Showing papers by "F. Frontera published in 2001"

Broadband Observations of the Afterglow of GRB 000926: Observing the Effect of Inverse Compton Scattering

Abstract: GRB 000926 has one of the best-studied afterglows to date, with multiple X-ray observations, as well as extensive multifrequency optical and radio coverage. Broadband afterglow observations, spanning from X-ray to radio frequencies, provide a probe of the density structure of the circumburst medium, as well as of the ejecta energetics, geometry, and physical parameters of the relativistic blast wave resulting from the explosion. We present an analysis of Chandra X-Ray Observatory observations of this event, along with Hubble Space Telescope and radio monitoring data. We combine these data with ground-based optical and IR observations and fit the synthesized afterglow light curve using models where collimated ejecta expand into a surrounding medium. We find that we can explain the broadband light curve with reasonable physical parameters if the cooling is dominated by inverse Compton scattering. For this model, an excess due to inverse Compton scattering appears above the best-fit synchrotron spectrum in the X-ray band. No previous bursts have exhibited this component, and its observation would imply that the GRB exploded in a moderately dense (n ~ 30 cm-3) medium, consistent with a diffuse interstellar cloud environment.

VLT Spectroscopy of GRB 990510 and GRB 990712: Probing the Faint and Bright Ends of the Gamma-Ray Burst Host Galaxy Population*

Abstract: We present time-resolved optical spectroscopy of the afterglows of the gamma-ray bursts GRB 990510 and GRB 990712. Through the identi—cation of several absorption lines in the —rst-epoch GRB 990510 spectrum, we determine the redshift for this burst at z " 1.619. No clear emission lines are detected. The strength of the Mg I feature is indicative of a dense environment, most likely the host galaxy of GRB 990510. Although the host is extremely faint the GRB afterglow allows us to probe its inter- (V Z 28), stellar medium andin principleto measure its metallicity. The optical spectrum of GRB 990712 (whose host galaxy is the brightest of the known GRB hosts at cosmological redshifts) shows clear fea- tures both in emission and absorption, at a redshift of z \ 0.4331 ^ 0.0004. On the basis of several line emission diagnostic diagrams, we conclude that the host galaxy of GRB 990712 is most likely an H II galaxy. We derive an unreddened (O II) star formation rate of 2.7 ^ 0.8 yr~1. Correcting for the M _ measured extinction intrinsic to the host galaxy this value increases to yr~1. (A V \ 3.4 ~1.72.4), 35 ~25178 M _ The (O II) equivalent width, compared to that of —eld galaxies at z " 1, also suggests that the host of GRB 990712 is vigorously forming stars. We employ the oxygen and Hb emission-line intensities to esti- mate the global oxygen abundance for the host of GRB 990712: log (O/H) \( 3.7 ^ 0.4, which is slight- ly below the lowest metallicity one —nds in nearby spiral galaxies. For both GRBs we study the time evolution of the absorption lines, whose equivalent width might be expected to change with time if the burst resides in a dense compact medium. We —nd no evidence for a signi—cant change in the Mg II width. Subject headings: cosmology: observationsgalaxies: distances and redshiftsgalaxies: starburst ¨ gamma rays: bursts

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

Abstract: 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.

Broadband Observations of the Afterglow of GRB 000926: Observing the Effect of Inverse Compton Scattering and Evidence for a High-Density Environment

Abstract: GRB 000926 has one of the best-studied afterglows to-date, with multiple X-ray observations, as well as extensive multi-frequency optical and radio coverage. Broadband afterglow observations, spanning from X-ray to radio frequencies, provide a probe of the density structure of the circumburst medium, as well as of the ejecta energetics, geometry, and the physical parameters of the relativistic blastwave resulting from the explosion. We present an analysis of {\em Chandra X-ray Observatory} observations of this event, along with {\em Hubble Space Telescope} and radio monitoring. We combine these data with ground-based optical and IR observations and fit the synthesized afterglow lightcurve using models where collimated ejecta expand into a surrounding medium. We find that we can explain the broadband lightcurve with reasonable physical parameters only if the cooling is dominated by inverse Compton scattering. Excess X-ray emission in the broadband spectrum indicates that we are directly observing a contribution from inverse Compton scattering. It is the first time this has been observed in a GRB afterglow, and it implies that the GRB exploded in a reasonably dense (n~30 cm^{-3}) medium, consistent with a diffuse interstellar cloud environment.

The REM telescope: detecting the near infra-red counterparts of Gamma-Ray Bursts and the prompt behavior of their optical continuum

Abstract: Observations of the prompt afterglow of Gamma Ray Burst (GRB) events are unanimously considered of paramount importance for GRB science and related cosmology. Such observations at NIR wavelengths are even more promis- ing allowing one to monitor high-z Ly- absorbed bursts as well as events occurring in dusty star-forming regions. In these pages we present REM (Rapid Eye Mount), a fully robotized fast slewing telescope equipped with a high throughput NIR (Z, J, H, K) camera dedicated to detecting the prompt IR afterglow. REM can discover objects at extremely high redshift and trigger large telescopes to observe them. The REM telescope will simultaneously feed ROSS (REM Optical Slitless Spectrograph) via a dichroic. ROSS will intensively monitor the prompt optical continuum of GRB afterglows. The synergy between REM-IR cam and ROSS makes REM a powerful observing tool for any kind of fast transient phenomena.

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

Abstract: 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.

A measurement of the broad-band spectrum of XTE J1118+480 with BeppoSAX and its astrophysical implications

Abstract: 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. Consistent with the weakness of the blackbody, Compton reflection is weak. Though 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 about 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 location of the system in the halo.

Response function of the Gamma-Ray Burst Monitor (GRBM) onboard the BeppoSAX satellite

Abstract: Preliminary results of a study aimed to the determination of the response function, in direction and energy, of the Gamma-Ray Burst Monitor (GRBM) onboard the BeppoSAX satellite are presented The study has been carried out by means of Monte Carlo techniques, taking into account on-ground calibration before launch, and the Crab spectrum measured during the flight The derived response matrix will be used to evaluate properties (eg, direction, spectrum, fluence) of all Gamma-Ray Bursts detected with the GRBM

A search for gamma-ray bursts in the GRBM/BeppoSAX database

Abstract: Preliminary results on a systematic analysis of the data from the Gamma-Ray Burst Monitor on-board BeppoSAX are presented. The purpose of this analysis is the identification of true celestial gamma-ray bursts. The study considers events that are recognized or not by the on-board trigger system logic. As the on-board logic triggers also spurious events which mimic GRBs (e.g., events due to high energy particles, atmospheric events, etc), it becomes necessary to impose additional constraints in order to recognize true GRBs. These further conditions will be the subject of this contribution.

What can BeppoSAX tell us about short GRBs: An update from the subsecond GRB project

Abstract: We present some statistical considerations on the BeppoSAX hunt for subsecond GRBs at the Scientific Operation Center. Archival 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 ≈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-γ-ray spectra of short vs. long GRBs may be inferred. Finally, the status of searching procedures at the SOC is reviewed.

Average properties of gamma-ray bursts spectra from 2 to 700 keV with BeppoSAX

Abstract: The co-alignment of two detectors of the BeppoSAX Gamma-Ray Burst Monitor (40–700 keV) with the Wide Field Cameras (2–26 keV) on-board the same satellite has permitted simultaneous detection of several GRBs in X and gamma rays and their localization with high precision These measurements allow us also to extend the study of GRB spectra down to 2 keV and with good accuracy thanks to the knowledge of source direction We show and discuss preliminary results of the analysis of X to gamma-rays average spectra based on a sample of 12 BeppoSAX events

A robust filter for the BeppoSAX Gamma-Ray Burst Monitor triggers

Abstract: The BeppoSAX Gamma Ray Burst Monitor (GRBM) is triggered any time a statistically significant counting excess is simultaneously revealed by at least two of its four independent detectors. Several spurious effects, including highly ionizing particles crossing two detectors, are recorded as onboard triggers. In fact, a large number of false triggers is detected, on the order of 10/day. A software code, based on an heuristic algorithm, was written to discriminate between real and false triggers. We present the results of the analysis on an homogeneous sample of GRBM triggers, thus providing an estimate of the efficiency of the GRB detection system consisting of the GRBM and the software.