Showing papers by "L. Piro published in 2002"

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

Abstract: 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

Abstract: 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-0918001-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

The bright Gamma-Ray Burst of February 10, 2000: a case study of an optically dark GRB

Abstract: The gamma-ray burst GRB000210 had the highest gamma-ray peak flux of any event localized by BeppoSAX as yet but it did not have a detected optical afterglow. It is therefore one of the events recently classified as dark GRBs or GHOST (GRB Hiding Optical Source Transient), whose origin is still unclear. Chandra observations allowed us to localize this GRB within ~1" and a radio transient was detected with the VLA. We identify the likely (P=0.01) host galaxy of this burst at z=0.846. The X-ray spectrum of the afterglow shows intrinsic absorption N_H=5x10**21 cm-2. The amount of dust needed to absorb the optical flux of this object is consistent with the above HI 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 (n>~10**9 cm-3) clouds, or that the GRB is located in a dusty, gas-rich region of the galaxy. Finally, if GRB000210 lies at z>5, its X-ray absorbing medium would have to be substantially different from that observed in GRBs with optical afterglows.

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

Abstract: We present ground-based optical observations of GRB 020124 starting 1.6 hours after the burst, as well as subsequent Very Large Array (VLA) and Hubble Space Telescope (HST) observations. The optical afterglow of GRB 020124 is one of the faintest afterglows detected to date, and it exhibits a relatively rapid decay, $F_ u\propto t^{-1.60\pm 0.04}$, followed by further steepening. In addition, a weak radio source was found coincident with the optical afterglow. The HST observations reveal that a positionally coincident host galaxy must be the faintest host to date, R>29.5 mag. The afterglow observations can be explained by several models requiring little or no extinction within the host galaxy, A_V~0-0.9 mag. These observations have significant implications for the statistics of the so-called dark bursts (bursts for which no optical afterglow is detected), which are usually attributed to dust extinction within the host galaxy. The faintness and relatively rapid decay of the afterglow of GRB 020124, combined with the low inferred extinction indicate that some dark bursts are intrinsically dim and not dust obscured. Thus, the diversity in the underlying properties of optical afterglows must be observationally determined before substantive inferences can be drawn from the statistics of dark bursts.

A comparative study of the X-ray afterglow properties of optically bright and dark GRBs

Abstract: We have examined the complete set of X-ray afterglow observations of dark and optically bright GRBs performed by BeppoSAX until February 2001. X-ray afterglows are detected in 90% of the cases. We do not find significant differences in the X-ray spectral shape, in particular no higher X-ray absorption in GRBs without optical transient (dark GRBs) compared to GRBs with optical transient (OTGRBs). Rather, we find that the 1.6-10 keV flux of OTGRBs is on average about 5 times larger than that of the dark GRBs. A K-S test shows that this difference is significant at 99.8% probability. Under the assumption that dark and OTGRB have similar spectra, this could suggest that the first are uncaught in the optical band because they are just faint sources. In order to test this hypothesis, we have determined the optical-to-X ray flux ratios of the sample. OTGRBs show a remarkably narrow distribution of flux ratios, which corresponds to an average optical-to-x spectral index 0.794\pm 0.054. We find that, while 75% of dark GRBs have flux ratio upper limits still consistent with those of OT GRBs, the remaining 25% are 4 - 10 times weaker in optical than in X-rays. The significance of this result is equal to or higher than 2.6 sigma. If this sub-population of dark GRBs were constituted by objects assimilable to OTGRBs, they should have shown optical fluxes higher than upper limits actually found. We discuss the possible causes of their behaviour, including a possible occurrence in high density clouds or origin at very high redshift and a connection with ancient, Population III stars.

Multiple BeppoSAX observations of IC 4329A to probe the origin of the Compton reflection component in Seyfert 1 galaxies

Abstract: IC 4329A is the brightest known Seyfert galaxy in hard (∼2–30 keV) X-rays and is likely to be representative of Seyfert 1 galaxies as a class. A recent 100 ks BeppoSAX observation (Perola et al. 1999) clearly confirmed the presence of a warm absorber, a reflection component (R∼0.6), and a high-energy cut-off in the power law at Ec∼270 keV. Its richness in spectral features, combined with its large flux (∼1.6×10−10 erg cm−2 s−1 between 2–10 keV), make this target ideal for multiple observations (in particular with BeppoSAX) to search for spectral variations. Results obtained from 3 follow-up observations (40 ks each) are presented here. The first and most important goal of this study was to probe the origin of the Compton reflection component observed in Seyfert galaxies by monitoring the variability of the reflection continuum and Fe Kα line in response to primary continuum variations. The second goal was to search for variability in the high energy cutoff. We obtain however no conclusive results on any o...