arXiv subjects

S. Covino

Publications and source records attributed to S. Covino.

At least 451 records · Page 25Linked 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

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

Out of the darkness: the infrared afterglow of the INTEGRAL burst GRB 040422 observed with the VLT

GRB 040422 was detected by the INTEGRAL satellite at an angle of only 3 degrees from the Galactic plane. Analysis of the prompt emission observed with the SPI and IBIS instruments on INTEGRAL are presented. The IBIS spectrum is well fit by the Band model with a break energy of Eo=56+/-2 keV and Epeak=41+/-3 keV. The peak flux is 1.8 10^(-7) erg/cm2/s and fluence 3.4 10^(-7) erg/cm2 in the range 20-200 keV. We then present the observations of the afterglow of GRB 040422, obtained with the ISAAC and FORS 2 instruments at the VLT less than 2 hours after the burst. We report the discovery of its near-infrared afterglow, for which we give here the astrometry and photometry. No detection could have been obtained in the R and I bands, partly due to the large extinction in the Milky Way. We re-imaged the position of the afterglow two months later in the Ks band, and detected a likely bright host galaxy. We compare the magnitude of the afterglow with a those of a compilation of promptly observed counterparts of previous GRBs, and show that the afterglow of GRB 040422 lies at the very faint end of the distribution, brighter only than that of GRB 021211, singled out later and in the optical bands, and GRB 040924 after accounting for Milky Way extinction. This observation suggests that the proportion of dark GRBs can be significantly lowered by a more systematic use of 8-m class telescopes in the infrared in the very early hours after the burst.

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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

Taking the pulse of the shortest orbital period binary system RX J0806.3+1527

RX J0806.3+1527 is thought to be a 321s orbital period (the shortest known) double white dwarf binary system. According to the double degenerate binary (DDB) scenario this source is expected to be one of the strongest gravitational wave (GW) emitter candidates. In the last years RX J0806.3+1527 has been studied in great details, through multiwavelength observational campaigns and from the point of view of data analysis result interpretations. We present here the timing results obtained thanks to a 3.5-year long optical monitoring campaign carried out by the Very Large Telescope (VLT) and the Telescopio Nazionale Galileo (TNG) which allowed us to detect and study the orbital period derivative (spin-up at a rate of about 10^-3 s/yr) of the 321s modulation, to detect the linear polarisation (at a level of about 2%), and to study the broad band energy spectrum. The VLT/TNG observational strategy we used allowed us to rely upon a P-Pdot coherent solution which we finally extended backward to the 1994 ROSAT observation of RX J0806.3+1527.

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arXiv2004

Indirect evidence for an active radio pulsar in the quiescent state of the transient ms pulsar SAX J1808.4-3658

Millisecond radio pulsars are neutron stars that have been spun-up by the transfer of angular momentum during the low-mass X-ray binary phase. The transition from an accretion-powered to a rotation-powered pulsar takes place on evolutionary timescales at the end of the accretion process, however it may also occur sporadically in systems undergoing transient X-ray activity. We have obtained the first optical spectrum of the low mass transient X-ray pulsar SAX J1808.4-3658 in quiescence. Similar to the black widow millisecond pulsar B1957+20, this X-ray pulsar shows a large optical modulation at the orbital period due to an irradiated companion star. Using the brightness of the companion star as a bolometer, we conclude that a very high irradiating luminosity, a factor of ~100 larger than directly observed, must be present in the system. This most likely derives from a rotation-powered neutron star that resumes activity during quiescence.

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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

Infrared and X-ray variability of the transient Anomalous X-ray Pulsar XTE J1810-197

We report on observations aimed at searching for flux variations from the proposed IR counterpart of the Anomalous X-ray Pulsar XTE J1810-197. These data, obtained in March 2004 with the adaptive optics camera NAOS-CONICA at the ESO VLT, show that the candidate proposed by Israel et al. (2004) was fainter by Delta H=0.7+/-0.2 and Delta Ks=0.5+/-0.1 with respect to October 2003, confirming it as the IR counterpart of XTE J1810-197. We also report on an XMM-Newton observation carried out the day before the VLT observations. The 0.5-10 keV absorbed flux of the source was 2.2x10^-11 erg/s/cm^2, which is less by a factor of about two compared to the previous XMM-Newton observation on September 2003. Therefore, we conclude that a similar flux decrease took place in the X-ray and IR bands. We briefly discuss these results in the framework of the proposed mechanism(s) responsible for the IR variable emission of Anomalous X-ray Pulsars.

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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

On the jet structure and magnetic field configuration of GRB 020813

The polarization curve of GRB 020813 is discussed and compared to different models for the structure, evolution and magnetisation properties of the jet and the interstellar medium onto which the fireball impacts. GRB 020813 is best suited for this kind of analysis for the smoothness of its afterglow light curve, ensuring the applicability of current models. The polarization dataset allows us to rule out the standard GRB jet, in which the energy and Lorentz factor have a well defined value inside the jet opening angle and the magnetic field is generated at the shock front. We explore alternative models finding that a structured jet or a jet with a toroidal component of the magnetic field can fit equally well the polarization curve. Stronger conclusions cannot be drawn due to the incomplete sampling of the polarization curve. A more dense sampling, especially at early times, is required to pin down the structure of the jet and the geometry of its magnetic field.

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arXiv2004

GRB 020813: polarization in the case of a smooth optical decay

We present the results of a VLT polarimetric monitoring campaign of the GRB 020813 optical afterglow carried out in three nights, from 0.88 to 4.20 days after the gamma-ray event. The mean values of the degree of linear polarization (P) and its position angle (Theta) resulting from our campaign are =1.18+/-0.10% and Theta = 148.7+/-2.3 deg after correcting for Galactic interstellar polarization. Our VLT data set is most naturally described by a constant degree of linear polarization and position angle, nonetheless a slow Theta evolution cannot be entirely ruled out by our data. The VLT monitoring campaign did not reveal either a significant Theta rotation respect to the Keck spectropolarimetric observations performed ~0.25 days after the GRB (Barth et al. 2003). However, is a factor of two lower than the polarization degree measured from Keck. Additionally, the VLT polarization data allowed us to construct an accurate V-band light curve. The V-band photometric data revealed a smooth light curve with a break located between the last Keck and our first VLT polarimetric measurement, 0.33 < t_(break, V) < 0.88 days after the GRB. The typical magnitude fluctuations of the VLT V-band lightcurve are 0.003 mag, 0.010 mag and 0.016 mag for our three observing nights, respectively. We speculate that the stability of Theta might be related to the smoothness of the light curve.

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arXiv2004

Accurate X-ray position of the Anomalous X-ray Pulsar XTE J1810-197 and identification of its likely IR counterpart

We report the accurate sub-arcsec X-ray position of the new Anomalous X-ray Pulsar (AXP) XTE J1810-197, derived with a Chndra-HRC Target of Opportunity observation carried out in November 2003. We also report the discovery of a likely IR counterpart based on a VLT (IR band) Target of Opportunity observation carried out in October 2003. Our proposed counterpart is the only IR source (Ks=20.8) in the X-ray error circle. Its IR colors as well as the X-ray/IR flux ratio, are consistent with those of the counterparts of all other AXPs (at variance with field star colors). Deep Gunn-i band images obtained at the 3.6m ESO telescope detected no sources down to a limiting magnitude of 24.3. Moreover, we find that the pulsed fraction and count rates of XTE J1810-197 remained nearly unchanged since the previous Chandra and XMM-Newton observations (2003 August 27th and September 8th, respectively). We briefly discuss the implications of these results. In particular, we note that the transient (or at least highly variable) nature of this AXP might imply a relatively large number of hidden members of this class.

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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

Intrinsic and dust-induced polarization in gamma-ray burst afterglows: the case of GRB 021004

Polarization measurements for the optical counterpart to GRB 021004 are presented and discussed. Our observations were performed with the TNG and the VLT-UT3 (Melipal) during the first and fourth night after the gamma-ray burst discovery. We find robust evidence of temporal evolution of the polarization, which is therefore, at least partially, intrinsic to the optical transient. We do not find convincing evidence of wavelength dependence for the intrinsic polarization of the transient, in agreement with current polarization models for optical afterglows. We discuss the role of dust, both in our galaxy and in the host, in modifying the transmitted polarization vector, showing how a sizable fraction of the observed polarized flux is due to Galactic selective extinction, while it is not possible to single out any clear contribution from dust in the host galaxy. We discuss how our data compare to those obtained by different groups showing that a two-component model is required to describe the complete dataset. This is not surprising given the complex lightcurve of GRB 021004.

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arXiv2003

Integral results on GRB030320: a long gamma-ray burst detected at the edge of the field of view

GRB030320 is the 5th Gamma-ray burst (GRB) detected by INTEGRAL in the field of view (FoV). It is so far the GRB with the largest off-axis angle with respect to the INTEGRAL pointing direction, near to the edge of the FoV of both main instruments, IBIS and SPI. Nevertheless, it was possible to determine its position and to extract spectra and fluxes. The GRB nature of the event was confirmed by an IPN triangulation. It is a ~ 60 s long GRB with two prominent peaks separated by ~ 35 s. The spectral shape of the GRB is best represented by a single power law with a photon index Gamma ~ 1.7. The peak flux in the 20 - 200 keV band is determined to ~ 5.7 photons cm-2 s-1 and the GRB fluence to 1.1 x 10-5 erg cm-2. Analysing the spectral evolution of the GRB, a ``hard-to-soft'' behaviour emerges. A search for an optical counterpart has been carried out, but none was found.

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arXiv2003

Unveiling the Nature of the 321 s Modulation in RX J0806.3+1527: Near-Simultaneous Chandra and VLT Observations

We report on the first near-simultaneous X-ray and optical observations of RX J0806.3+1527. The source is believed to be a 321s orbital period ultra-compact binary system hosting an X-ray emitting white dwarf. Data were obtained with Chandra and the ESO Very Large Telescope (VLT) in November 2001. We found an optical/X-ray phase-shift in the periodic modulation of about 0.5, strongly favoring the existance of two distinct emission regions in the two bands (for the pulsed fluxes). The Chandra data allow us to study, for the first time, the spectral continuum of RX J0806.3+1527 in soft X-rays. This was well fitted by a blackbody spectrum with kT about 65eV and hydrogen column density of N_H about 5 x 10^20 cm^-2. The average (unabsorbed) source 0.1-2.5 keV luminosity during the modulation-on is L_X about 5 x 10^32 erg s^-1 (assuming a distance of 500pc). Such a value is lower than the luminosity expected if stable mass transfer between two white dwarfs were driven by gravitational radiation. Evidence for absorption-like features are present in the phase-averaged spectrum at about 0.53, 0.64, and 1.26 keV, which may be attributed to heavy elements (C and N). We compare and discuss these findings with other binary systems hosting an accreting white dwarf.

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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

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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