BeppoSAX Narrow Field Instruments Research Articles

TheBeppoSAX WFC X-ray source catalogue

Aims. We present the catalogue of X-ray sources detected by the two Wide Field Cameras (WFCs) in complete observations on board BeppoSAX during its 6 years of operational lifetime, between April 1996 and April 2002. Methods. The BeppoSAX WFCs were coded mask instruments sensitive in the 2–28 keV energy band with a 40 × 40 square degree fields of view, pointing in opposite directions and perpendicularly to the BeppoSAX Narrow Field Instruments (NFI). The WFCs were usually operated simultaneously to NFI observations, each lasting up to several days. WFCs observed thus the entire sky several times with a typical sensitivity of 2 to 10 mCrab. A systematic analysis of all WFC observations in the BeppoSAX archive has been carried out using the latest post-mission release of the WFC analysis software and calibrations. Results. The catalogue includes 253 distinct sources, obtained from a total sample of 8283 WFC detections. We describe the basic statistical properties of the sample and present a six-year history of two celestial calibration X-ray sources.

Prompt and afterglow X-ray emission from the X-Ray Flash of 2002 April 27

We report on the X-ray observations of the X-ray flash (XRF) which occurred on 2002 April 27, three days before BeppoSAX was switched off. The event was detected with the BeppoSAX Wide Field Cameras but not with the Gamma ray Burst Monitor. A follow-up observation with the BeppoSAX Narrow Field Instruments was soon performed and a candidate afterglow source was discovered. We present the results obtained. We also include the results obtained from the observations of the XRF field with the Chandra X-ray satellite. The spectral analysis of the prompt emission shows that the peak energy of the EF(E) spectrum is lower than 5.5 keV, with negligible spectral evolution. The X-ray afterglow spectrum is consistent with a power law model with photon index of about 2, while the 2-10 keV flux fades as a power law with a decay index -1.33. Both these indices are typical of GRBs. A very marginal excess around 4.5-5 keV is found in the afterglow spectrum measured by BeppoSAX . As for many GRBs, the extrapolation of the 2-10 keV fading law back to the time of the prompt emission is consistent with the X-ray flux measured during the second part of the event. We estimate a possible range of values of the redshift and discuss our results in the light of current models of XRFs.

A Decreasing Column Density during the Prompt Emission from GRB 000528 Observed withBeppoSAX

We report observation results of the prompt X- and gamma-ray emission from GRB 000528. This event was detected with the Gamma-Ray Burst Monitor and one of the two Wide Field Cameras aboard the BeppoSAX satellite. The gamma-ray burst (GRB) was promptly followed on with the BeppoSAX narrow field instruments and with ground optical and radio telescopes. The X-ray afterglow source was identified, but no optical or radio counterpart found. We report here results from the prompt and afterglow emission analysis. The main feature derived from spectral evolution of the prompt emission is a high hydrogen equivalent column density with evidence of its decrease with time. We model this behavior in terms of a time-dependent photoionization of the local circumburst medium, finding that a compact and dense environment is required by the data. We also find a fading of the late part of the 2‐10 keV prompt emission, which is consistent with afterglow emission. We discuss this result in the light of the external shock model scenario. Subject headingg gamma rays: bursts — gamma rays: observations — X-rays: bursts

BeppoSAX confirmation of beamed afterglow emission from GRB 990510

We compare the prompt X-ray (2-10 keV) emission of GRB 990510 measured by the BeppoSAX Wide Field Cameras (WFC) during the burst to the X-ray afterglow detected by the BeppoSAX Narrow Field Instruments. A single power-law model for the afterglow, , is ruled out. Provided the initial time of the afterglow is assumed to coincide with the last short pulse in the X-ray prompt event (i.e., 72 s after the GRB trigger time), the X-ray emission from ~80 to 105 s after the GRB trigger is well described by an external shock expanding in a decelerating jet, in which synchrotron radiation takes place. This model, represented by a double power-law of indices and before and after a jet collimation break time of ~0.5 days after GRB, respectively, is consistent with the second and third upper limits measured by the WFC, but not with the first. This may be related to inhomogeneities in the circumburst medium. Our finding indicates that the temporal behavior of the GRB 990510 X-ray afterglow is similar to that at optical wavelengths, and thus strengthens the interpretation of the multiwavelength afterglow as synchrotron emission in a jet with decreasing Lorentz factor. GRB 990510 is thus the only burst in which evidence of a spreading jet has been found in X-rays.

The Prompt Emission of GRB 990712 with [ITAL]B[/ITAL][CSC][ITAL]eppo[/ITAL][/CSC][ITAL]SAX[/ITAL]: Evidence of a Transient X-Ray Emission Feature

We report on the prompt X- and γ-ray observations of GRB 990712 with the BeppoSAX Gamma-Ray Burst Monitor and Wide Field Camera 2. As a result of Sun constraints, we could not perform a follow-up observation with the BeppoSAX narrow-field instruments. The light curve of the prompt emission shows two pulses and a total duration of about 40 s in X-rays. In γ-rays, the event is even shorter. The 2-700 keV spectral emission with time shows a discontinuity in the peak energy Ep of the EF(E) spectrum: Ep is above our energy passband during the first pulse and goes down to ~10 keV during the second pulse. Another peculiarity is noted in this event for the first time: the evidence of a 2 s duration emission feature during the tail of the first pulse. The feature is consistent with either a Gaussian profile with centroid energy of 4.5 keV or a blackbody spectrum with kTbb ~ 1.3 keV. We discuss the possible origin of the feature. The most attractive possibility is that we are observing the thermal emission of a baryon-loaded expanding fireball when it becomes optically thin.

Broadband Observations of the New X-Ray Burster SAX J1747.0−2853 during the 1998 March Outburst

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 performed a follow-up target of opportunity observation with the BeppoSAX Narrow Field Instruments on March 23 for a total elapsed time of 72 ks. The source persistent luminosity was 2.6x10^36 erg/s in the 2-10 keV energy range. The wide band spectral data (1-200 keV) are consistent with a remarkable hard X-ray spectrum detected up to 150 keV, highly absorbed at low energies (Nh of the order of 10^23 cm^-2) and with clear evidence for an absorption edge near 7 keV. A soft thermal component is also observed, which can be described by single temperature blackbody emission at about 0.6 keV.

Detection of exceptional X-ray spectral variability in the TeV BL Lac 1ES 2344+514

We present the results of six BeppoSAX observations of 1ES 2344+514, five of which were taken within one week. 1ES 2344+514, one of the few known TeV BL Lac objects, was detected by the BeppoSAX Narrow Field Instruments (NFI) in the range 0.1 and ≈50 keV. During the first five closely spaced observations 1ES 2344+514 showed large-amplitude luminosity variability, associated with spectacular spectral changes: in particular, the last observation found the source to be several times fainter, with a much steeper X-ray spectrum. The energy-dependent shape of the light curve and the spectral changes both imply a large frequency shift (by a factor of 30 or more) of the peak of synchrotron emission. At maximum flux the peak was located at, or above, 10 keV, making 1ES 2344+514 the second blazar (after Mrk 501) to have a synchrotron peak in the hard X-ray band. The frequency shift, and the corresponding increase in luminosity, might be caused by the onset of a second synchrotron component extending from the soft to the hard X-ray band where most of the power is emitted. Rapid variability on a time-scale of approximately 5000 s has also been detected when the source was brightest.

Prompt and Afterglow Emission from the X‐Ray–Rich GRB 981226 Observed withBeppoSAX

We discuss observations of the prompt X- and c-ray emission and X-ray afterglow from GRB 981226. This event has the weakest gamma-ray peak —ux detected with the BeppoSAX Gamma-Ray Burst Monitor. It shows an isolated X-ray precursor and the highest X-ray to gamma-ray —uence ratio mea- sured thus far with the BeppoSAX Wide Field Cameras. The event was followed up with the BeppoSAX Narrow Field Instruments, and the X-ray afterglow was detected up to 10 keV. The afterglow —ux is observed to rise from a level below the sensitivity of the MECS/LECS telescopes up to a peak —ux of (5 ^ 1) ) 10~13 ergs cm~2 s~1 in the 2¨10 keV energy band. This rise is followed by a decline according to a power law with an index of We discuss these results in the light of the current GRB 1.31 ~0.390.44. models. Subject headings: gamma rays: burstsgamma rays: observationsshock wavesX-rays: general

Observational status of Gamma-Ray bursts with BeppoSAX

An overview of the results on Gamma-Ray Bursts obtained with the X-ray Astronomy satellite BeppoSAX is given. Eighteen events were detected and localized with the wide field instruments, with sixteen of them that were also promptly followed-up with the BeppoSAX Narrow Field Instruments (NFIs). X-ray afterglow emission was discovered in twelve cases. Most of the GRBs with X-ray afterglow also show optical emission, while radio emission has been observed in about half of these cases. There is a GRB (981226), from which radio but not optical afterglow emission has been discovered. I will discuss the properties of these events in the light of the fireball models of GRBs.

BeppoSAX discovery of the X-ray afterglow of GRB 971227

The Beppo SAX Narrow Field Instruments observed the region of sky containing GRB 971227 14 hours after the initial burst. A faint source (1SAXJ1257.3+5924) is detected in a position consistent with the Beppo SAX Wide Field Camera position, and disappears during the second half of the observation. In contrast to some other GRBs observed by Beppo SAX no optical and radio afterglows were unambiguously observed.

The X-ray afterglow of GRB 980519

Over a total of 20 gamma-ray bursts local- ized with arcmin accuracies, GRB 980519 represents the 13 th detected by the BeppoSAX Wide Field Cameras (WFCs). An X-ray TOO observation performed by the BeppoSAX Narrow Field Instruments (NFIs), starting about 9.5 hours after the high energy event, revealed X-ray afterglow emission in the 0:1 10 keV energy range. The flux decay was particularly fast with a power-law in- dex of ' 1:8. This is the fastest decay so far measured. Signs of bursting activity are evident. The power-law spec- tral index of 2:8 +0:6 0:5 is quite soft but not unique among GRB afterglows. VR cI c optical emission was detected as soon as 8 hours after the GRB and the power-law flux decay in all these bands were all consistent with ' 2:0. As for the X-ray, this is the fastest of all the 9 optically identied afterglows but GRB 980326. A candidate host galaxy with magnitude Rc = 26 has been reported and variable radio emission detected.