arXiv subjects

J. Gorosabel

Publications and source records attributed to J. Gorosabel.

At least 127 records · Page 7Linked to original sources

arXiv2006

Evidence for a Supernova Associated with the X-ray Flash 020903

We present ground-based and Hubble Space Telescope optical observations of the X-ray flash (XRF) 020903, covering 300 days. The afterglow showed a very rapid rise in the first day, followed by a relatively slow decay in the next few days. There was a clear bump in the light curve after ~25 days, accompanied by a drastic change in the spectral energy distribution. The light curve and the spectral energy distribution are naturally interpreted as the emergence -- and subsequent decay -- of a supernova (SN), similar to SN 1998bw. At peak luminosity, the SN is estimated to be 0.8 +/- 0.1 mag fainter than SN1998bw. This argues in favor of the existence of a supernova associated with this X-ray flash. A spectrum obtained 35 days after the burst shows emission lines from the host galaxy. We use this spectrum to put an upper limit on the oxygen abundance of the host at [O/H] < -0.6 dex. We also discuss a possible trend between the softness of several bursts and the early behavior of the optical afterglow, in the sense that XRFs and X-ray rich GRBs seem to have a plateau phase or even a rising light curve. This can be naturally explained in models where XRFs are similar to GRBs but seen off the jet axis.

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arXiv2006

The short-duration GRB 050724 host galaxy in the context of the long-duration GRB hosts

We report optical and near-infrared broad band observations of the short-duration GRB 050724 host galaxy, used to construct its spectral energy distribution (SED). Unlike the hosts of long-duration gamma-ray bursts (GRBs), which show younger stellar populations, the SED of the GRB 050724 host galaxy is optimally fitted with a synthetic elliptical galaxy template based on an evolved stellar population (age ~2.6 Gyr). The SED of the host is difficult to reproduce with non-evolving metallicity templates. In contrast, if the short GRB host galaxy metallicity enrichment is considered, the synthetic templates fit the observed SED satisfactorily. The internal host extinction is low (A_v \~< 0.4 mag) so it cannot explain the faintness of the afterglow. This short GRB host galaxy is more massive (~5x10^10 Mo) and luminous (~1.1 L*) than most of the long-duration GRB hosts. A statistical comparison based on the ages of short- and long-duration GRB host galaxies strongly suggests that short-duration GRB hosts contain, on average, older progenitors. These findings support a different origin for short- and long-duration GRBs.

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arXiv2005

A mean redshift of 2.8 for Swift gamma-ray bursts

The exceptionally high luminosities of gamma-ray bursts (GRBs), gradually emerging as extremely useful probes of star formation, make them promising tools for exploration of the high-redshift Universe. Here we present a carefully selected sample of Swift GRBs, intended to estimate in an unbiased way the GRB mean redshift (z_mean), constraints on the fraction of high-redshift bursts and an upper limit on the fraction of heavily obscured afterglows. We find that z_mean = 2.8 and that at least 7% of GRBs originate at z > 5. In addition, consistent with pre-Swift observations, at most 20% of afterglows can be heavily obscured. The redshift distribution of the sample is qualitatively consistent with models where the GRB rate is proportional to the star formation rate in the Universe. We also report optical, near-infrared and X-ray observations of the afterglow of GRB 050814, which was seen to exhibit very red optical colours. By modelling its spectral energy distribution we find that z = 5.3 +/- 0.3. The high mean redshift of GRBs and their wide redshift range clearly demonstrates their suitability as efficient probes of galaxies and the intergalactic medium over a significant fraction of the history of the Universe.

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arXiv2005

The galaxies in the field of the nearby GRB980425/SN1998bw

We present spectroscopic observations of ESO 184-G82, the host galaxy of GRB980425/SN1998bw, and six galaxies in its field. A host redshift of z=0.0087+/-0.0006 is derived, consistent with that measured by Tinney et al. (1998). Redshifts are obtained for the six surrounding galaxies observed. Three of these galaxies lie within 11 Mpc of each other, confirming the suggestion that some of these galaxies form a group. However, all of the field galaxies observed lie at significantly greater distances than ESO 184-G82 and are therefore not associated with it. The host galaxy of GRB980425/SN1998bw thus appears to be an isolated dwarf galaxy and interactions with other galaxies do not seem to be responsible for its star formation.

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arXiv2005

Multi-wavelength analysis of the field of the dark burst GRB 031220

We have collected and analyzed data taken in different spectral bands (from X-ray to optical and infrared) of the field of GRB031220 and we present results of such multiband observations. Comparison between images taken at different epochs in the same filters did not reveal any strong variable source in the field of this burst. X-ray analysis shows that only two of the seven Chandra sources have a significant flux decrease and seem to be the most likely afterglow candidates. Both sources do not show the typical values of the R-K colour but they appear to be redder. However, only one source has an X-ray decay index (1.3 +/- 0.1) that is typical for observed afterglows. We assume that this source is the best afterglow candidate and we estimate a redshift of 1.90 +/- 0.30. Photometric analysis and redshift estimation for this object suggest that this GRB can be classified as a Dark Burst and that the obscuration is the result of dust extinction in the circum burst medium or inside the host galaxy.

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arXiv2005

Low-resolution VLT spectroscopy of GRBs 991216, 011211 and 021211

We present low-resolution VLT spectroscopy of the afterglow of the gamma-ray bursts (GRBs) 991216, 011211 and 021211. Our spectrum of GRB991216 is the only optical spectrum for this afterglow. It shows two probable absorption systems at z=0.80 and z=1.02, where the highest redshift most likely reflects the distance to the host galaxy. A third system may be detected at z=0.77. HST imaging of the field, obtained 4 months after the burst, has resulted in the detection of two amorphous regions of emission, one at the projected afterglow position, and the other 0.6" away. The spectrum shows a depression in flux in between 4000 A and 5500 A. This could be the result of a 2175 A-type extinction feature in the host of GRB991216, but at a rather red wavelength of 2360 A. If this interpretation is correct, it is the first time the extinction feature is seen in a GRB afterglow spectrum. It is centered at a wavelength similar to that of the ultra-violet (UV) bumps inferred from observations of a few UV-strong, hydrogen-poor stars in the Galaxy. All significant absorption lines (except for one) detected in the spectrum of GRB011211 are identified with lines originating in a single absorption system at z=2.142+/-0.002, the redshift of the GRB011211 host galaxy. We also detect the Lyman alpha absorption line in the host, to which we fit a neutral hydrogen column density of log N(HI)=20.4+/-0.2, which indicates that it is a damped Lyman alpha system. Using a curve-of-growth analysis, we estimate the Si, Fe and Al metallicity at the GRB011211 redshift. For GRB021211, we detect a single emission line in a spectrum obtained tens of days after the burst, which we identify as [OII] 3727 at z=1.006. The corresponding unobscured [OII] star-formation rate is 1.4 Msun/yr.

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arXiv2005

The host galaxy cluster of the short gamma-ray burst GRB 050509B

The first arcsecond localization of a short gamma-ray burst, GRB 050509B, has enabled detailed studies of a short burst environment. We here report on studies of the environment of GRB 050509B using the Swift X-ray Telescope (XRT). The XRT error circle of the burst overlaps with an elliptical galaxy in the cluster of galaxies ZwCl 1234.0+02916. Based on the measured X-ray flux of the cluster we estimate that the probability for a chance superposition of GRB 050509B and a cluster at least as X-ray bright as this cluster is $< 2\times 10^{-3}$, presenting the first strong case of a short burst located in a cluster of galaxies. We also consider the case for GRB 050509B being located behind ZwCl 1234.0+02916 and gravitationally lensed. From the velocity dispersion of the elliptical galaxy and the temperature of hot intracluster gas, we model the mass distribution in the elliptical galaxy and the cluster, and calculate the gravitational lensing magnification within the XRT error circle. We find that, if GRB050509B would be positioned significantly behind the cluster, it is most likely magnified by a factor less than two, but that the burst could be strongly lensed if it is positioned within 2 arcsec of the center of the bright elliptical galaxy. Further mapping of arcsecond size short burst error boxes is a new promising route to determine the spatial distribution of old stars throughout the Universe.

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arXiv2005

Discovery and identification of the very high redshift afterglow of GRB 050904

In 2000, Lamb and Reichart predicted that gamma-ray bursts (GRBs) and their afterglows occur in sufficient numbers and at sufficient brightnesses at very high redshifts (z > 5) to eventually replace quasars as the preferred probe of element formation and reionization in the early universe and to be used to characterize the star-formation history of the early universe, perhaps back to when the first stars formed. Here we report the discovery of the afterglow of GRB 050904 and the identification of GRB 050904 as the first very high redshift GRB. We measure its redshift to be 6.39(+0.11,-0.12), which is consistent with the reported spectroscopic redshift (6.29 +/- 0.01). Furthermore, just redward of Ly-alpha the flux is suppressed by a factor of three on the first night, but returns to expected levels by the fourth night. We propose that this is due to absorption by molecular hydrogen that was excited to rovibrational states by the GRB's prompt emission, but was then overtaken by the jet. Now that very high redshift GRBs have been shown to exist, and at least in this case the afterglow was very bright, observing programs that are designed to capitalize on this science will likely drive a new era of study of the early universe, using GRBs as probes.

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arXiv2005

Multi-Wavelength Studies of the Optically Dark Gamma-Ray Burst 001025A

We identify the fading X-ray afterglow of GRB 001025A from XMM-Newton observations obtained 1.9-2.3 days, 2 years, and 2.5 years after the burst. The non-detection of an optical counterpart to an upper limit of R=25.5, 1.20 days after the burst, makes GRB 001025A a ``dark'' burst. Based on the X-ray afterglow spectral properties of GRB 001025A, we argue that some bursts appear optically dark because their afterglow is faint and their cooling frequency is close to the X-ray band. This interpretation is applicable to several of the few other dark bursts where the X-ray spectral index has been measured. The X-ray afterglow flux of GRB 001025A is an order of magnitude lower than for typical long-duration gamma-ray bursts. The spectrum of the X-ray afterglow can be fitted with an absorbed synchrotron emission model, an absorbed thermal plasma model, or a combination thereof. For the latter, an extrapolation to optical wavelengths can be reconciled with the R-band upper limit on the afterglow, without invoking any optical circumburst absorption, provided the cooling frequency is close to the X-ray band. Alternatively, if the X-ray afterglow is due to synchrotron emission only, seven magnitudes of extinction in the observed R-band is required to meet the R-band upper limit, making GRB 001025A much more obscured than bursts with detected optical afterglows. Based on the column density of X-ray absorbing circumburst matter, an SMC gas-to-dust ratio is insufficient to produce this amount of extinction. The X-ray tail of the prompt emission enters a steep temporal decay excluding that the tail of the prompt emission is the onset of the afterglow (abridged).

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arXiv2005

Spectro-photometric study of the GRB 030329 host galaxy

In this study optical/near-infrared(NIR) broad band photometry and optical spectroscopic observations of the GRB 030329 host galaxy are presented. The Spectral Energy Distribution (SED) of the host is consistent with a starburst galaxy template with a dominant stellar population age of ~150 Myr and an extinction Av ~0.6. Analysis of the spectral emission lines shows that the host is likely a low metallicity galaxy. Two independent diagnostics, based on the restframe UV continuum and the [OII] line flux, provide a consistent unextincted star formation rate of SFR ~0.6 Mo yr^-1. The low absolute magnitude of the host (M_B ~ -16.5) implies a high specific star formation rate value, SSFR = ~34 Mo yr^-1 (L/L*)^-1.

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arXiv2005

Photometric redshift of the GRB 981226 host galaxy

No optical afterglow was found for the dark burst GRB 981226 and hence no absorption redshift has been obtained. We here use ground-based and space imaging observations to analyse the spectral energy distribution (SED) of the host galaxy. By comparison with synthetic template spectra we determine the photometric redshift of the GRB 981226 host to be z_phot = 1.11+/-0.06 (68% confidence level). While the age-metallicity degeneracy for the host SED complicates the determination of accurate ages, metallicity, and extinction, the photometric redshift is robust. The inferred z_phot value is also robust compared to a Bayesian redshift estimator which gives z_phot=0.94+/-0.13. The characteristics for this host are similar to other GRB hosts previously examined. Available low resolution spectra show no emission lines at the expected wavelengths. The photometric redshift estimate indicates an isotropic energy release consistent with the Amati relation for this GRB which had a spectrum characteristic of an X-ray flash.

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arXiv2005

Gamma-Ray Burst Selected High Redshift Galaxies: Comparison to Field Galaxy Populations to z~3

We study the internal structural properties of 37 long duration gamma-ray burst (GRB) host galaxies imaged with the Hubble Space Telescope. Fitting exponential disk and r^1/4 models to the surface brightness profiles of eight z < 1.2 bright host galaxies, we find that the disk model is slightly preferred for most hosts, although two galaxies are fit best with an r^1/4 profile. We furthermore measure the central concentrations and asymmetries of all 37 host galaxies using the CAS system, and compare with field galaxies in the Hubble Deep Field. Our first main conclusion is that GRB hosts exhibit a surprisingly high diversity of galaxy types. A significant fraction (68%) of host galaxies are situated in a region of the concentration-asymmetry diagram occupied by spirals or peculiar/merging galaxies. Twelve hosts (32%) are situated in the region occupied by elliptical galaxies, having high concentration indices. We also find some evidence for evolution in GRB host galaxy morphology, such that hosts at z > 1 have a relatively high light concentration, indicating that these systems are perhaps progenitors of massive galaxies, or are compact blue star forming galaxies. We find that GRB hosts at z > 1 are different from the general field population at z > 1 in terms of light concentration at >99.5% confidence, yet have sizes similar to the general z > 1 population. This is the opposite of the effect at z < 1 where GRB hosts are smaller than average. We argue that GRB hosts trace the starburst population at high redshift, as similarly concentrated galaxies at z > 1 are undergoing a disproportionate amount of star formation for their luminosities. Our results show that GRBs are perhaps ideal tracers of typical galaxies undergoing star formation at any epoch.

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arXiv2005

GRB 050509B: Constraints on short gamma-ray burst models

We have obtained deep optical images with the Very Large Telescope at ESO of the first well-localized short-duration gamma-ray burst, GRB 050509B. From V and R imaging, initiated ~2 days after the GRB trigger and lasting up to three weeks, we detect no variable object inside the small Swift/XRT X-ray error circle down to 2sigma limits of V = 26.5 and R = 25.1. The X-ray error circle includes a giant elliptical galaxy at z = 0.225, which has been proposed as the likely host of this GRB. Our limits indicate that if the GRB originated at z = 0.225, any supernova-like event accompanying the GRB would have to be over 100 times fainter than normal Type Ia SNe or Type Ic hypernovae, 5 times fainter than the faintest known Ia or Ic SNe, and fainter than the faintest known Type II SNe. Moreover, we use the optical limits to constrain the energetics of the GRB outflow. Simple models indicate that, unless the intrinsic energy in the outflow from GRB 050509B was << 10^51 erg, there was very little radioactive material with efficient decay timescales for generating a large luminosity. These limits strongly constrain progenitor models for this short GRB.

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arXiv2005

The GRB 030329 host: a blue low metallicity subluminous galaxy with intense star formation

We present broad band photometry and spectroscopic observations of the host galaxy of GRB 030329. Analysis of the spectral emission lines shows that the host is likely a low metallicity galaxy (Z~0.004). The spectral energy distribution (SED) constructed with the photometric points has been fitted using synthetic and observational templates. The best SED fit is obtained with a starburst template with an age of ~150 Myr and an extinction Av~0.6. We find that the GRB 030329 host galaxy is a subluminous galaxy (L~0.016 L*) with a stellar mass of >~10^8 Mo. Three independent diagnostics, based on the restframe UV continuum, the [OII], and the Balmer emission lines, provide a consistent unextincted star formation rate of ~0.6 Mo yr^-1, implying a high unextincted specific star formation rate (~34 Mo yr^-1 (L/L*)^-1). We estimate that the unextincted specific star formation rate of the GRB 030329 host is higher than ~93.5% of the galaxies at a similar redshift.

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arXiv2005

GRB 050509b: the elusive optical/nIR/mm afterglow of a short-duration GRB

We present multiwavelength (optical/near infrared/millimetre) observations of a short duration gamma-ray burst detected by Swift (GRB 050509b) collected between 0 seconds and ~18.8 days after the event. No optical, near infrared or millimetre emission has been detected in spite of the well localised X-ray afterglow, confirming the elusiveness of the short duration events. We also discuss the possibility of the burst being located in a cluster of galaxies at z= 0.225 or beyond. In the former case, the spectral energy distribution of the neighbouring, potential host galaxy, favours a system harbouring an evolved dominant stellar population (age ~360 Myr), unlike most long duration GRB host galaxies observed so far, i.e. thus giving support to a compact binary merger origin. Any underlying supernova that could be associated with this particular event should have been at least 3 magnitudes fainter than the type Ib/c SN 1998bw and 2.3 magnitudes fainter than a typical type Ia SN.

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arXiv2005

The search for the host galaxy of the gamma-ray burst GRB 000214

We present UBVRI-band observations taken ~300 days after the BeppoSAX gamma-ray burst GRB 000214. This GRB did not show a detectable optical afterglow, however due to the localization of a previously unknown, fading, X-ray source at a tentative redshift in the range 0.37-0.47, we have searched with the ESO 3.6 m telescope for objects with photometric redshifts consistent with the mentioned X-ray redshift range. We report four host galaxy candidates, which might be the subject of future spectroscopic observations in order to confirm their redshifts.

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arXiv2005

GRB 021004 modelled by multiple energy injections

GRB 021004 is one of the best sampled gamma-ray bursts (GRB) to date, although the nature of its light curve is still being debated. Here we present a large amount (107) of new optical, near-infrared (NIR) and millimetre observations, ranging from 2 hours to more than a year after the burst. Fitting the multiband data to a model based on multiple energy injections suggests that at least 7 refreshed shocks took place during the evolution of the afterglow, implying a total energy release (collimated within an angle of 1.8 deg) of ~ 8x10^51 erg. Analysis of the late photometry reveals that the GRB 021004 host is a low extinction (Av ~ 0.1) starburst galaxy with M_B ~ -22.0.

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arXiv2005

Radio, millimeter and optical monitoring of GRB030329 afterglow: Constraining the double jet model

We present radio, millimeter and optical observations of the afterglow of GRB030329. UBVR_{C}I_{C} photometry is presented for a period of 3 hours to 34 days after the burst. Radio monitoring at 1280 MHz has been carried out using the GMRT for more than a year. Simultaneous millimeter observations at 90 GHz and 230 GHz have been obtained from the Swedish-ESO Submillimeter Telescope (SEST) and the IRAM-PdB interferometer over more than a month following the burst. We use these data to constrain the double jet model proposed by Berger et al. (2003) for this afterglow. We also examine whether instead of the two jets being simultaneously present, the wider jet could result from the initially narrow jet, due to a fresh supply of energy from the central engine after the ``jet break''.

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