arXiv subjects

E. Rol

Publications and source records attributed to E. Rol.

At least 91 records · Page 5Linked to original sources

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

The bright optical afterglow of the long GRB 001007

We present optical follow up observations of the long GRB 001007 between 6.14 hours and ~468 days after the event. An unusually bright optical afterglow (OA) was seen to decline following a steep power law decay with index alpha = -2.03 +/- 0.11, possibly indicating a break in the light curve at t - to < 3.5 days, as found in other bursts. Upper limits imposed by the LOTIS alerting system 6.14 hours after the gamma ray event provide tentative (1.2 sigma) evidence for a break in the optical light curve. The spectral index beta of the OA yields -1.24 +/- 0.57. These values may be explained both by several fireball jet models and by the cannonball model. Fireball spherical expansion models are not favoured. Late epoch deep imaging revealed the presence of a complex host galaxy system, composed of at least two objects located 1.2" (1.7 sigma) and 1.9" (2.7 sigma) from the afterglow position.

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arXiv2004

On the constraining observations of the dark GRB 001109 and the properties of a z = 0.398 radio selected starburst galaxy contained in its error box

We present optical and NIR (near infrared) follow up observations of the GRB 001109 from 1 to 300 days after the burst. No transient emission was found at these wavelengths within this GRB's (Gamma Ray Burst) 50" radius BeppoSAX error box. Strong limits (3 sigma) are set with: R >~ 21, 10.2 hr after the GRB; I >~ 23, 11.4 hr after the GRB; H >~ 20.7, 9.9 hr after the GRB; and Ks >~ 20, 9.6 hours after the GRB. We discuss whether the radio source found in the GRB's error box (Taylor et al. 2000) might be related to the afterglow. We also present a multiwavelength study of a reddened starburst galaxy, found coincident with the potential radio and the X ray afterglow. We show that our strong I band upper limit makes of the GRB 001109 the darkest one localised by the BeppoSAX's NFI (Narrow Field Instrument), and it is one of the most constraining upper limits on GRB afterglows to date. Further to it, the implications of these observations in the context of dark GRBs are considered.

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arXiv2004

The submm properties of GRB host galaxies

Long duration gamma-ray bursts (GRBs) accompany the deaths of some massive stars and hence, since massive stars are short lived, are a tracer of star formation activity. Given that GRBs are bright enough to be seen to very high redshifts, and detected even in dusty environments, they should therefore provide a powerful probe of the global star formation history of the universe. The potential of this approach can be investigated via submm photometry of GRB host galaxies. Submm luminosity also correlates with star formation rate, so the distribution of host galaxy submm fluxes should allow us to test the two methods for consistency. Here, we report new JCMT/SCUBA 850 micron measurements for 15 GRB hosts. Combining these data with results from previous studies we construct a sample of 21 hosts with <1.4 mJy errors. We show that the distribution of apparent 850 micron flux densities of this sample is reasonably consistent with model predictions, but there is tentative evidence of a dearth of submm bright (>4 mJy) galaxies. Furthermore, the optical/infrared properties of the submm brightest GRB hosts are not typical of the galaxy population selected in submm surveys, although the sample size is still small. Possible selection effects and physical mechanisms which may explain these discrepancies are discussed.

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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

On the Afterglow of the X-Ray Flash of July 23 2003: Photometric evidence for an off-axis Gamma-Ray Burst with an associated Supernova?

We present optical and near-infrared follow-up observations of the X-Ray Flash (XRF) of July 23 2003. Our observations in the R-band cover the temporal range from 4.2 h to 64 days after the high energy event. We also present the results of multicolor imaging extending to the K-band on three epochs. The lightcurve of the R-band afterglow the first week after the burst is similar to the lightcurve for long duration Gamma-Ray Bursts (GRBs), i.e., a broken power-law with a late time slope of alpha=-2.0 (F_nu propto t^alpha). Furthermore, the spectral energy distribution (SED) has a power-law (F_nu propto nu^beta) shape with slope beta=-1.0. However, the decay slope at t<1 day is shallow, consistent with zero. This is in qualitative agreement with the prediction that XRFs are off-axis classical GRBs. After the first week there is a strong bump in the lightcurve, which peaks at around 16 days. The SED after the peak becomes significantly redder. We discuss the possible interpretations of this bump, and conclude that an underlying supernova is the most likely explanation since no other model appears consistent with the evolution of the SED. Finally, we present deep spectroscopy of the burst both in the afterglow and in the bump phase. A firm upper limit of z=2.3 is placed on the redshift of XRF030723 from the lack of Ly-alpha forest lines in the spectrum of the afterglow. The lack of significant absorption and emission lines in either of the two spectra excludes a spectroscopic redshift determination.

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arXiv2004

The host of GRB 030323 at z=3.372: a very high column density DLA system with a low metallicity

We present photometry and spectroscopy of the afterglow of GRB 030323. VLT spectra of the afterglow show damped Lya (DLA) absorption and low- and high-ionization lines at a redshift z=3.3718+-0.0005. The inferred neutral hydrogen column density, log N(HI)=21.90+-0.07, is larger than any (GRB- or QSO-) DLA HI column density inferred directly from Lya in absorption. From the afterglow photometry, we derive a conservative upper limit to the host-galaxy extinction: A(V)<0.5 mag. The iron abundance is [Fe/H]=-1.47+-0.11, while the metallicity of the gas as measured from sulphur is [S/H]=-1.26+-0.20. We derive an upper limit on the H2 molecular fraction of 2N(H2)/(2N(H2)+N(HI))<~10^-6. In the Lya trough, a Lya emission line is detected, which corresponds to a star-formation rate (not corrected for dust extinction) of roughly 1 Msun per year. All these results are consistent with the host galaxy of GRB 030323 consisting of a low metallicity gas with a low dust content. We detect fine-structure lines of silicon, SiII*, which have never been clearly detected in QSO-DLAs; this suggests that these lines are produced in the vicinity of the GRB explosion site. Under the assumption that these fine-structure levels are populated by particle collisions, we estimate the HI volume density to be n_HI=100-10000 cm^-3. HST/ACS imaging 4 months after the burst shows an extended AB(F606W)=28.0+-0.3 mag object at a distance of 0.14" (1kpc) from the early afterglow location, which presumably is the host galaxy of GRB 030323.

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arXiv2003

GRB 030227: the first multiwavelength afterglow of an INTEGRAL GRB

We present multiwavelength observations of a gamma-ray burst detected by INTEGRAL (GRB 030227) between 5.3 hours and ~1.7 days after the event. Here we report the discovery of a dim optical afterglow (OA) that would not have been detected by many previous searches due to its faintess (R~23). This OA was seen to decline following a power law decay with index Alpha_R= -0.95 +/- 0.16. The spectral index Beta_opt/NIR yielded -1.25 +/- 0.14. These values may be explained by a relativistic expansion of a fireball (with p = 2.0) in the cooling regime. We also find evidence for inverse Compton scattering in X-rays.

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arXiv2003

GRB 011121: A collimated outflow into wind-blown surroundings

We discover a break in the GRB 011121 afterglow light curve after 1.3 days, which implies an initial jet opening angle of about 9 deg. The SED during the first four days is achromatic, and supports the jet origin of this break. The SED during the supernova bump can be best represented by a black body with a temperature of 6000 K. The deduced parameters for the decay slope as well as the spectral index favor a wind scenario, i.e. an outflow into a circum-burst environment shaped by the stellar wind of a massive GRB progenitor. Due to its low redshift of z=0.36, GRB 011121 has been the best example for the GRB-supernova connection until GRB 030329, and provides compelling evidence for a circum-burster wind region expected to exist if the progenitor was a massive star.

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arXiv2003

The blue host galaxy of the red GRB 000418

We report on multi-band (UBVRIZJsKs) observations of the host galaxy of the April 18, 2000 gamma-ray burst. The Spectral Energy Distribution (SED) is analysed by fitting empirical and synthetic spectral templates. We find that: (i) the best SED fit is obtained with a starburst template, (ii) the photometric redshift is consistent with the spectroscopic redshift, (iii) the colours of the host are inconsistent with an old stellar population, and (iv) the global extinction is constrained to be in the range Av=0.12-0.61 mag. The derived global extinction agrees with the one reported for the afterglow (Av=0.4-0.9 mag), suggesting a homogeneous distribution of the interstellar medium (ISM) in the host galaxy. These findings are supplemented by morphological information from Hubble Space Telescope (HST) imaging: the surface brightness profile is smooth, symmetric and compact with no underlying structures (like dust lanes, spiral arms or disks). A natural scenario which accounts of all the above results is a nuclear starburst that harbours a young population of stars from which the GRB originated.

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arXiv2003

Very high column density and small reddening towards GRB 020124 at z = 3.20

We present optical and near-infrared observations of the dim afterglow of GRB 020124, obtained between 2 and 68 hours after the gamma-ray burst. The burst occurred in a very faint (R > 29.5) Damped Ly-alpha Absorber (DLA) at a redshift of z = 3.198 +- 0.004. The derived column density of neutral hydrogen is log(N_H) = 21.7 +- 0.2 and the rest-frame reddening is constrained to be E(B-V) < 0.065, i.e., A_V < 0.20 for standard extinction laws with R_V ~ 3. The resulting dust-to-gas ratio is less than 11 % of that found in the Milky Way, but consistent with the SMC and high-redshift QSO DLAs, indicating a low metallicity and/or a low dust-to-metals ratio in the burst environment. A grey extinction law (large R_V), produced through preferential destruction of small dust grains by the GRB, could increase the derived A_V and dust-to-gas ratio. The dimness of the afterglow is however fully accounted for by the high redshift: If GRB 020124 had been at z = 1 it would have been approximately 1.8 mag brighter--in the range of typical bright afterglows.

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arXiv2003

Variable polarization in the optical afterglow of GRB 021004

We present polarimetric observations of the afterglow of gamma-ray burst (GRB) 021004, obtained with the Nordic Optical Telescope (NOT) and the Very Large Telescope (VLT) between 8 and 17 hours after the burst. Comparison among the observations shows a 45 degree change in the position angle from 9 hours after the burst to 16 hours after the burst, and comparison with published data from later epochs even shows a 90 degree change between 9 and 89 hours after the burst. The degree of linear polarization shows a marginal change, but is also consistent with being constant in time. In the context of currently available models for changes in the polarization of GRBs, a homogeneous jet with an early break time of t_b ~ 1 day provides a good explanation of our data. The break time is a factor 2 to 6 earlier than has been found from the analysis of the optical light curve. The change in the position angle of the polarization rules out a structured jet model for the GRB.

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arXiv2003

Optical and near-infrared observations of the GRB020405 afterglow

(Abridged) We report on observations of the optical and NIR afterglow of GRB020405. Ground-based optical observations started about 1 day after the GRB and spanned a period of ~10 days; archival HST data extended the coverage up to 70 days after the GRB. We report the first detection of the afterglow in NIR bands. The detection of emission lines in the optical spectrum indicates that the GRB is located at z = 0.691. Absorptions are also detected at z = 0.691 and at z = 0.472. The latter system is likely caused by clouds in a galaxy located 2 arcsec southwest of the GRB host. Hence, for the first time, the galaxy responsible for an intervening absorption system in the spectrum of a GRB afterglow is identified. Optical and NIR photometry indicates that the decay in all bands follows a single power law of index alpha = 1.54. The late-epoch VLT and HST points lie above the extrapolation of this power law, so that a plateau is apparent in the VRIJ light curves at 10-20 days after the GRB. The light curves at epochs later than day ~20 after the GRB are consistent with a power-law decay with index alphaprime = 1.85. We suggest that this deviation can be modeled with a SN having the same temporal profile as SN2002ap, but 1.3 mag brighter at peak, and located at the GRB redshift. Alternatively, a shock re-energization may be responsible for the rebrightening. A polarimetric R-band measurement shows that the afterglow is polarized, with P = 1.5 % and theta = 172 degrees. Optical-NIR spectral flux distributions show a change of slope across the J band which we interpret as due to the presence of nu_c. The analysis of the multiwavelength spectrum within the fireball model suggests that a population of relativistic electrons produces the optical-NIR emission via synchrotron in an adiabatically expanding blastwave, and the X-rays via IC.

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arXiv2003

The optical/near-IR spectral energy distribution of the GRB 000210 host galaxy

We report on UBVRIZJsHKs-band photometry of the dark GRB 000210 host galaxy. Fitting a grid of spectral templates to its Spectral Energy Distribution (SED), we derived a photometric redshift (z=0.842\+0.0540.042) which is in excellent agreement with the spectroscopic one (z=0.8463+/-0.0002; Piro et al. 2002). The best fit to the SED is obtained with a blue starburst template with an age of 0.181\+0.0370.026 Gyr. We discuss the implications of the inferred low value of Av and the age of the dominant stellar population for the non detection of the GRB 000210 optical afterglow.

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arXiv2002

A multi-colour study of the dark GRB 000210 host galaxy and its environment

We present UBVRIZJsHKs broad band photometry of the host galaxy of the dark gamma-ray burst (GRB) of February 10, 2000. These observations represent the most exhaustive photometry given to date of any GRB host galaxy. A grid of spectral templates have been fitted to the Spectral Energy Distribution (SED) of the host. The derived photometric redshift is z=0.842^+0.054_-0.042, which is in excellent agreement with the spectroscopic redshift (z=0.8463+/-0.0002) proposed by Piro et al. (2002) based on a single emission line. Furthermore, we have determined the photometric redshift of all the galaxies in an area of 6'x6' around the host galaxy, in order to check for their overdensity in the environment of the host. We find that the GRB 000210 host galaxy is a subluminous galaxy (L ~ 0.5+/-0.2 L*), with no companions above our detection threshold of 0.18+/-0.06 L*. Based on the restframe ultraviolet flux a star formation rate of 2.1+/-0.2 Solar Masses per year is estimated. The best fit to the SED is obtained for a starburst template with an age of 0.181^+0.037_-0.026 Gyr and a very low extinction (Av~0). We discuss the implications of the inferred low value of Av and the age of the dominant stellar population for the non detection of the GRB 000210 optical afterglow.

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arXiv2002

SCUBA observations of the host galaxies of four dark gamma-ray bursts

We present the results of a search for submillimetre-luminous host galaxies of optically dark gamma-ray bursts (GRBs) using the Submillimetre Common-User Bolometer Array (SCUBA) on the James Clerk Maxwell Telescope (JCMT). We made photometry measurements of the 850-micron flux at the location of four `dark bursts', which are those with no detected optical afterglow despite rapid deep searches, and which may therefore be within galaxies containing substantial amounts of dust. We were unable to detect any individual source significantly. Our results are consistent with predictions for the host galaxy population as a whole, rather than for a subset of dusty hosts. This indicates that optically dark GRBs are not especially associated with very submillimetre-luminous galaxies and so cannot be used as reliable indicators of dust-enshrouded massive star-formation activity. Further observations are required to establish the relationship between the wider GRB host galaxy population and SCUBA galaxies.

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arXiv2001

The host galaxy of GRB010222: The strongest damped Lyman-alpha system known

Analysis of the absorption lines in the afterglow spectrum of the gamma-ray burst GRB010222 indicates that its host galaxy (at a redshift of z=1.476) is the strongest damped Lyman-alpha (DLA) system known, having a very low metallicity and modest dust content. This conclusion is based on the detection of the red wing of Lyman-alpha plus a comparison of the equivalent widths of ultraviolet Mg I, Mg II, and Fe II lines with those in other DLAs. The column density of H I, deduced from a fit to the wing of Lyman-alpha, is (5 +/- 2) 10^22 cm^-2. The ratio of the column densities of Zn and Cr lines suggests that the dust content in our line of sight through the galaxy is low. This could be due to either dust destruction by the ultraviolet emission of the afterglow or to an initial dust composition different to that of the diffuse interstellar material, or a combination of both.

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