arXiv subjects

E. Palazzi

Publications and source records attributed to E. Palazzi.

At least 181 records · Page 10Linked to original sources

arXiv2001

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

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.

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arXiv2001

Multiband optical polarimetry of BL Lac objects with the Nordic Optical Telescope

Optical polarization of seven selected BL Lac objects in UBVRI bands was studied with the Nordic Optical Telescope from December 10-14, 1999. Two of them, 3C 66A and PKS 0735+178, were monitored for 4 nights for a total integration time of 4.75 and 5.5 hours, respectively. Other objects (1Jy 0138-097, H 0414+009, PKS 0823-223, OJ287 and BL Lac) were observed sparsely during the run. Apart from PKS 0823-223 (more polarized than observed in the past), the sources show levels of flux and polarization consistent with results at previous epochs. 3C 66A and PKS 0735+178 were intensively observed during December 11 and 12 and exhibited variability of polarization, both on internight and intranight time scales. Wavelength dependence of polarization has been investigated, as well as circular polarization. The results are discussed within the standard model for BL Lacs.

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arXiv2001

GRB000615 in X-rays

GRB000615 was detected simultaneously by the BeppoSAX GRBM and WFC 1 with a localization uncertainty of 2' (error circle radius). X-ray emission was detected only in the 0.1-4 keV range during a NFI observation started about 10 hours after the trigger time. The positional and temporal analysis shows the presence of two sources, one of which may be related to the GRB.

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arXiv2001

The prompt emission of GRB990712 with BeppoSAX: evidence of a transient X-ray emission feature

We report on the prompt X- and gamma-ray observations of GRB990712 with the BeppoSAX Gamma-Ray Burst Monitor and Wide Field Camera No. 2. Due to Sun constraints, we could not perform a follow-up observation with the BeppoSAX Narrow Field Instruments. The light curve of the prompt emission shows two pulses and a total duration of about 40s in X-rays. In gamma-rays the event is even shorter. The 2-700 keV spectral emission with time shows a discontinuity in the peak energy Ep of the E F(E) spectrum: Ep is above our energy passband during the first pulse and goes down to ~10 keV during the second pulse. Another peculiarity is noted in this event for the first time: the possible evidence of a 2s duration emission feature during the tail of the first pulse. The feature is consistent with either a Gaussian profile with centroid energy of 4.5 keV or a blackbody spectrum with kTbb ~1.3 keV. We discuss the possible origin of the feature. The most attractive possibility is that we are observing the thermal emission of a baryon-loaded expanding fireball, when it becomes optically thin.

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arXiv2001

The extraordinarily bright optical afterglow of GRB 991208 and its host galaxy

Observations of the extraordinarily bright optical afterglow (OA) of GRB 991208 started 2.1 d after the event. The flux decay constant of the OA in the R-band is -2.30 +/- 0.07 up to 5 d, which is very likely due to the jet effect, and after that it is followed by a much steeper decay with constant -3.2 +/- 0.2, the fastest one ever seen in a GRB OA. A negative detection in several all-sky films taken simultaneously to the event implies either a previous additional break prior to 2 d after the occurrence of the GRB (as expected from the jet effect). The existence of a second break might indicate a steepening in the electron spectrum or the superposition of two events. Once the afterglow emission vanished, contribution of a bright underlying SN is found, but the light curve is not sufficiently well sampled to rule out a dust echo explanation. Our determination of z = 0.706 indicates that GRB 991208 is at 3.7 Gpc, implying an isotropic energy release of 1.15 x 10E53 erg which may be relaxed by beaming by a factor > 100. Precise astrometry indicates that the GRB coincides within 0.2" with the host galaxy, thus given support to a massive star origin. The absolute magnitude is M_B = -18.2, well below the knee of the galaxy luminosity function and we derive a star-forming rate of 11.5 +/- 7.1 Mo/yr. The quasi-simultaneous broad-band photometric spectral energy distribution of the afterglow is determined 3.5 day after the burst (Dec 12.0) implying a cooling frequency below the optical band, i.e. supporting a jet model with p = -2.30 as the index of the power-law electron distribution.

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arXiv2001

Detection of the optical afterglow of GRB 000630: Implications for dark bursts

We present the discovery of the optical transient of the long-duration gamma-ray burst GRB000630. The optical transient was detected with the Nordic Optical Telescope 21.1 hours after the burst. At the time of discovery the magnitude of the transient was R = 23.04+-0.08. The transient displayed a power-law decline characterized by a decay slope of alpha = -1.035+-0.097. A deep image obtained 25 days after the burst shows no indication of a contribution from a supernova or a host galaxy at the position of the transient. The closest detected galaxy is a R=24.68+-0.15 galaxy 2.0 arcsec north of the transient. The magnitudes of the optical afterglows of GRB980329, GRB980613 and GRB000630 were all R>=23 less than 24 hours from the burst epoch. We discuss the implications of this for our understanding of GRBs without detected optical transients. We conclude that i) based on the gamma-ray properties of the current sample we cannot conclude that GRBs with no detected OTs belong to another class of GRBs than GRBs with detected OTs and ii) the majority (>75%) of GRBs for which searches for optical afterglow have been unsuccessful are consistent with no detection if they were similar to bursts like GRB000630 at optical wavelengths.

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arXiv2000

Discovery of a Transient Absorption Edge in the X-ray Spectrum of GRB 990705

We report the discovery of a transient equivalent hydrogen column density with an absorption edge at ~3.8 kiloelectron volts in the spectrum of the prompt x-ray emission of gamma-ray burst (GRB) 990705. This feature can be satisfactorily modeled with a photoelectric absorption by a medium located at a redshift of ~0.86 and with an iron abundance of ~75 times the solar one. The transient behavior is attributed to the strong ionization produced in the circumburst medium by the GRB photons. The high iron abundance points to the existence of a burst environment enriched by a supernova along the line of sight. The supernova explosion is estimated to have occurred about 10 years before the burst. Our results agree with models in which GRBs originate from the collapse of very massive stars and are preceded by a supernova event

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arXiv2000

VLT identification of the optical afterglow of the gamma-ray burst GRB 000131 at z=4.50

We report the discovery of the gamma-ray burst GRB 000131 and its optical afterglow. The optical identification was made with the VLT 84 hours after the burst following a BATSE detection and an Inter Planetary Network localization. GRB 000131 was a bright, long-duration GRB, with an apparent precursor signal 62 s prior to trigger. The afterglow was detected in ESO VLT, NTT, and DK1.54m follow-up observations. Broad-band and spectroscopic observations of the spectral energy distribution reveals a sharp break at optical wavelengths which is interpreted as a Ly-alpha absorption edge at 6700 A. This places GRB 000131 at a redshift of 4.500 +/- 0.015. The inferred isotropic energy release in gamma rays alone was approximately 10^54 erg (depending on the assumed cosmology). The rapid power-law decay of the afterglow (index alpha=2.25, similar to bursts with a prior break in the lightcurve), however, indicates collimated outflow, which relaxes the energy requirements by a factor of < 200. The afterglow of GRB 000131 is the first to be identified with an 8-m class telescope.

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arXiv2000

Broad band 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 K$α$ line and Compton reflection are clearly detected in both states. The soft-state broad-band continuum is well modeled by a disk blackbody (accounting for the soft excess) and Compton upscattering of the disk photons by a hybrid, thermal/non-thermal, 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 $\sim 60$ 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.

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arXiv2000

Hard X-rays from Type II bursts of the Rapid Burster and its transition toward quiescence

We report on 4 BeppoSAX Target Of Opportunity observations of MXB 1730-335, the Rapid Burster (RB), made during the 1998 February-March outburst. In the first observation, approximately 20 days after the outburst peak, the X-ray light curve showed Type II bursts at a rate of 43 per hour. Nine days later, during the second BeppoSAX pointing, only 5 Type II bursts were detected at the beginning of the observation. During the third pointing no X-ray bursts were detected and in the fourth and final observation the RB was not detected at all. Persistent emission from the RB was detected up to 10 keV during the first three pointings. The spectra of the persistent and bursting emissions below 10 keV were best fit with a model consisting of two blackbodies. An additional component (a power law) was needed to describe the 1-100 keV bursting spectrum when the persistent emission was subtracted. To our knowledge, this is the first detection of the RB beyond 20 keV. We discuss the evolution of the spectral parameters for the bursting and persistent emission during the outburst decay. The light curve, after the second BeppoSAX pointing, showed a steepening of the previous decay trend, and a sharper decay rate leading to quiescence was observed with BeppoSAX in the two subsequent observations. We interpret this behaviour as caused by the onset of the propeller effect. Finally, we infer a neutron star magnetic field B ~ 4 10^8 Gauss.

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arXiv2000

VLT spectroscopy of GRB 990510 and GRB 990712; probing the faint and bright end of the GRB host galaxy population

We present time-resolved optical spectroscopy of the afterglows of the gamma-ray bursts GRB 990510 and GRB 990712. Through the identification of several absorption lines in the first 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 MgI feature is indicative of a dense environment, most likely the host galaxy of GRB 990510. Although the host is extremely faint (V >~ 28), the GRB afterglow allows us to probe its interstellar medium and - in principle - to 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 features 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 HII galaxy. We derive a unreddened [OII] star formation rate of 2.7 +/- 0.8 Msun/yr. Correcting for the measured extinction intrinsic to the host galaxy (A(V) = \gpm{3.4}{2.4}{1.7}), this value increases to \gpm{35}{178}{25} Msun/yr. The [OII] equivalent width, compared to that of field galaxies at z <= 1, also suggests that the host of GRB 990712 is vigorously forming stars. We employ the oxygen and Hbeta emission-line intensities to estimate the global oxygen abundance for the host of GRB 990712: log(O/H) = -3.7 +- 0.4, which is slightly below the lowest metallicity one finds 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 find no evidence for a significant change in the MgII width.

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arXiv2000

The very red afterglow of GRB 000418 - further evidence for dust extinction in a GRB host galaxy

We report near-infrared and optical follow-up observations of the afterglow of the Gamma-Ray Burst 000418 starting 2.5 days after the occurrence of the burst and extending over nearly seven weeks. GRB 000418 represents the second case for which the afterglow was initially identified by observations in the near-infrared. During the first 10 days its R-band afterglow was well characterized by a single power-law decay with a slope of 0.86. However, at later times the temporal evolution of the afterglow flattens with respect to a simple power-law decay. Attributing this to an underlying host galaxy we find its magnitude to be R=23.9 and an intrinsic afterglow decay slope of 1.22. The afterglow was very red with R-K=4 mag. The observations can be explained by an adiabatic, spherical fireball solution and a heavy reddening due to dust extinction in the host galaxy. This supports the picture that (long) bursts are associated with events in star-forming regions.

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arXiv2000

Discovery of hard X-ray emission from Type II bursts of the Rapid Burster

We report on results of BeppoSAX Target Of Opportunity (TOO) observations of the source MXB 1730-335, also called the Rapid Burster (RB), made during its outburst of February-March 1998. We monitored the evolution of the spectral properties of the RB from the outburst decay to quiescence. During the first TOO, the X-ray light curve of the RB showed many Type II bursts and its broadband (1-100 keV) spectrum was acceptably fit with a two blackbody plus power law model. Moreover, to our knowledge, this is the first time that this source is detected beyond 30 keV.

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arXiv2000

The jet and circumburst stellar wind of GRB 980519

We present extensive multi-colour (UBVR_CI_C) photometry of the optical afterglow of GRB 980519. Upon discovery, 8.3 hours after the burst, the source was decaying as a power law, (t-t_GRB)^alpha, with a rapid decay rate alpha_1 = - 1.73+-0.04. About 13 hours after the burst a steepening of the light-curve to alpha_2 = -2.22+-0.04 was observed. Within the framework of current afterglow models, we argue that the rapid initial decline, the `break' in the light curve, and the spectral properties of the afterglow are best interpreted as being due to a collimated ultra-relativistic jet of fixed opening angle expanding into an inhomogeneous medium. In this scenario, we find that the circumburst medium has a density structure that goes as r^(-2.05+-0.22). This is characteristic of a preexisting wind expelled from a massive star. A possible physical scenario is that the progenitor star collapsed to form a black hole (i.e., a `collapsar'), producing the observed burst and afterglow. However, the supernova signature expected in the light curve in such a scenario is not detected. This either implies that the redshift of GRB980519 is greater than 1.5 or that supernovae accompanying GRBs are not standard candles.

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arXiv2000

BeppoSAX Observations of Markarian 501 in June 1999

We present the preliminary results of a long BeppoSAX observation of the BL Lac object Mkn501 carried out in June 1999. The source was fainter than found during the BeppoSAX pointings of 1997 and 1998, but is still detected with a good signal-to-noise ratio up to ~40 keV. The X-ray spectrum in the energy range 0.1-40 keV, produced through synchrotron radiation, is steeper than in the previous years, it is clearly curved, and peaks (in nu*F_nu) at ~0.5 keV. This energy is much lower than those at which the synchrotron component was found to peak in 1997 and 1998. Some intraday variability suggests that activity of the source on small time scales accompanies the large long time scale changes of brightness and spectrum.

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arXiv2000

GRB990712: First Indication of Polarization Variability in a Gamma-ray Burst Afterglow

We report the detection of significant polarization in the optical afterglow of GRB990712 on three instances 0.44, 0.70 and 1.45 days after the gamma-ray burst, with (P, theta) being (2.9% +- 0.4%, 121.1 degr +- 3.5 degr), (1.2% +- 0.4%, 116.2 degr +- 10.1 degr) and (2.2% +- 0.7%, 139.2 degr +- 10.4 degr) respectively. The polarization is intrinsic to the afterglow. The degree of polarization is not constant, and smallest at the second measurement. The polarization angle does not vary significantly during these observations. We find that none of the existing models predict such polarization variations at constant polarization angle, and suggest ways in which these models might be modified to accommodate the observed behavior of this afterglow.

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arXiv2000

ISO detection of a 60micron source near GRB970508

The Infrared Space Observatory observed the field of the $γ$--ray burst GRB 970508 with the CAM and PHT instruments on May 21 and 24, 1997 and with PHT in three filters in November 1997. A source at 60 $μ$m (flux in May of $66\pm 10$ mJy) was detected near the position of the host galaxy of this $γ$--ray burst. The source was detected again in November 1997, at a marginally lower flux ($43\pm 13$ mJy). A Galactic cirrus origin and a stellar origin for the emission can be ruled out on the basis of the infrared colours. The marginal evidence for variability in the 60 $μ$m flux between May and November is not sufficient to warrant interpretation of the source as transient fireball emission. However, the infrared colours are physically reasonable if attributed to conventional dust emission from a single blackbody source. The probability of detecting a 60 $μ$m by chance in a PHT beam down to a detection limit of 50 mJy is $\sim 5\times 10^{-3}$. If the source is at the redshift of the host galaxy of the $γ$--ray burst the fluxes and upper limits at wavelengths from 12 $μ$m to 170 $μ$m indicate it is an ultraluminous infrared galaxy (L$_{\rm ir} \sim 2\times 10^{12}$ L$_{\sun}$). The star formation rate is estimated to be several hundred solar masses per year, depending significantly on model-dependent parameters. If this source is associated with the host galaxy of GRB 970508, progenitor models which associate GRBs with star-forming regions are favoured.

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arXiv2000

Prompt and afterglow emission from the X-ray rich GRB981226 observed with BeppoSAX

We discuss observations of the prompt X and gamma-ray emission and X-ray afterglow from GRB981226. This event has the weakest gamma-ray peak flux detected with the Gamma-Ray Burst Monitor. It shows an isolated X-ray precursor and the highest X-ray to gamma-ray fluence ratio measured thus far with the Wide Field Cameras. The event was followed up with the Narrow Field Instruments, and the X-ray afterglow was detected up to 10 keV. The afterglow flux is observed to rise from a level below the sensitivity of the MECS/LECS telescopes up to a peak flux of (5 +/-1) * 10^(-13) erg cm-2 s-1, in the 2-10 keV energy band. This rise is followed by a decline according to a power law with index of 1.31 (+0.44,-0.39). We discuss these results in the light of the current GRB models.

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