arXiv subjects

L. A. Antonelli

Publications and source records attributed to L. A. Antonelli.

At least 325 records · Page 18Linked to original sources

arXiv2005

XMM-Newton and VLT observations of the afterglow of GRB040827

The field of the Gamma-Ray Burst GRB 040827 was observed with XMM-Newton and with the ESO/VLT starting ~6 and ~12 hours after the burst, respectively. A fading X-ray afterglow is clearly detected with the XMM-Newton/EPIC instrument, with a time decay t^(-delta), with delta=1.41+/-0.10. Its spectrum is well described by a power law (photon index Gamma=2.3+/-0.1) affected by an absorption largely exceeding (by a factor ~5) the expected Galactic one, requiring the contribution of an intrinsic, redshifted absorber. In the optical/NIR range, the afterglow emission was observed in the Ks band, as a weak source superimposed to the host galaxy, with magnitude Ks=19.44+/-0.13 (12 hours after the GRB, contribution from the host subtracted); in other bands the flux is dominated by the host galaxy. Coupling constraints derived from X-ray spectral fitting and from photometry of the host, we estimated a gas column density in the range (0.4-2.6)x10^22 cm^-2 in the GRB host galaxy, likely located at a redshift 0.5<z<1.7. GRB 040827 stands out as the best example of an X-ray afterglow with intrinsic absorption.

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arXiv2005

GRB990123: multiwavelength afterglow study

We report on the BeppoSAX and multiwavelength data analysis of the afterglow of Gamma-Ray Burst (GRB) 990123. Mainly due to its exceptional brightness, this is the only source for which the Wide Field Cameras have allowed an early detection of the X-ray afterglow between about 20 and 60 min after the GRB trigger. Besides, again for the first time, high-energy emission from the afterglow was detected up to 60 keV. The backwards extrapolation of the afterglow decay smoothly reconnects with the late GRB emission, thus indicating that both are consistent with being produced by the same phenomenon. For the X-ray afterglow we found a power-law decay with index alpha_X = 1.46 +/- 0.04; the spectrum has a power-law shape with photon index Gamma about 2. An extensive set of multiwavelength observations on the afterglow, collected from the literature and made during the BeppoSAX pointing, allowed constructing a Spectral Flux Distribution. We performed an analysis of the GRB afterglow emission in the context of the "fireball" model.

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arXiv2005

The puzzling case of GRB 990123: multiwavelength afterglow study

We report on the BeppoSAX data analysis of the afterglow of Gamma-Ray Burst (GRB) 990123, one of the brightest GRBs detected by BeppoSAX. Mainly due to its exceptional brightness, this is the only source for which the Wide Field Cameras have allowed an early detection of the X-ray afterglow between about 20 and 60 min after the GRB trigger. Besides, again for the first time, high-energy emission from the afterglow was detected up to 60 keV. For the X-ray afterglow we found a power-law decay with index alpha = 1.46 +/- 0.04; the spectrum has a power-law shape with photon index Gamma about 1.9. The backward extrapolation of the afterglow decay smoothly reconnects with the late GRB emission, thus suggesting that both emissions are produced by the same phenomenon. An extensive set of multiwavelength observations for the GRB 990123 afterglow made during the BeppoSAX pointing was collected from the literature. The hard X-ray to radio range coverage allowed to construct a spectral flux distribution and to perform an analysis of the GRB afterglow in the context of the "fireball" model. We also report the results of temporal and spectral analysis of an X-ray source serendipitously observed about 22 arcmin north of the GRB afterglow, along with the optical spectroscopy of its possible counterpart to establish the nature of this source.

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arXiv2005

Swift observations of GRB050128: the early X-ray afterglow

Swift discovered GRB050128 with the Burst Alert Telescope and promptly pointed its narrow field instruments to monitor the afterglow. X-ray observations started 108 s after the trigger time. The early decay of the afterglow is relatively flat with a temporal decay modeled with a power law with index ~ -0.3. A steepening occurs at later times (~ 1500 s) with a power law index of ~ -1.3. During this transition, the observed X-ray spectrum does not change. We interpret this behaviour as either an early jet break or evidence for a transition from the fast cooling regime to the slow cooling regime in a wind environment.

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arXiv2005

A flash in the dark: UVES/VLT high resolution spectroscopy of GRB afterglows

We present the first high resolution (R=20000--45000, corresponding to 14 km/s at 4200A to 6.6 km/s at 9000A) observations of the optical afterglow of Gamma Ray Bursts. GRB020813 and GRB021004 were observed by UVES@VLT 22.19 hours and 13.52 hours after the trigger, respectively. These spectra show that the inter--stellar matter of the GRB host galaxies is complex, with many components contributing to each main absorption system, and spanning a total velocity range of up to about 3000 km/s. Several narrow components are resolved down to a width of a few tens of km/s. In the case of GRB021004 we detected both low and high ionization lines. Combined with photoionization results obtained with CLOUDY, the ionization parameters of the various systems are consistent with a remarkably narrow range with no clear trend with system velocity. This can be interpreted as due to density fluctuations on top of a regular R^-2 wind density profile.

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arXiv2004

Probing a GRB progenitor at a redshift of z=2: a comprehensive observing campaign of the afterglow of GRB 030226

We report results from a comprehensive follow-up observing campaign of the afterglow of GRB 030226, including VLT spectroscopy, VLT polarimetry, and Chandra X-ray observations. In addition, we present BOOTES-1 wide-field observations at the time of the occurrence of the burst. First observations at ESO started 0.2 days after the event when the GRB afterglow was at a magnitude of R~19 and continued until the afterglow had faded below the detection threshold (R>26). No underlying host galaxy was found. The optical light curve shows a break around 0.8 days after the burst, which is achromatic within the observational errors, supporting the view that it was due to a jetted explosion. Close to the break time the degree of linear polarization of the afterglow light was less than 1.1%, which favors a uniform jet model rather than a structured one. VLT spectra show two absorption line systems at redshifts z=1.962+/-0.001 and at z=1.986+/-0.001, placing the lower limit for the redshift of the GRB close to 2. We emphasize that the kinematics and the composition of the absorbing clouds responsible for these line systems is very similar to those observed in the afterglow of GRB 021004. This corroborates the picture in which at least some GRBs are physically related to the explosion of a Wolf-Rayet star.

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arXiv2004

Prompt and afterglow X-ray emission from the X-Ray Flash of 2002 April 27

We report on the X-ray observations of the X-ray flash (XRF) which occurred on 2002 April 27, three days before BeppoSAX was switched off. The event was detected with the BeppoSAX Wide Field Cameras but not with the Gamma ray Burst Monitor. A follow-up observation with the BeppoSAX Narrow Field Instruments was soon performed and a candidate afterglow source was discovered. We present the results obtained. We also include the results obtained from the observations of the XRF field with the Chandra X-ray satellite. The spectral analysis of the prompt emission shows that the peak energy of the EF(E) spectrum is lower than 5.5 keV, with negligible spectral evolution. The X-ray afterglow spectrum is consistent with a power law model with photon index of about 2, while the 2-10 keV flux fades as a power law with a decay index -1.33. Both these indices are typical of GRBs. A very marginal excess around 4.5-5 keV is found in the afterglow spectrum measured by BeppoSAX . As for many GRBs, the extrapolation of the 2-10 keV fading law back to the time of the prompt emission is consistent with the X-ray flux measured during the second part of the event. We estimate a possible range of values of the redshift and discuss our results in the light of current models of XRFs.

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arXiv2004

SN 2003lw and GRB 031203: A Bright Supernova for a Faint Gamma-ray Burst

Optical and near-infrared observations of the gamma-ray burst GRB 031203, at z = 0.1055, are reported. A very faint afterglow is detected superimposed to the host galaxy in our first infrared JHK observations, carried out ~9 hours after the burst. Subsequently, a rebrightening is detected in all bands, peaking in the R band about 18 rest-frame days after the burst. The rebrightening closely resembles the light curve of a supernova like SN 1998bw, assuming that the GRB and the SN went off almost simultaneously, but with a somewhat slower evolution. Spectra taken close to the maximum of the rebrightening show extremely broad features as in SN 1998bw. The determination of the absolute magnitude of this SN (SN 2003lw) is difficult owing to the large and uncertain extinction, but likely this event was brighter than SN 1998bw by 0.5 mag in the VRI bands, reaching an absolute magnitude M_V = -19.75+-0.15.

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arXiv2004

Absorption in Gamma Ray Burst afterglows

We studied the X-ray and optical absorption properties of 13 Gamma Ray Burst (GRB) afterglows observed by BeppoSAX. We found that X-ray absorption in addition to the Galactic one along the line of sight is highly statistically significant in the two cases with the best statistics (probability >99.9%). In three other cases the presence of X-ray absorption is marginally significant (probability 97%). Measured rest frame equivalent hydrogen column densities NH range from 0.1 to 10.0 x 10^22 cm-2 (at 90% confidence level) assuming a solar metal abundance. X-ray absorption may be common, although the quality of present data does not allow us to reach a firm conclusion. The rest frame NH derived from XMM and Chandra data as quoted in the literature are in good agreement with the BeppoSAX estimates. For the same GRB afterglow sample we evaluated the rest frame visual extinction AVr. We fitted the optical-NIR afterglow photometry with a power law model corrected by four different extinction curves. By comparing X-ray absorptions and optical extinction, we found that if a Galactic-like dust grain size distribution is assumed, a dust to gas ratio lower than the one observed in the Galaxy is required by the data. A dust to gas ratio 1/10 than the Galactic one, has been tested using the Small Magellanic Cloud extinction curve, which produces inconsistent results. Alternative scenarios to explain simultaneously the optical and X-ray data involve dust with grain size distributions biased toward large grains. Possible mechanisms that can bring to such a grain size distribution are discussed.

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arXiv2004

The high energy X-ray tail of Eta Car revealed by BeppoSAX

We report on the June 2000 long (100 ks) BeppoSAX exposure that has unveiled above 10 keV a new very high energy component of the X-ray spectrum of Eta Car extending to at least 50 keV. We find that the 2-150 keV spectrum is best reproduced by a thermal + non-thermal model. The thermal component dominates the 2-10 keV spectral range with kT_h=5.5 +/- 0.3 keV and log NH_h=22.68 +/- 0.01. The spectrum displays a prominent iron emission line centred at 6.70 keV. Its equivalent width of 0.94 keV, if produced by the thermal source, gives a slightly sub-solar iron abundance ([Fe/H]=-0.15 +/- 0.02). The high energy tail above 10 keV is best fitted by a power law with a photon index of 2.42 +/- 0.04. The integrated 13-150 keV luminosity of ~12 L-sun is comparable to that of the 2-10 keV thermal component. The present result can be explained, in the Eta Car binary star scenario, by Comptonisation of low frequency radiation by high energy electrons, probably generated in the colliding wind shock front, or in instabilities in the wind of the S Dor primary star. It is possible that the high energy tail had largely weakened near the minimum of the 5.53 yr cycle. With respect to the thermal component, it probably has a longer recovering time like that of the highest excitation optical emission lines. Both features can be associated with the large absorption measured by BeppoSAX at phase 0.05.

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arXiv2003

SN 2002lt and GRB 021211: a SN/GRB Connection at z = 1

We present spectroscopic and photometric observations of the afterglow of GRB 021211 and the discovery of its associated supernova, SN 2002lt. The spectrum shows a broad feature (FWHM = 150 A), around 3770 A (in the rest-frame of the GRB), which we interpret as Ca H+K blueshifted by 14400 km/s. Potential sources of contamination due to the host galaxy and/or residuals of telluric absorption have been analyzed and ruled out. Overall, the spectrum shows a suggestive resemblance with the one of the prototypical type-Ic SN 1994I. This might indicate that GRBs are produced also by standard type-Ic supernovae.

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arXiv2003

Evidence for Supernova Signatures in the Spectrum of the Late-time Bump of the Optical Afterglow of GRB 021211

We present photometric and spectroscopic observations of the gamma-ray burst GRB 021211 obtained during the late stages of its afterglow. The light curve shows a rebrightening occurring ~25 days after the GRB. The analysis of a VLT spectrum obtained during the bump (27 days after the GRB) reveals a suggestive resemblance with the spectrum of the prototypical type-Ic SN 1994I, obtained about ~10 days past maximum light. Particularly we have measured a strong, broad absorption feature at 3770 A, which we have identified with Ca II blueshifted by ~14400 km/s, thus indicating that a supernova (SN) component is indeed powering the `bump' in the afterglow decay. Assuming SN 1994I as a template, the spectroscopic and photometric data together indicate that the SN and GRB explosions were at most separated by a few days. Our results suggest that GRBs might be associated also to standard type-Ic supernovae.

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arXiv2003

Chandra Discovery of an X-ray jet and Extended X-ray Structure in z=0.63 quasar, B2 0738+313

We have made a 30 ksec Chandra observation of the redshift z=0.63 GPS quasar B2 0738+313. We detected X-ray emission from the core and have discovered a 200 kpc (projected on the sky) X-ray jet. The X-ray jet is narrow and curves, following the extended radio structure to the south of the quasar, and ending with a hot spot at the southernmost part of the radio lobe. The jet has a knot at ~13 arcsec away from the core. The knot emission is consistent with the X-rays being created by the inverse Compton scattering of the cosmic microwave background (CMB) photons and requires jet bulk Lorentz factors of a few (Gamma_{bulk} ~ 5-7). We discuss the emission mechanisms that may be responsible for the jet emission. We present new VLA data of the core and jet, and discuss the relation between the extended radio and X-ray emission. Extended emission observed in several GPS sources has been interpreted as a signature of the source past activity, while the GPS source is young and newly expanded. We argue that B2~0738+313 may be an example of a new class of radio sources similar to the FRII radio galaxies in their high jet bulk velocities, but with the powerful GPS-like nucleus. B2 0738+313 also has two damped Lyman-alpha systems along the line of sight, at z_{abs} = 0.0912 and 0.2212. We discuss the possible connection between the X-ray absorption (7.2+/-0.9 e20 cm(-2)) detected in the ACIS spectrum and these two intervening absorbers. We also investigate an extended structure within the central 10 arcsec of the core in the relation to structure seen in the optical.

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arXiv2003

Optical and NIR Observations of the Afterglow of GRB 020813

We present optical and near-infrared (NIR) photometry of the bright afterglow of GRB 020813. Our data span from 3 hours to 4 days after the GRB event. A rather sharp achromatic break is present in the light curve, 14 hours after the trigger. In the framework of jetted fireballs, this break corresponds to a jet half opening angle of (1.9 +- 0.2) deg, the smallest value ever inferred for a GRB. We discuss our results in the framework of currently available models, and find that they have problems in explaining the joint temporal and spectral properties, and in particular the slow decay before the break.

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arXiv2003

The dark burst 010214 with BeppoSAX: possible variable absorption and jet X-ray emission

We report on the prompt and afterglow emission observations of the dark burst GRB010214 with BeppoSAX. The prompt emission shows possible evidence of variable absorption from N_H = 3.0^{+5.1}_{-2.0} x 10^23 cm-2 in the first 6 s of the event to a value consistent with the Galactic column density (N_H^(G) = 2.66 x 10^20 cm-2) in the GRB direction. An X-ray afterglow emission in the 2-10 keV energy band was detected with BeppoSAX, but an analogue search at lower wavelengths (optical, IR and radio) was unsuccessful. The X-ray afterglow spectrum is consistent with a power-law with Galactic absorption. The light curve shows a complex decay, if the tail of the prompt emission is assumed as the onset of the afterglow: if the origin of the afterglow is coincident with the GRB onset, a bump before ~ 3 x 10^4 s is inferred, while if the afterglow is assumed to start later, a steepening of the power-law light curve at t ~ 3 x 10^4 s is deduced. We discuss these results in the light of the current models of afterglows and the possible origin of the GRB darkness. Finally, we tentatively derive an estimate of the burst redshift.

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arXiv2002

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

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.

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arXiv2002

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

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.

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arXiv2001

GRB 990704: the most X-ray rich BeppoSAX gamma-ray burst

We present the X- and $γ$-ray detection of GRB 990704 and the discovery and study of its X-ray afterglow, 1SAX J1219.5-0350. Two pointed BeppoSAX observations with the narrow field instruments were performed on this source, separated in time by one week. The decay of the X-ray flux within the first observation appears unusually slow, being best-fit by a power law with negative index 0.83$\pm$0.16. Such a slow decay is consistent with the non-detection in our second observation, but its back-extrapolation to the time of the GRB largely underestimates the detected GRB X-ray prompt emission. In addition, the GRB prompt event shows, among the BeppoSAX-WFC detected sample, unprecedentedly high ratios of X- and gamma-ray peak fluxes (F$_{2-10 keV}$/F$_{40-700 keV}$$\sim$0.6, and F$_{2-26 keV}$/F$_{40-700 keV}$$\sim$1.6) and fluences (S$_{2-10 keV}$/S$_{40-700 keV}$$\sim$1.5 and S$_{2-26 keV}$/S$_{40-700 keV}$$\sim$2.8), making it, among the BeppoSAX arcminute-localized GRBs, the closest to the recently discovered class of Fast X-ray Transients.

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