtained with the NFI show a remarkable hard X-ray spectrum detected up to ~200 keV, whi…
We use Swift/BAT Earth occultation data at different geomagnetic latitudes to derive a sensitive measurement of the Cosmic X-ray background (CXB) and of the Earth albedo emission in the 15--200 keV band. We compare our CXB spectrum with recent (INTEGRAL, BeppoSAX) and past results (HEAO-1) and find good agreement. Using an independent measurement of the CXB spectrum we are able to confirm our results. This study shows that the BAT CXB spectrum has a normalization ~8(+/-3)% larger than the HEAO-1 measurement. The BAT accurate Earth albedo spectrum can be used to predict the level of photon back…
We review the results obtained with the Galactic center campaigns of the BeppoSAX Wide Field X-ray Cameras (WFCs). This pertains to the study of luminous low-mass X-ray binaries (LMXBs). When pointed at the Galactic center, the WFC field of view contains more than half of the Galactic LMXB population. The results exemplify the excellent WFC capability to detect brief X-ray transients. Firstly, the WFCs expanded the known population of Galactic thermonuclear X-ray bursters by 50%. At least half of all LMXBs are now established to burst and, thus, to contain a neutron star as compact accretor ra…
We present Chandra/ACIS observations of the bursting X-ray transient SAX J1747.0-2853 performed on 18 July 2001. We detected a bright source at the position of R.A = 17^h 47^m 02.60^s and Dec. = -28 52' 58.9'' (J2000.0; with a 1 sigma error of ~0.7 arcseconds), consistent with the BeppoSAX and ASCA positions of SAX J1747.0-2853 and with the Ariel V position of the transient GX +0.2,-0.2, which was active during the 1970's. The 0.5-10 keV luminosity of the source during our observations was ~3 x 10^{35} erg/s (assuming a distance of 9 kpc) demonstrating that the source was in a low-level accret…
The gamma-ray burst of 1997 February 28 was localized using the arrival-time analysis method with the Ulysses, BeppoSAX, and WIND spacecraft. The result is a plus-or-minus 31.5 arcsec (3 sigma) wide annulus of possible arrival directions which intersects both the position of the burst determined independently by the SAX Wide Field Camera, and the position of a fading X-ray source detected by the SAX focussing X-ray telescopes, and reduces these source location areas by factors of 7 and 1.5 respectively. The combination of the annulus and the SAX locations, a 0.76 square arcminute error box, is…
A BeppoSAX observation of the z=2.34 quasar PKS2149-306 produced a strong signal in the high energy PDS instrument up to a maximum observed energy of nearly 50 keV, 150 keV in the quasar frame. The Beppo-SAX spectrum spans almost 3 decades (0.3-150 keV, quasar frame) and shows an extremely hard (alpha=0.4+/-0.05) X-ray spectrum above 3 keV (comparable to the X-ray background slope), and either a softer (alpha=1.0(+0.6, -0.3)) low energy component, or an ionized absorber at zero redshift. No evidence is seen of an Fe-K emission line (EW<167 eV at 6.5 keV quasar frame) or a Compton hump (R<0.3).…
The neutron star binary SAX J1747.0-2853, located in the Galactic Center region at about 0.5 deg from Sgr A* and at a distance of 9 kpc, has been observed in outburst four times (1998, 1999, 2000 and 2001) by BeppoSAX and RossiXTE. At the time of its discovery in 1998 the source was observed in a low/hard state, showing a hard tail with a high energy cutoff of 70 keV. About two years later the source reappeared about one order of magnitude brighter in the X-rays (0.5-10 keV) and with a significantly steeper spectrum. As was the case for the low state, the data could be fitted by an input model…
We report on results of a target of opportunity observation of the X-ray transient XTE J1118+480 performed on 2000 April 14-15 with the Narrow Field Instruments (0.1-200 keV) of the SAX satellite. The measured spectrum is a power law with a photon index of ~1.7 modified by an ultrasoft X-ray excess and a high-energy cutoff above ~100 keV. The soft excess is consistent with a blackbody with temperature of ~40 eV and a low flux, while the cut-off power law is well fitted by thermal Comptonization in a plasma with an electron temperature of 100 keV and an optical depth of order of unity. Consiste…
We report on the results of INTEGRAL observations of the neutron star low mass X-ray binary SAX J1810.8-2609 during its latest active phase in August 2007. The current outburst is the first one since 1998 and the derived luminosity is 1.1-2.6x10^36 erg s-1 in the 20-100 keV energy range. This low outburst luminosity and the long-term time-average accretion rate of ~5x10^-12Msolar/yr suggest that SAXJ 1810.8-2609 is a faint soft X-ray transient. During the flux increase, spectra are consistent with a thermal Comptonization model with a temperature plasma of ~23-30 keV and an optical depth of ~1…
(Abridged) We present results of several X-ray observations of the X-ray binary 4U 1954+31 performed with the satellites BeppoSAX, EXOSAT, ROSAT, RXTE, and Swift. We also studied the RXTE ASM data over a period of more than 10 years. Light curves of all observations show an erratic behaviour with sudden increases in the source emission on timescales variable from hundreds to thousands of seconds. There are no indications of changes in the source spectral hardness, with the possible exception of the RXTE pointed observation. Timing analysis does not reveal the presence of coherent pulsations or…
GRB011121 is analyzed as a prototype to understand the ``flares'' recently observed by Swift in the afterglow of many GRB sources. Detailed theoretical computation of the GRB011121 light curves in selected energy bands are presented and compared and contrasted with observational BeppoSAX data.
SAX J1747.0-2853 is an X-ray transient which exhibited X-ray outbursts yearly between 1998 and 2001, and most probably also in 1976. The outburst of 2000 was the longest and brightest. We have analyzed X-ray data sets that focus on the 2000 outburst and were obtained with BeppoSAX, XMM-Newton and RXTE. The data cover unabsorbed 2--10 keV fluxes between 0.1 and 5.3 X 10^-9 erg/s/cm^2. The equivalent luminosity range is 6 X 10^35 to 2 X 10^37 erg/s. The 0.3--10 keV spectrum is well described by a combination of a multi-temperature disk blackbody, a hot Comptonization component and a narrow Fe-K …
We have made BeppoSAX observations of the Seyfert 2/1.9 galaxy ESO103-G35, which contains a nuclear maser source and is known to be heavily absorbed in the X-rays. Analysis of the X-ray spectra observed by SAX in October 1996 and 1997 yields a spectral index 0.74+/-0.07, typical of Seyfert galaxies and consistent with earlier observations of this source. The strong, soft X-ray absorption has column density 1.79E(23)/cm^2, again consistent with earlier results. The best fitting spectrum is that of a power law with a high energy cutoff at 29+/-10 keV, a cold, marginally resolved Fe Kalpha line w…
We present a detailed analysis of the selection effects that plague GRB observations. We find that these effects may partially explain the different redshift distributions between BeppoSax/HETE2 and Swift bursts. It is mandatory to consider these effects to determine the redshift evolution of GRBs.
We present the results of a spectral analysis of 5 Swift XRT and UVOT observations of the BL Lac object PKS 0548-322 carried out over the period April-June 2005. The X-ray flux of this high energy peaked BL Lac (HBL) source was found to be approximately constant at a level of F(2-10 keV) ~ 4x10^-11 erg cm^-2 s^-1, a factor of 2 brighter than when observed by BeppoSAX in 1999 and close to the maximum intensity reported in the Einstein Slew Survey. The very good statistics obtained in the 0.3-10 keV Swift X-ray spectrum allowed us to detect highly significant deviations from a simple power law s…
Gamma-ray burst afterglow observations in the Swift era have a perceived lack of achromatic jet breaks compared to the BeppoSAX, or pre-Swift era. Specifically, relatively few breaks, consistent with jet breaks, are observed in the X-ray light curves of these bursts. If these breaks are truly missing, it has serious consequences for the interpretation of GRB jet collimation and energy requirements, and the use of GRBs as cosmological tools. Here we address the issue of X-ray breaks that are possibly `hidden' and hence the light curves are misinterpreted as being single power laws. We do so by …
We present optical, near-IR, and radio follow up of sixteen Swift bursts, including our discovery of nine afterglows and a redshift determination for three. These observations, supplemented by data from the literature, provide an afterglow recovery rate of 60% in the optical/near-IR, much higher than in previous missions (BeppoSAX, HETE-2, INTEGRAL, and IPN). The optical/near-IR afterglows of Swift events are on average 1.7 mag fainter at t=12 hr than those of previous missions. The X-ray afterglows are similarly fainter compared to those of pre-Swift bursts. In the radio the limiting factor i…
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