J. M. Muller

Discovery of an X-ray afterglow associated with the γ-ray burst of 28 February 1997

Establishing the nature of γ-ray bursts is one of the greatest challenges in high-energy astrophysics. The distribution of these bursts is isotropic across the sky, but inhomogeneous in space, with a deficit of faint bursts. It is currently unknown whether γ-ray bursts are produced in our Galaxy or at cosmological distances. The detection and identification of counterparts at other wavelengths are seen as crucial for resolving the origin of the events. Here we report the detection by the Beppo-SAX satellite of an X-ray ‘afterglow’, associated with the γ-ray burst of 28 February 1997 (GRB970228; ref. 3)—the first such detection for any γ-ray burst. The X-ray transient was found to contain a significant fraction of the total energy of the γ-ray burst and, following the initial detection eight hours after the main burst, faded within a few days with a power-law decay function. The rapid locating of this γ-ray burst instigated a multi-wavelength observational campaign that culminated in the identification of a fading optical transient in a position consistent with the X-ray transient reported here.The invention is a three-piece contact assembly for an electrical connector. The contact assembly is characterized by an inner sleeve (10) captivated between a forward outer sleeve (20) and a rear outer sleeve (30) to eliminate deformation and relative movement between the sleeves of a contact assembly.

Prompt and delayed emission properties of gamma-ray bursts observed with BeppoSAX

We investigated the spectral evolution in the 2-700 keV energy band of gamma-ray bursts (GRBs) detected by the Gamma-Ray Burst Monitor (GRBM) and localized with the Wide Field Cameras (WFCs) aboard the BeppoSAX satellite before 1998 May. Most of them have been followed up with the Narrow Field Instruments aboard the same satellite. In the light of these results we discuss open issues on the GRB phenomenon. We find that the optically thin synchrotron shock model (SSM) provides an acceptable representation of most of the time-resolved GRB spectra extending down to 2 keV, except in the initial phases of several bursts and during the whole duration of the quite strong GRB 970111, where a low-energy photon depletion with respect to the thin SSM spectrum is observed. A strong and time-variable low-energy cutoff, consistent with absorption effect, is observed during the prompt emission of GRB 980329. We find that the X-ray afterglow starts at about 50% of the GRB duration and that its fluence, as computed from the WFC light curve, is consistent with the decay law found from the afterglow NFI observations. We also investigate the hydrodynamical evolution of the GRB in our sample and their associated afterglow, when it was detected. We find that the photon index of the latest spectrum of the GRB prompt emission is correlated with the index of the afterglow fading law, when available, as expected on the basis of an external shock of a relativistic fireball. We also find that for most of the GRBs in our sample the late emission is consistent with a slow cooling of the shock. Adiabatic shocks appear more likely than radiative shocks. Parameters of the shocks at earliest times have been derived.

The first outburst of SAX J1808.4-3658 revisited

Data of the 1996 outburst of the single-known accreting millisecond pulsar SAX J1808.4-3658, taken with the Wide Field Cameras (WFCs) on BeppoSAX , are revisited with more complete data coverage and more comprehensive analysis techniques than in a previous report. An additional type-I X-ray burst was identified which occurred at a time when the persistent emission is below the detection limit, roughly 30 days after outburst maximum. This burst is three times longer than the first two bursts, and 50% brighter. It is the brightest burst within the ~1700 type-I bursts detected so far with the WFCs. A spectral analysis of the data reveals a distance to SAX J1808.4-3658 of ~2.5 kpc. This is an update from a previously reported value of 4 kpc. We present the evidence that we have for the presence of oscillations at the pulsar frequency during part of the newly found burst. Such an oscillation would lend support to the idea that the frequency of millisecond burst oscillations in other objects is very close to the neutron star rotation frequency.

Spectral Properties of the Prompt X-ray Emission and Afterglow from the Gamma-Ray Burst of 1997 February 28

We report high-energy spectral data of the prompt emission of GRB 970228 and its X-ray afterglow. We establish that the nature of the X-ray afterglow emission is nonthermal and similar to the later portion of GRB 970228. Our data can be used to discriminate different emission models of GRB afterglows. While cooling of excited compact objects can be ruled out, fireball models are constrained in the physics of the radiation emission processes and their evolution.

Bursts from GS 1826–238: A Clocked Thermonuclear Flashes Generator

The transient X-ray source GS 1826-238 was monitored during five different observing periods between 1996 August and 1998 October with the BeppoSAX Wide Field Camera instrument in the framework of a deep observation of the Galactic center region. During this time, a first detection of X-ray bursts from this source occurred, ruling out its previously suggested black hole candidacy and strongly suggesting the compact object to be a weakly magnetized neutron star. During the 2.5 yr of monitoring, corresponding to ~2.0 Ms of observing time, 70 bursts were detected from the source. Here we report a quasi-periodicity of 5.76 hr in the burst occurrence time that is present during all observations. This is the first example of quasi-periodic bursting over a period of years. It is in line with the history of a rather constant flux since the turn-on in 1988 and points to a very stable accretion.

Prompt and Afterglow Emission from the X-Ray-Rich GRB 981226 Observed with BeppoSAX

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

The 0.1–100 keV spectrum and variability of Mrk 421 in a high state

ABSTRA C T The results of a BeppoSAX target of opportunity (TOO) observation of the BL Lac object Mrk 421 during a high-intensity state are reported and compared with monitoring X-ray data collected with the BeppoSAX Wide Field Cameras (WFC) and the RXTE All Sky Monitor (ASM). The 0.1‐100 keV spectrum of Mrk 421 shows continuous convex curvature that can be interpreted as the high-energy end of the synchrotron emission. The source shows significant short-term temporal and spectral variability, which can be interpreted in terms of synchrotron cooling. The comparison of our results with those of previous observations when the source was a factor 3‐5 fainter shows evidence for strong spectral variability, with the maximum of the synchrotron power shifting to higher energy during high states. This behaviour suggests an increase in the number of energetic electrons during high states.

X-Ray Afterglow Detection of the Short Gamma-Ray Burst GRB 991014

GRB 991014 is one of the shortest gamma-ray bursts detected so far with the Wide Field Cameras aboard BeppoSAX, in both gamma rays and X-rays. The duration is 9.6 s in 2¨28 keV and 3.2 s in 40 to 700 keV (as measured between the times when 5% and 95% of the burst photons have been accumulated). We have re—ned the InterPlanetary Network annulus of the burst; we present the detec- tion of the X-ray afterglow of GRB 991014 within this re—ned annulus and discuss X-ray and gamma- ray observations of the prompt and afterglow emission. Except for the briefness of the prompt event, no other unusual aspects were found in the prompt and afterglow observations as compared to such mea- surements in previous gamma-ray bursts. Subject heading: gamma rays: burstsX-rays: general

Discovery of SAX J1753.5-2349 and SAX J1806.5-2215: two X-ray bursters without detectable steady emission

Abstract 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 flas on a neutron star. The first of two burst detected from SAX J1806.5-2215 does not show spectral softening and cannot be confirmed as a thermonuclear flash.

Bursting sources in the Galactic Centre region

Abstract A continuous monitoring of the Galactic Centre region has been performed with the WFC s on board BeppoSAX on a time spread of about 7 months for a total of 500 ks exposure time. The long repeated observations allowed for the identification of 14 bursting sources, 2 of which uncataloged ( SAXJ1750-290, SAXJ1806-221 ) and 3 observed bursting for the first time ( GS1826-238, SLX1735-269, XTE1709-267 ). A summary of the monitoring of the Galactic Centre bursters is reported.