arXiv subjects

L. Nicastro

Publications and source records attributed to L. Nicastro.

At least 109 records · Page 6Linked to original sources

arXiv1999

BeppoSAX Detection and Follow-up of GRB980425

We present BeppoSAX GRBM and WFC light curves of GRB980425 and NFI follow-up data taken in 1998 April, May, and November. The first NFI observation has detected within the 8' radius error box of the GRB an X-ray source positionally consistent with the supernova SN 1998bw, exploded within a day of GRB980425, and a fainter X-ray source, not consistent with the position of the supernova. The former source is detected in the following NFI pointings and exhibits a decline of a factor of two in six months. If it is associated with SN 1998bw, this is the first detection of hard X-ray emission from a Type I supernova. The latter source exhibits only marginally significant variability. Based on these data, it is not possible to select either source as a firm candidate for the GRB counterpart.

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arXiv1999

Integrating the BeppoSAX Gamma-Ray Burst Monitor into the 3rd Interplanetary Network

We have added the BeppoSAX Gamma-Ray Burst Monitor to the 3rd Interplanetary Network of burst detectors. We analyze 16 bursts whose positions are known to good accuracy from measurements at other wavelengths. We show that there is excellent agreement between the Ulysses/BeppoSAX triangulation annuli and the known positions of these events, and that these annuli can in many cases provide useful constraints on the positions of bursts detected by the BeppoSAX Wide Field Camera and Narrow Field Instruments.

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arXiv1999

The discovery of polarization in the afterglow of GRB 990510 with the ESO Very Large Telescope

Following a BeppoSAX alert (Piro 1999a) and the discovery of the OT at SAAO (Vreeswijk et al. 1999a), we observed GRB 990510 with the FORS instrument on ESO's VLT Unit 1 (`Antu'). The burst is unremarkable in gamma rays, but in optical is the first one to show good evidence for jet-like outflow (Stanek et al. 1999, Harrison et al. 1999). We report the detection of significant linear polarization in the afterglow: it is (1.6 +/- 0.2)% 0.86 days after trigger, and after 1.81 days is consistent with that same value, but much more uncertain. The polarization angle is constant on a time scale of hours, and may be constant over one day. We conclude that the polarization is intrinsic to the source and due to the synchrotron nature of the emission, and discuss the random and ordered field geometries that may be responsible for it.

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arXiv1999

The X-ray afterglow of GRB980519

Over a total of 20 gamma-ray bursts localized with arcmin accuracies, GRB980519 represents the 13th detected by the BeppoSAX Wide Field Cameras. An X-ray TOO observation performed by the BeppoSAX Narrow Field Instruments, starting about 9.5 hours after the high energy event, revealed X-ray afterglow emission in the 0.1--10 keV energy range. The flux decay was particularly fast with a power-law index of ~ 1.8. This is the fastest decay so far measured. Signs of bursting activity are evident. The power-law spectral index of 2.8 (+0.6, -0.5) is quite soft but not unique among GRB afterglows.

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arXiv1999

The SPOrt Project: an Experimental Overview

The Sky Polarization Observatory (SPOrt) is presented as a project aimed to measure the diffuse sky polarized emission, from the International Space Station, in the frequency range 20-90 GHz with 7 degrees of HPBW. The SPOrt experimental configuration is described with emphasis on the aspects that make SPOrt the first European scientific payload operating at microwave wavelengths.

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arXiv1999

The SPOrt Project: Cosmological and Astrophysical Goals

We present the cosmological and astrophysical objectives of the SPOrt mission, which is scheduled for flying on the International Space Station (ISS) in the year 2002 with the purpose of measuring the diffuse sky polarized radiation in the microwave region. We discuss the problem of disentangling the cosmic background polarized signal from the Galactic foregrounds.

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arXiv1998

The optical/IR counterpart of the 3 July 1998 gamma-ray burst and its evolution

We imaged the X-ray error box of GRB 980703, beginning 22.5 hours after the gamma--ray event, in both the optical R and near-infrared H bands. A fading optical/IR object was detected within the X-ray error box, coincident with the variable radio source reported by Frail et al. (1998a), who also detected the optical transient independently of us. Further imagery revealed the GRB host galaxy, with R = 22.49 +/- 0.04 and H = 20.5 +/- 0.25, the brightest so far detected. When excluding its contribution to the total flux, both the R and H-band light curves are well-fit by a power-law decay with index alpha~1.4. Our data suggest an intrinsic column density in the host galaxy of ~ 3.5 x 10^21 cm^-2 which indicates the existence of a dense and rich-gas medium in which the GRB occurred, thus supporting the hypernova model scenarios.

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arXiv1998

Properties of the host galaxy of the gamma-ray burst 970508 and local star-forming galaxies

Late-time observations of GRB970508 with the SAO-RAS 6-m telescope in July-August 1998 show that the optical transient (OT) varies very little since November 1997. Here we report the $BVR_cI_c$ light-curves behaviour up to 470 days from the GRB occurence with independent $BVR_cI_c$ power-law fits. After ~=200 days the power-law decay has ceased and the OT contamination to the host galaxy flux is already less than the observational errors of the brightness estimates. Spectral energy distribution for the host galaxy alone (without OT) and comparison of its broad-band spectrum to the average continuum spectra of different Hubble types galaxies are shown. The best fits to the BVR_cI_c spectral energy distribution of the GRB host with local starburst galaxies are found for the blue compact S0 dwarf galaxy NGC1510, the Scd starburst HII galaxy NGC7793 and the blue compact galaxy Mrk 1267. Position of the host galaxy in the M_B vs. log(D_25) diagram for local late-type LEDA galaxies allows us to attribute GRB970508 host galaxy to the blue compact galaxies.

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arXiv1998

The absorption properties of gamma-ray burst afterglows measured by BeppoSAX

We present an analysis of the X-ray absorption properties of 6 gamma-ray burst (GRB) afterglows measured with BeppoSAX. Between 8 hrs and 20 hrs after the initial GRB trigger, individual spectra can be described by a power-law with a photon index of ~2 and absorption marginally consistent with the galactic value. Taken collectively, the data are inconsistent with zero absorption at the >99.999 confidence level. The data are only marginally consistent with a distribution of column densities varying as the total galactic absorption in the direction of each of the bursts (chi^2 =9.6 for 6 degrees of freedom). The data are consistent with cosmological models in which GRB occur within host galaxies. By simultaneously fitting a power-law spectral model with absorption fixed at the galactic value and additional, redshifted, absorption to all 6 afterglow spectra, the best-fit average absorption within the host objects is found to be (1.01 ^0.28_0.51) 10^22 atom cm^-2. This value is compatible with the host galaxy column densities inferred from optical data for GRB970508, GRB971214 and GRB980329, supporting the hypothesis that GRB occur within heavily absorbed star forming regions of their host galaxies.

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arXiv1998

BeppoSAX observations of GRB970402

GRB970402 is the fourth gamma-ray burst detected by BeppoSAX simultaneously in the Gamma Ray Burst Monitor (GRBM) and one of the two Wide Field Cameras (WFCs). A rapid pointing of the BeppoSAX Narrow Field Instruments (NFIs) 8 hours after the GRB led to the identification of an unknown weak X-ray source: 1SAX J1450.1-6920. Its position was approximately at the center of the 3 arcmin error circle derived from the WFC image. Both the Medium Energy (MECS, 2-10 keV) and Low Energy (LECS, 0.1-10 keV) concentrators detected the source. A follow-up observation performed 1.5 days later and lasting 54 ks showed that the source had faded almost to, but not below the detectability threshold. The flux decrease between the two observations was a factor ~= 2.5. 1SAX J1450.1-6920 was the second X-ray afterglow associated with a GRB. Searches promptly started at other wavelengths (optical, IR, radio) did not reveal any transient event within the 3 arcmin error circle.

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arXiv1998

Gamma-Ray Burst 980329 and its X-Ray Afterglow

GRB 980329 is the brightest gamma-ray burst detected so far with the Wide Field Cameras aboard BeppoSAX, both in gamma-rays and X-rays. With respect to its fluence (2.6 X 10**-5 erg/s/cm**2 in 50 to 300 keV) it would be in the top 4% of gamma-ray bursts in the 4B catalog (Meegan et al. 1998). The time-averaged burst spectrum from 2 to 20 and 70 to 650 keV can be well described by the empirical model of Band et al. (1993). The resulting photon index above the break energy is exceptionally hard at -1.32 +/- 0.03. An X-ray afterglow was detected with the narrow-field instruments aboard BeppoSAX 7 h after the event within the error box as determined with the Wide Field Cameras. Its peak flux is (1.4 +/- 0.2) X 10**-12 erg/s/cm**2 (2 to 10 keV). The afterglow decayed according to a power law function with an index of -1.35 +/- 0.03. GRB 980329 is characterized by being bright and hard, and lacking strong spectral evolution.

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arXiv1998

Discovery of the peculiar supernova 1998bw in the error box of GRB980425

The discovery of X-ray, optical and radio afterglows of gamma-ray bursts (GRBs) and the measurements of the distances to some of them have established that these events come from Gpc distances and are the most powerful photon emitters known in the Universe, with peak luminosities up to 10^52 erg/s. We here report the discovery of an optical transient, in the BeppoSAX Wide Field Camera error box of GRB980425, which occurred within about a day of the gamma-ray burst. Its optical light curve, spectrum and location in a spiral arm of the galaxy ESO 184-G82, at a redshift z = 0.0085, show that the transient is a very luminous type Ic supernova, SN1998bw. The peculiar nature of SN1998bw is emphasized by its extraordinary radio properties which require that the radio emitter expand at relativistical speed. Since SN1998bw is very different from all previously observed afterglows of GRBs, our discovery raises the possibility that very different mechanisms may give rise to GRBs, which differ little in their gamma-ray properties.

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arXiv1998

Detection of the near-infrared counterpart of GRB 971214 3.2 hours after the gamma-ray event

We report the detection of the GRB 971214 counterpart in the near-infrared by means of two images in the K'-band taken at Calar Alto only ~3.5 and ~5 hours after the gamma-ray event. We detect the transient at K'=18.03+/-0.18 and K'=18.00+/-0.22 respectively. Our data seem to indicate the existence of a plateau with duration ~1.5 < T < 6.7 hours (between 3.5 and 10.2 hours after the high-energy event). Moreover the power-law decline should be steeper than the one given by the index ak'=0.45. There is also a change in the slope of the broad-band spectrum at some wavelength between the J and K' bands (possibly around the H-band).

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arXiv1998

BeppoSAX follow-up search for the X-ray afterglow of GRB970111

The BeppoSAX satellite has recently opened a new way towards the solution of the long standing gamma-ray bursts' (GRBs) enigma, providing accurate coordinates few hours after the event thus allowing for multiwavelength follow-up observational campaigns. The BeppoSAX Narrow Field Instruments observed the region of sky containing GRB970111 16 hours after the burst. In contrast to other GRBs observed by BeppoSAX no bright afterglow was unambiguously observed. A faint source (1SAXJ1528.1+1937) is detected in a position consistent with the BeppoSAX Wide Field Camera position, but unconsistent with the IPN annulus. Whether 1SAXJ1528.1+1937 is associated with GRB970111 or not, the X-ray intensity of the afterglow is significantly lower than expected, based on the properties of the other BeppoSAX GRB afterglows. Given that GRB970111 is one of the brightest GRBs observed, this implies that there is no obvious relation between the GRB gamma-ray peak flux and the intensity of the X-ray afterglow.

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arXiv1998

BVRcIc photometry of GRB970508 optical remnant: May-August, 1997

We present the results of photometric observations of the variable optical source associated to the remnant of the gamma-ray burst GRB970508 performed at SAO RAS (Russia) from May to August 1997. The observations were carried out with the standard (Johnson-Kron-Cousins) photometric BVRcIc system using the 1-meter and 6-meter telescopes. The brightness of the optical source increased from Rc=21.19 (May 9.75 UT) to Rc=19.70 (May 10.77 UT), whereupon it was decaying in all the four BVRcIc bands during about one month after the burst. Between the 3rd and the 6th night after the GRB970508 event, the flux decrease of the optical source follows an exponential law in all the four bands. In that period the broad-band spectrum of the object does not change and can be approximately described by a power with index -1.1. The subsequent observations showed a reduction of the source brightness decrease rate in all bands. The source flux decay after the maximum in the Rc band is well described by a single power-law with index -1.171 for the whole set of observations during 86 days. In the other bands the decay seems to slow down from the 31th day onwards. In particular, in the Ic band the source magnitude is about 23.1 from the 36th day after the GRB to the end of the observations (August 4, 1997).

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arXiv1998

High Energy Properties of X-ray Sources observed with BeppoSAX

We report on highlight results on celestial sources observed in the high energy band (>20 keV) with BeppoSAX. In particular we review the spectral properties of sources that belong to different classes of objects, i.e. stellar coronae (Algol), supernova remnants (Cas A), low mass X-ray binaries (Cygnus X-2 and the X-ray burster GS1826-238), black hole candidates (Cygnus X-1) and Active Galactic Nuclei (Mkn 3). We detect, for the first time, the broad-band spectrum of a stellar corona up to 100 keV; for Cas A we report upper limits to the ^44Ti line intensities that are lower than those available to date; for Cyg X-2 we report the evidence of a high energy component; we report a clear detection of a broad Fe K line feature from Cygnus X-1 in soft state and during its transition to hard state; Mkn 3 is one of several Seyfert 2 galaxies detected with BeppoSAX at high energies, for which Compton scattering process is important.

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arXiv1998

Evidence for a late-time outburst of the X-ray afterglow of GB970508 from BeppoSAX

The gamma-ray burst GB970508 was observed simultaneously by the Gamma Ray Burst Monitor (GRBM) and one of the X-ray Wide Field Cameras (WFC) aboard BeppoSAX. The latter provided a position within 1.9' radius. A series of follow-up observations with the Narrow Field Intruments (NFI) was then performed in a period from 6 hours to 6 days after the main event. A previously unknown source, which we associate with the afterglow of the GRB, was discovered in the error box. We find that, after the initial burst, X-ray emission is still present and decays as t^(-1.1) up to 6 times 10^4 s. This is followed by a burst of activity with a duration 10^5 s. The energy produced in this event is a substantial fraction of the total energy of the GRB, which means that the afterglow is not a remnant of the initial burst (the GRB) that fades away smoothly. Our results support the idea that the processes generating the GRB and its afterglow are the same.

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arXiv1997

Broad band X-ray spectral properties of Gamma-ray bursts with BeppoSAX

In about one year, five gamma-ray bursts were simultaneously observed with the Wide Field Cameras and Gamma Ray Burst Monitor aboard the BeppoSAX satellite. From some of them X-ray afterglow emission has been clearly detected with the same satellite. In order to understand how GRB emission is related to the X-ray afterglow, we are performing a systematic study of the spectral properties of these events. We report here preliminary results of this study.

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