L. Piro

Intrinsic spectra and energetics of BeppoSAX Gamma-Ray Bursts with known redshifts

We present the main results of a study of spectral and energetics properties of twelve gamma-ray bursts (GRBs) with redshift estimates. All GRBs in our sample were detected by BeppoSAX in a broad energy range (2-700 keV). From the redshift estimates and the good-quality BeppoSAX time-integrated spectra we deduce the main properties of GRBs in their cosmological rest frames. All spectra in our sample are satisfactorily represented by the Band model, with no significant soft X-ray excesses or spectral absorptions. We find a positive correlation between the estimated total (isotropic) energies in the 1-10 000 keV energy range (Erad) and redshifts z. Interestingly, more luminous GRBs are characterized also by larger peak energies Ep so f theirEF(E) spectra. Furthermore, more distant GRBs appear to be systematically harder in the X-ray band compared to GRBs with lower redshifts. We discuss how selection and data truncation eects could bias our results and give possible explanations for the correlations that we found.

Compton reflection and iron fluorescence in BeppoSAX observations of Seyfert type 1 galaxies

A sample of nine bright Seyfert 1 and NELG type galaxies, observed with BeppoSAX, is analyzed to assess on a truly broad band basis (0.1{200 keV) the issue of the spectral contributions of Compton reflection and iron line fluorescence from circumnuclear gas. The empirical description adopted for the direct continuum is the commonly used power law with an exponential cut{o. The most direct test of the theoretical predictions, namely that the equivalent width of the line, W, and the strength R of the reflection relative to the direct continuum are closely related to each other, gives a substantially positive result, that is their mean ratio is very close to expectation, and only a modest spread in the iron abundance seems implied. The existence of a steep correlation between R and the slope of the power law is not conrmed. A weak evidence is found that the existence of a very shallow trend to increase on average with cannot be altogether excluded in both R and W, but needs to be tested with a larger sample. The energy Ef in the exponential cut{o spans a range from about 80 to more than 300 keV. A possible correlation is found, with Ef increasing on average with : if ignored, for instance by keeping Ef at a xed value in a sample study, it could be cause of articial steepening in a correlation between R and .

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.

Absorption in gamma-ray burst afterglows

We studied the X-ray and optical absorption properties of 13 gamma-ray burst (GRB) afterglows observed by BeppoSAX. We found that X-ray absorption in addition to the Galactic one along the line of sight is highly statistically significant in the two cases with the best statistics (probability >99.9%). In three other cases the presence of X-ray absorption is marginally significant (probability ~97%). Measured rest-frame equivalent column densities of hydrogen, NH, range from 0.1 × 1022 to 10.0 × 1022 cm-2 (at 90% confidence level) assuming a solar metal abundance. X-ray absorption may be common, although the quality of present data does not allow us to reach a firm conclusion. We found that the rest-frame column densities derived from XMM and Chandra data as quoted in the literature are in good agreement with the BeppoSAX estimated rest-frame NH range, supporting our result. For the same GRB afterglow sample we evaluated the rest-frame visual extinction AVr. We fitted the optical-NIR afterglow photometry with a power-law model corrected at short wavelengths by four different extinction curves. By comparing X-ray absorptions and optical extinction, we found that if a Galactic-like dust grain size distribution is assumed, a dust-to-gas ratio lower than the one observed in the Galaxy is required by the data. A dust-to-gas ratio ~1/10 that of the Galactic one, as in the Small Magellanic Cloud (SMC) environment, has been tested using the SMC extinction curve, which produces good agreement between the best-fit NH and AVr. We note, however, that the best-fit NH values have been obtained by assuming solar metal abundances, while the metallicity of the SMC ISM is ~ the solar one (Pei 1992). If such low metallicity were assumed, the best-fit NH values would be higher by a factor of ~7, providing a significant increase of the χ2. Alternative scenarios to explain simultaneously the optical and X-ray data involve dust with grain size distributions biased toward large grains. Possible mechanisms that can bring about such a grain size distribution are discussed.

PROBING THE ENVIRONMENT IN GAMMA-RAY BURSTS: THE CASE OF AN X-RAY PRECURSOR, AFTERGLOW LATE ONSET, AND WIND VERSUS CONSTANT DENSITY PROFILE IN GRB 011121 AND GRB 011211

In this paper we present BeppoSAX and XMM-Newton observations of two long gamma-ray bursts (GRBs), the X-ray-rich event of 2001 December 11 (GRB 011211) and the hard and very bright event of 2001 November 21 (GRB 011121). In both events we find evidence of a late X-ray burst taking place several minutes after the prompt emission. In the November burst the spectrum of the X-ray burst is much softer than that of the preceding prompt phase and consistent with the spectrum of the afterglow at 1 day. In addition, the tail of the X-ray burst and the light curve of the afterglow at 1 day are connected by a single power law ?(t - t0), when t0 corresponds with the onset of the X-ray burst. These evidences suggest that the late X-ray burst represents the onset of the afterglow. A similar conclusion is drawn for the December burst. The temporal and spectral behavior of the X-ray and optical afterglows indicate that the fireball evolution in the December burst takes place in an interstellar medium (ISM) environment. In contrast, in the November burst the wind case is revealed by an X-ray decay slower than that observed in the optical (?X = 1.29 ? 0.04 vs. ?O = 1.66 ? 0.06). The wind profile should change into a constant-density profile at large radii in order to reconcile late-time radio data with a jet. Two other results are obtained for this burst. An X-ray burst precedes the much harder GRB by about 30 s. Contrary to the prediction of simple models of precursor activity for collapsars, the precursors spectrum is not consistent with a blackbody. Finally, a substantial absorption column [NH = (7 ? 2) ? 1022?cm-2] is detected during the early part of the prompt emission. This is much greater than that of the wind, and it is thus likely associated with the region surrounding the burst.

Discovery of a Redshifted Iron K Line in the X-Ray Afterglow of GRB 000214

We report the detection (3 ? significance level) of a strong iron emission line in the X-ray spectrum of the afterglow of GRB 000214 (Valentines Day Burst) observed by BeppoSAX. An emission-line feature was observed with a centroid energy of 4.7 ? 0.2 keV, which, if interpreted as K? emission from hydrogen-like iron, corresponds to a redshift of z = 0.47. The intensity (EW ~ 2 keV) and duration (tens of hours) of the line give information on the distance, from the burst region, of the emitting material (R ? 3 ? 1015 cm) and its mass (M ? 1.4 M?). These results are not easily reconciled with the binary merger and hypernova models for gamma-ray bursts because they require large amounts of mass (about 1 M?) at large distances (?1016 cm) and at Newtonian speeds.

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.

The Bright Gamma-Ray Burst of 2000 February 10: A Case Study of an Optically Dark Gamma-Ray Burst

The gamma-ray burst GRB 000210 had the highest gamma-ray peak flux of any event localized by BeppoSAX as yet, but it did not have a detected optical afterglow, despite prompt and deep searches down to Rlim � 23:5. It is therefore one of the events recently classified as dark GRBs, whose origin is still unclear. Chandra observations allowed us to localize the X-ray afterglow of GRB 000210 to within � 1 00 , and a radio transient was detected with the Very Large Array. The precise X-ray and radio positions allowed us to identify the likely host galaxy of this burst and to measure its redshift, z ¼ 0:846. The probability that this galaxy is a field object is � 1:6 � 10 � 2 . The X-ray spectrum of the afterglow shows significant absorption in excess of the Galactic one corresponding, at the redshift of the galaxy, to NH ¼ð 5 � 1 Þ� 10 21 cm � 2 . The amount of dust needed to absorb the optical flux of this object is consistent with the above H i column density, given a dust-to-gas ratio similar to that of our Galaxy. We do not find evidence for a partially ionized absorber expected if the absorption takes place in a giant molecular cloud. We therefore conclude that either the gas is local to the GRB but is condensed in small-scale high-density (ne10 9 cm � 3 ) clouds, or the GRB is located in a dusty, gas-rich region of the Galaxy. Finally, we examine the hypothesis that GRB 000210 lies at ze5 (and

The Faint Optical Afterglow and Host Galaxy of GRB 020124: Implications for the Nature of Dark Gamma-ray Bursts

F. A. H. acknowledges support from a Presidential Early Career award. S. R. K. and S. G. D. thank the NSF for support. R. S. is grateful for support from a NASA ATP grant. R. S. and T. J. G. acknowledge support from the Sherman Fairchild Foundation. J. C. W. acknowledges support from NASA grant NAG 59302. K. H. is grateful for Ulysses support under JPL contract 958056 and for IPN support under NASA grants FDNAG 5-11451 and NAG 5-17100. Support for Proposal HST-GO-09180.01-A was provided by NASA through a grant from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555.

Discovery of GRB 020405 and Its Late Red Bump

We present the discovery of GRB 020405 made with the Interplanetary Network (IPN). With a duration of 60 s, the burst appears to be a typical long-duration event. We observed the 75 arcmin2 IPN error region with the Mount Stromlo Observatorys 50 inch robotic telescope and discovered a transient source that subsequently decayed and was also associated with a variable radio source. We identify this source as the afterglow of GRB 020405. Subsequent observations by other groups found varying polarized flux and established a redshift of 0.690 to the host galaxy. Motivated by the low redshift, we triggered observations with WFPC2 on board the Hubble Space Telescope (HST). Modeling the early ground-based data with a jet model, we find a clear red excess over the decaying optical light curves that is present between day 10 and day 141 (the last HST epoch). This bump has the spectral and temporal features expected of an underlying supernova (SN). In particular, the red color of the putative SN is similar to that of the SN associated with GRB 011121 at late time. Restricting the sample of GRBs to those with z < 0.7, a total of five bursts, red bumps at late times are found in GRB 970228, GRB 011121, and GRB 020405. It is possible that the simplest idea, namely, that all long-duration γ-ray bursts have underlying SNe with a modest dispersion in their properties (especially peak luminosity), is sufficient to explain the nondetections.