M. Orlandini

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.

Discovery of a Transient Absorption Edge in the X-ray Spectrum of GRB 990705

We report the discovery of a transient equivalent hydrogen column density with an absorption edge at approximately 3.8 kiloelectron volts in the spectrum of the prompt x-ray emission of gamma-ray burst (GRB) 990705. This feature can be satisfactorily modeled with a photoelectric absorption by a medium located at a redshift of approximately 0.86 and with an iron abundance of approximately 75 times the solar one. The transient behavior is attributed to the strong ionization produced in the circumburst medium by the GRB photons. The high iron abundance points to the existence of a burst environment enriched by a supernova along the line of sight. The supernova explosion is estimated to have occurred about 10 years before the burst. Our results agree with models in which GRBs originate from the collapse of very massive stars and are preceded by a supernova event.

Broadband Spectrum of Cygnus X-1 in Two Spectral States with BeppoSAX

We report on the 0.5-200 keV spectral properties of Cyg X-1 observed at different epochs with the Narrow Field Instruments of the BeppoSAX satellite. The source was in its soft state during the first observation of 1996 June. In the second observation of 1996 September, the source had parameters characteristic to its hard state. A soft X-ray excess, a broad Fe Kα line and Compton reflection are clearly detected in both states. The soft-state broadband continuum is well modeled by a disk blackbody (accounting for the soft excess) and Compton upscattering of the disk photons by a hybrid, thermal/nonthermal plasma, probably forming a corona above the disk (also giving rise to the Compton-reflection component). In the hard state, the primary hard X-ray spectrum can be well modeled by Compton upscattering of a weak blackbody emission by a thermal plasma at a temperature of ~60 keV. The soft excess is then explained by thermal Comptonization of the same blackbody emission by another hot plasma cloud characterized by a low value of its Compton parameter. Finally, we find the characteristic ratio of the bolometric flux in the soft state to that in the hard state to be about 3. This value is much more compatible with theories of state transitions than the previously reported (and likely underestimated) value of 1.5.

Confirmation of Nonthermal Hard X-Ray Excess in the Coma Cluster from Two Epoch Observations

We report the hard X-ray spectrum of the Coma Cluster obtained using the Phoswich Detection System data of two independent BeppoSAX observations performed with a time interval of about 3 yr. In both the spectra a nonthermal excess with respect to the thermal emission is present at a confidence level of ~3.4 σ. The combined spectrum obtained by adding up the two spectra allows a measurement of the excess at the level of ~4.8 σ at energies above 20 keV. The analysis of the full BeppoSAX data set provides a revised nonthermal X-ray flux that is slightly lower than that previously estimated (Fusco-Femiano et al.) and in agreement with that measured by two Rossi X-Ray Timing Explorer observations. The analysis of the offset fields in our Coma observations provides a possible flux determination of the BL Lac object 1ES 1255+244.

A BEPPOSAX Study of the Pulsating Transient X0115+63: The First X-Ray Spectrum with Four Cyclotron Harmonic Features

The recurrent hard pulsating X-ray transient X0115+63 was observed with BeppoSAX on 1999 March 19, when the source was at a 2-10 keV flux level of ~310 mcrab. We report on the high-energy spectrum of the source, concentrating on cyclotron resonant scattering features. The spectrum is strongly pulse phase dependent, and absorption features are detected at virtually all phases. In particular, four absorption-like features at 12.74, 24.16, 35.74, and 49.5 keV are observed in the descending edge of the main peak of the pulse profile. The ratios between the centroid energies of the lines with respect to the first are 1 : (1.9) : (2.8) : (3.9). These values are close to the harmonic relation expected from cyclotron resonant scattering in a strong magnetic field when relativistic effects are taken into account. The equivalent widths of the second, third, and fourth harmonics are found to be larger than that of the first harmonic, confirming the key role of two-photon processes in the spectral formation. These results provide the first evidence for four harmonically spaced lines in the spectrum of an accreting X-ray pulsar, yielding the clearest confirmation to date of their magnetic origin.

The Cosmic X-Ray Background and the Population of the Most Heavily Obscured AGNs

We report on an accurate measurement of the CXB in the 15-50 keV range performed with the Phoswich Detection System (PDS) instrument aboard the BeppoSAX satellite. We establish that the most likely CXB intensity level at its emission peak (26-28 keV) is ≈40 keV cm-2 s-1 sr-1, a value consistent with that derived from the best available CXB measurement obtained over 25 years ago with the first High Energy Astronomical Observatory (HEAO-1) satellite mission (Gruber et al.), whose intensity, lying well below the extrapolation of some lower energy measurements performed with focusing telescopes, was questioned in recent years. We find that 90% of the acceptable solutions of our best-fit model to the PDS data give a 20-50 keV CXB flux lower than 6.5 × 10-8 ergs cm-2 s-1 sr-1, which is 12% higher than that quoted by Gruber et al. when we use our best calibration scale. In combination with the CXB synthesis models we infer that about 25% of the intensity at ~30 keV arises from extremely obscured, Compton-thick AGNs (absorbing column density NH > 1024 cm-2), while a much larger population would be implied by the highest intensity estimates. We also infer a mass density of supermassive black holes of ~3 × 105 M☉ Mpc-3. The summed contribution of resolved sources (Moretti et al.) in the 2-10 keV band exceeds our best-fit CXB intensity extrapolated to lower energies, but it is within our upper limit, so that any significant contribution to the CXB from sources other than AGNs, such as star-forming galaxies and diffuse warm-hot intergalactic medium (WHIM), is expected to be mainly confined below a few keV.

A measurement of the broadband spectrum of XTE J1118+480 with BeppoSAX and its astrophysical implications

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 BeppoSAX 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 a temperature of ~40 eV and a low flux, while the cutoff power law is well fitted by thermal Comptonization in a plasma with an electron temperature of ~102 keV and an optical depth of order unity. Consistent with the weakness of the blackbody, Compton reflection is weak. Although the data are consistent with various geometries of the hot and cold phases of the accreting gas, we conclude that a hot accretion disk is the most plausible model. The Eddington ratio implied by recent estimates of the mass and the distance is ~10-3, which may indicate that advection is probably not the dominant cooling mechanism. We finally suggest that the reflecting medium has a low metallicity, consistent with the location of the system in the halo.

The Zoo of X-Ray Sources in the Galactic Center Region: Observations with BeppoSAX

We report the results of a survey of the Galactic Center region (| l |< 2◦, | b |< 0.5◦) performed with the BeppoSAX satellite. The flux from the center of our Galaxy corresponds to a luminosity of ∼3 10 erg s−1 in the 2–10 keV range. Due to the limited angular resolution ( > ∼ 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 (LX > ∼ 10 ergs s−1) point sources have been observed, both persistent (A 1742–294, SLX 1744–299, SLX 1744–300, 1E 1743.1–2843, 1E 1740.7–2942) and transient (XTE J1748–288, SAX J1747.0–2853 and KS 1741–293). The Low Mass X–ray Binary AX J1745.6–2901, discovered with ASCA at only 1.3′ from SgrA* was detected in a low luminosity state in August 1997. The 1–150 keV spectrum of the hard X–ray source 1E 1740.7–2942 is well described by a Comptonization model, typical of black hole candidates in their low/hard state, with no evidence for strong Fe lines. The detection of a type I burst shows that the transient source SAX J1747.0– 2853 (probably the same as the 1976 transient GX 0.2–0.2) is a LMXRB containing a neutron star. The transient black hole candidate XTE J1748–288 was detected at a luminosity (∼ 10 ergs s−1) consistent with the extrapolation of the exponential decay of the outburst observed with the XTE All Sky Monitor. Two fainter sources are very likely associated with young neutron stars: the (possibly diffuse) X–ray source at the center of the composite supernova remnant G0.9+0.1, and the “head” of the axially symmetric radio source G359.23–0.92. The latter has been detected above ∼ 6 keV, supporting a non–thermal emission mechanism.

The Quiescent X-Ray Emission of Three Transient X-Ray Pulsars

We report on BeppoSAX and Chandra observations of three hard X-ray transients in quiescence containing fast-spinning (P < 5 s) neutron stars: A0538-66, 4U 0115+63, and V0332+53. These observations allowed us to study these transients at the faintest flux levels thus far. The spectra are remarkably different from the ones obtained at luminosities more than a factor of 10 higher, testifying that the quiescent emission mechanism is different. Pulsations were not detected in any of the sources, indicating that accretion of matter down to the neutron star surface has ceased. We conclude that the quiescent emission of the three X-ray transients likely originates from accretion onto the magnetospheric boundary in the propeller regime and/or from deep crustal heating resulting from pycnonuclear reactions during the outbursts.