arXiv subjects

E. Palazzi

Publications and source records attributed to E. Palazzi.

At least 217 records · Page 12Linked to original sources

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

Diffuse Thermal Emission from Very Hot Gas in Starburst Galaxies: Spectral Results

New BeppoSAX observations of the nearby archetypical starburst galaxies (SBGs) NGC253 and M82 are presented. The main observational result is the unambiguous evidence that the hard (2-10 keV) component is (mostly) produced in both galaxies by thermal emission from a metal-poor (~ 0.1-0.3 solar), hot (kT \~ 6- 9 keV) and extended (see companion paper: Cappi et al. 1998) plasma. Possible origins of this newly discovered component are briefly discussed. A remarkable similarity with the (Milky Way) Galactic Ridge's X-ray emission suggests, nevertheless, a common physical mechanism.

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arXiv1998

BeppoSAX detection of the Fe K line in the nearby starburst galaxy NGC 253

We present BeppoSAX results on the nearby starburst galaxy NGC 253. Although extended, a large fraction of the X-ray emission comes from the nuclear region. Preliminary analysis of the LECS/MECS/PDS ~0.2-60 keV data from the central 4' region indicates that the continuum is well fitted by two thermal models: a ``soft'' component with kT ~ 0.9 keV, and a ``hard'' component with kT ~ 6 keV absorbed by a column density of ~ 1.2 x10**22 cm-2. For the first time in this object, the Fe K line at 6.7 keV is detected, with an equivalent width of ~ 300 eV. This detection, together with the shape of the 2--60 keV continuum, implies that most of the hard X-ray emission is thermal in origin, and constrains the iron abundances of this component to be ~0.25 of solar. Other lines clearly detected are Si, S and Fe L/Ne, in agreement with previous ASCA results. We discuss our results in the context of the starburst-driven galactic superwind model.

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arXiv1998

An optical study of the GRB 970111 field beginning 19 hours after the Gamma-Ray Burst

We present the results of the monitoring of the GRB 970111 field that started 19 hours after the event. This observation represents the fastest ground-based follow-up performed for GRB 970111 in all wavelengths. As soon as the detection of the possible GRB 970111 X-ray afterglow was reported by Feroci et al. (1998) we reanalyzed the optical data collected for the GRB 970111 field. Although we detect small magnitude variability in some objects, no convincing optical counterpart is found inside the WFC error box. Any change in brightness 19 hours after the GRB is less than 0.2 mag for objects with B < 21 and R < 20.8. The bluest object found in the field is coincident with 1SAXJ1528.8+1937. Spectroscopic observations revealed that this object is a Seyfert-1 galaxy with redshift z=0.657, which we propose as the optical counterpart of the X-ray source. Further observations allowed to perform multicolour photometry for objects in the GRB 970111 error box. The colour-colour diagrams do not show any object with unusual colours. We applied a photometric classification method to the objects inside the GRB error box, that can distinguish stars from galaxies and estimate redshifts. We were able to estimate photometric redshifts in the range 0.2 < z < 1.4 for several galaxies in this field and we did not find any conspicuous unusual object. We note that GRB 970111 and GRB 980329 could belong to the same class of GRBs, which may be related to nearby sources (z ~1) in which high intrinsic absorption leads to faint optical afterglows.

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arXiv1998

Spectroscopy of the Host Galaxy of the Gamma--Ray Burst 980703

We present spectroscopic observations of the host galaxy of the gamma-ray burst (GRB) 980703. Several emission and absorption features are detected, making the redshift, z = 0.966, completely unambiguous. This is only the third known redshift for a GRB host. The implied isotropic gamma-ray energy release from the burst is in excess of 10^{53} erg, for a reasonable choice of cosmological parameters. The spectroscopic properties of the host galaxy are typical for a star formation powered object. Using the observed value of the Balmer decrement, we derived the extinction in the galaxy's restframe, A_V = 0.3 +- 0.3 mag. Using three different star formation rate indicators, we estimate SFR ~ 10 Msun/yr, or higher, depending on the extinction, with a lower limit of SFR > 7 Msun/yr. This is the highest value of the star formation rate measured for a GRB galaxy so far, and it gives some support to the idea that GRBs are closely related to massive star formation.

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arXiv1998

A `Hypernova' model for SN 1998bw associated with gamma-ray burst of 25 April 1998

The discovery of the peculiar supernova (SN) 1998bw and its possible association with the gamma-ray burst (GRB) 980425$^{1,2,3}$ provide new clues to the understanding of the explosion mechanism of very massive stars and to the origin of some classes of gamma-ray bursts. Its spectra indicate that SN~1998bw is a type Ic supernova$^{3,4}$, but its peak luminosity is unusually high compared with typical type Ic supernovae$^3$. Here we report our findings that the optical spectra and the light curve of SN 1998bw can be well reproduced by an extremely energetic explosion of a massive carbon+oxygen (C+O) star. The kinetic energy is as large as $\sim 2-5 \times 10^{52}$ ergs, more than ten times the previously known energy of supernovae. For this reason, the explosion may be called a `hypernova'. Such a C+O star is the stripped core of a very massive star that has lost its H and He envelopes. The extremely large energy, suggesting the existence of a new mechanism of massive star explosion, can cause a relativistic shock that may be linked to the gamma-ray burst.

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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

BeppoSAX detection of the Fe K line in the starburst galaxy NGC253

Preliminary results obtained from BeppoSAX observation of the starburst galaxy NGC253 are presented. X-ray emission from the object is clearly extended but most of the emission is concentrated on the optical nucleus. Preliminary analysis of the LECS and MECS data obtained using the central 4' region indicates that the continuum is well fitted by two thermal components at 0.9 keV and 7 keV. Fe K line at 6.7 keV is detected for the first time in this galaxy; the line has an equivalent width of ~300eV. The line energy and the shape of the 2-10 keV continuum strongly support thermal origin of the hard X-ray emission of NGC253. From the measurement of the Fe K line the abundances can be unambiguously constrained to ~0.25 the solar value. Other lines clearly detected are Si, S and Fe XVIII/Ne, in agreement with ASCA results.

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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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arXiv1997

Hubble Space Telescope Imaging of the Field of GRB970508

We report on Hubble Space Telescope observations of the optical transient discovered in the error box of GRB970508. The object was imaged on 1997 June 2 with the Space Telescope Imaging Spectrograph and Near-Infrared Camera and Multi-Object Spectrometer. The observations reveal a point-like source with R = 23.1 +- 0.2 and H = 20.6 +- 0.3, in agreement with the power-law temporal decay seen in previous ground-based monitoring. Unlike the case of GRB970228, no nebulosity is detected surrounding the OT of GRB970508, although Mg I absorption and [O II] emission seen in Keck spectra at a redshift of z = 0.835 suggest the presence of a dense, star-forming medium. The HST observations set very conservative upper limits of R ~24.5 and H ~22.2 on the brightness of any underlying extended source. If this subtends a substantial fraction of an arcsecond, then the R band limit is ~25.5. Subsequent photometry suggests a flattening of the light curve at later epochs. Assuming the OT decline follows a pure power-law and ascribing the flattening to the presence of an underlying component of constant flux, we find that this must have R = 25.4, consistent with the upper limits determined by HST. At z = 0.8, this would correspond to an absolute magnitude in the U band of ~-18, similar to that of the Large Magellanic Cloud. We propose a scenario in which the host galaxy of the GRB is of Magellanic type, possibly being a ``satellite'' of one of the bright galaxies located at few arcseconds from the optical transient.

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arXiv1997

The pulsed light curves of Her X-1 as observed by BeppoSAX

We report on the timing analysis of the observation of the X-ray binary pulsar Her X-1 performed during the BeppoSAX Science Verification Phase. The observation covered more that two full orbital cycles near the maximum of the main-on in the 35 day cycle of Her X-1. We present the pulse profiles from 0.1 to 100 keV. Major changes are present below 1 keV, where the appearance of a broad peak is interpreted as re-processing from the inner part of the accretion disk, and above 10 keV, where the pulse profile is less structured and the main peak is appreciably harder. The hardness ratios show complex changes with pulse phase at different energies.

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arXiv1997

The broad-band (0.1-200 keV) spectrum of Her X-1 observed with BeppoSAX

The X-ray pulsar Her X-1 was observed for more than two orbital cycles near the maximum of the 35 day X-ray intensity cycle by the Narrow Field Instruments on-board the BeppoSAX satellite. We present the first simultaneous measurement of the 0.1-200 keV spectrum. Three distinct continuum components are evident in the phase averaged spectrum: a low energy excess, modeled as a 0.1 keV blackbody; a power-law and a high energy cut-off. Superposed on this continuum are Fe L and K emission features at 1.0 and 6.5 keV, respectively, and a ~40 keV cyclotron absorption feature. The cyclotron feature can be clearly seen in raw count spectra. We present the properties of the cyclotron feature with unprecedented precision and discuss the indications given by this measurement on the physical properties of the emitting region.

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arXiv1997

Spectral Properties of the Prompt X-ray emission and Afterglow from the Gamma-Ray Burst of 28 February 1997

We report high-energy spectral data of the prompt emission of GRB970228 and its X-ray afterglow. We establish that the nature of the X-ray afterglow emission is non-thermal and similar to the later portion of GRB970228. Our data can be used to discriminate different emission models of GRB afterglows. While cooling of excited compact objects can be ruled out, fireball models are constrained in the physics of the radiation emission processes and their evolution.

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