arXiv subjects

A. J. Castro-Tirado

Publications and source records attributed to A. J. Castro-Tirado.

At least 181 records · Page 10Linked to original sources

arXiv2006

An optical supernova associated with the X-ray flash XRF 060218

Long-duration gamma-ray bursts (GRBs) are associated with type Ic supernovae that are more luminous than average and that eject material at very high velocities. Less-luminous supernovae were not hitherto known to be associated with GRBs, and therefore GRB-supernovae were thought to be rare events. Whether X-ray flashes - analogues of GRBs, but with lower luminosities and fewer gamma-rays - can also be associated with supernovae, and whether they are intrinsically 'weak' events or typical GRBs viewed off the axis of the burst, is unclear. Here we report the optical discovery and follow-up observations of the type Ic supernova SN 2006aj associated with X-ray flash XRF 060218. Supernova 2006aj is intrinsically less luminous than the GRB-supernovae, but more luminous than many supernovae not accompanied by a GRB. The ejecta velocities derived from our spectra are intermediate between these two groups, which is consistent with the weakness of both the GRB output and the supernova radio flux. Our data, combined with radio and X-ray observations, suggest that XRF 060218 is an intrinsically weak and soft event, rather than a classical GRB observed off-axis. This extends the GRB-supernova connection to X-ray flashes and fainter supernovae, implying a common origin. Events such as XRF 060218 are probably more numerous than GRB-supernovae.

astro-ph↗

arXiv2006

Multi-wavelength afterglow observations of the high redshift GRB 050730

GRB 050730 is a long duration high-redshift burst (z=3.967) discovered by Swift. The afterglow shows variability and is well monitored over a wide wavelength range. We present comprehensive temporal and spectral analysis of the afterglow of GRB 050730 including observations from the millimeter to X-rays. We use multi-wavelength afterglow data to understand the temporal and spectral decay properties with superimposed variability of this high redshift burst. Five telescopes were used to study the decaying afterglow of GRB 050730 in the B, V, r', R, i', I, J and K photometric pass bands. A spectral energy distribution was constructed at 2.9 hours post-burst in the K, J, I, R, V and B bands. X-ray data from the satellites Swift and XMM-Newton were used to study the afterglow evolution at higher energies. The early afterglow shows variability at early times and shows a steepening at ~0.1 days (8.6 ks) in the B, V, r', R, i', I, J and K passbands. The early afterglow light curve decayed with alpha_1 = -0.60+/-0.07 and alpha_2 = -1.71+/-0.06 based on R and I band data. A millimeter detection of the afterglow around 3 days after the burst shows an excess in comparison to predictions. The early X-ray light curve observed by Swift is complex and contains flares. At late times the X-ray light curve can be fit by a powerlaw alpha_x = -2.5+/-0.15 which is steeper than the optical light curve. A spectral energy distribution (SED) was constructed at \~2.9 hours after the burst. An electron energy index, p, of ~ 2.3 was calculated using the SED and the photon index from the X-ray afterglow spectra and indicates that the synchrotron cooling frequency nu_c is above observed frequencies.

astro-ph↗

arXiv2006

The variable X-ray light curve of GRB 050713A: the case of refreshed shocks

We present a detailed study of the spectral and temporal properties of the X-ray and optical emission of GRB050713a up to 0.5 day after the main GRB event. The X-ray light curve exhibits large amplitude variations with several rebrightenings superposed on the underlying three-segment broken powerlaw that is often seen in Swift GRBs. Our time-resolved spectral analysis supports the interpretation of a long-lived central engine, with rebrightenings consistent with energy injection in refreshed shocks as slower shells generated in the central engine prompt phase catch up with the afterglow shock at later times. Our sparsely-sampled light curve of the optical afterglow can be fitted with a single power law without large flares. The optical decay index appears flatter than the X-ray one, especially at later times.

astro-ph↗

arXiv2006

GRB 060121: Implications of a Short/Intermediate Duration Gamma-Ray Burst at High Redshift

Since the discovery of the first short-hard gamma-ray burst afterglows in 2005, the handful of observed events have been found to be embedded in nearby (z < 1), bright underlying galaxies. We present multiwavelength observations of the short-duration burst GRB 060121, which is the first observed to clearly outshine its host galaxy (by a factor >10^2). A photometric redshift for this event places the progenitor at a most probable redshift of z = 4.6, with a less probable scenario of z = 1.7. In either case, GRB 060121 could be the farthermost short-duration GRB detected to date and implies an isotropic-equivalent energy release in gamma-rays comparable to that seen in long-duration bursts. We discuss the implications of the released energy on the nature of the progenitor. These results suggest that GRB 060121 may belong to a family of energetic short-duration events, lying at z > 1 and whose optical afterglows would outshine their host galaxies, unlike the first short-duration GRBs observed in 2005. The possibility of GRB 060121 being an intermediate duration burst is also discussed.

astro-ph↗

arXiv2006

Long gamma-ray bursts and core-collapse supernovae have different environments

When massive stars exhaust their fuel they collapse and often produce the extraordinarily bright explosions known as core-collapse supernovae. On occasion, this stellar collapse also powers an even more brilliant relativistic explosion known as a long-duration gamma-ray burst. One would then expect that long gamma-ray bursts and core-collapse supernovae should be found in similar galactic environments. Here we show that this expectation is wrong. We find that the long gamma-ray bursts are far more concentrated on the very brightest regions of their host galaxies than are the core-collapse supernovae. Furthermore, the host galaxies of the long gamma-ray bursts are significantly fainter and more irregular than the hosts of the core-collapse supernovae. Together these results suggest that long-duration gamma-ray bursts are associated with the most massive stars and may be restricted to galaxies of limited chemical evolution. Our results directly imply that long gamma-ray bursts are relatively rare in galaxies such as our own Milky Way.

astro-ph↗

arXiv2006

Revealing the Jet Structure of GRB 030329 with High Resolution Multicolor Photometry

We present multicolor optical observations of the nearby (z=0.1685) GRB 030329 obtained with the same instrumentation over a time period of 6 hours for a total of an unprecedented 475 quasi-simultaneous BVR observations. The achromatic steepening in the optical, which occurs at t~0.7 days, provides evidence for a dynamic transition of the source, and can be most readily explained by models in which the GRB ejecta are collimated into a jet. Since the current state-of-the-art modeling of GRB jets is still flawed with uncertainties, we use these data to critically assess some classes of models that have been proposed in the literature. The data, especially the smooth decline rate seen in the optical afterglow, are consistent with a model in which GRB 030329 was a homogeneous, sharp-edged jet, viewed near its edge interacting with a uniform external medium, or viewed near its symmetry axis with a stratified wind-like external environment. The lack of short timescale fluctuations in the optical afterglow flux down to the 0.5 per cent level puts stringent constraints on possible small scale angular inhomogeneities within the jet or fluctuations in the external density.

astro-ph↗

arXiv2006

Physics of the GRB 030328 afterglow and its environment

We report on the photometric, spectroscopic and polarimetric, monitoring of the optical afterglow of Gamma-Ray Burst (GRB) 030328 detected by HETE-2. Photometry, collected at 7 different telescopes, shows that a smoothly broken powerlaw decay, with indices alpha_1 = 0.76 +/- 0.03, alpha_2 = 1.50 +/- 0.07 and a break at t_b = 0.48 +/- 0.03 days after the GRB, provides the best fit of the optical afterglow decline. This shape is interpreted as due to collimated emission, for which we determine a jet opening angle theta_{jet} of about 3.2 degrees. An achromatic bump starting around 0.2 d after the GRB is possibly marginally detected in the optical light curves. Optical spectroscopy shows the presence of two rest-frame ultraviolet metal absorption systems at z = 1.5216 +/- 0.0006 and at z = 1.295 +/- 0.001, the former likely associated with the GRB host galaxy. Analysis of the absorption lines at z = 1.5216 suggests that the host of this GRB may be a Damped Lyman-alpha Absorber. The optical V-band afterglow appears polarized, with P= (2.4 +/- 0.6) % and theta = (170 +/- 7) degrees, suggesting an asymmetric blastwave expansion. An X-ray-to-optical spectral flux distribution of the GRB 030328 afterglow was obtained at 0.78 days after the GRB and fitted using a broken powerlaw, with an optical spectral slope beta_{opt} = 0.47 +/- 0.15, and an X-ray slope beta_{X} = 1.0 +/- 0.2. The discussion of these results in the context of the "fireball model" shows that the preferred scenario for this afterglow is collimated structured jet with fixed opening angle in a homogeneous medium.

astro-ph↗

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.

astro-ph↗

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.

astro-ph↗

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.

astro-ph↗

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.

astro-ph↗

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.

astro-ph↗

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.

astro-ph↗

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.

astro-ph↗

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''.

astro-ph↗

arXiv2005

The earliest spectroscopy of the GRB 030329 afterglow with 6-m telescope

The earliest BTA (SAO RAS 6-m telescope) spectroscopic observations of the GRB 030329 optical transient (OT) are presented, which almost coincide in time with the "first break" ($t\sim 0.5$ day after the GRB) of the OT light curve. The beginning of spectral changes are seen as early as $\sim 10-12$ hours after the GRB. So, the onset of the spectral changes for $t<1$ day indicates that the contribution from Type Ic supernova (SN) into the OT optical flux can be detected earlier. The properties of early spectra of GRB 030329/SN 2003dh can be consistent with a shock moving into a stellar wind formed from the pre-SN. Such a behavior (similar to that near the UV shock breakout in SNe) can be explained by the existence of a dense matter in the immediate surroundings of massive stellar GRB/SN progenitor). The urgency is emphasized of observation of early GRB/SN spectra for solving a question that is essential for understanding GRB physical mechanism: {\it Do all} long-duration gamma-ray bursts are caused by (or physically connected to) {\it ordinary} core-collapse supernovae? If clear association of normal/ordinary core-collapse SNe (SN Ib/c, and others SN types) and GRBs would be revealed in numbers of cases, we may have strong observational limits for gamma-ray beaming and for real energetics of the GRB sources.

astro-ph↗

arXiv2005

Late-epoch optical and near-infrared observations of the GRB000911 afterglow and its host galaxy

We present the results of an optical and near-infrared (NIR) monitoring campaign of the counterpart of Gamma-Ray Burst (GRB) 000911, located at redshift z=1.06, from 5 days to more than 13 months after explosion. Our extensive dataset is a factor of 2 larger and spans a time interval about 4 times longer than the ones considered previously for this GRB afterglow; this allows a more thorough analysis of its light curve and of the GRB host galaxy properties. The afterglow light curves show a single power-law temporal decline, modified at late times by light from a host galaxy with moderate intrinsic extinction, and possibly by an emerging supernova (SN). The afterglow evolution is interpreted within the classical "fireball" scenario as a weakly collimated adiabatic shock propagating in the interstellar medium. The presence of a SN light curve superimposed on the non-thermal afterglow emission is investigated: while in the optical bands no significant contribution to the total light is found from a SN, the NIR J-band data show an excess which is consistent with a SN as bright as the known hypernova SN1998bw. If the SN interpretation is true, this would be the farthest GRB-associated SN, as well as the farthest core-collapse SN, discovered to date. However, other possible explanations of this NIR excess are also investigated. Finally, we studied the photometric properties of the host, and found that it is likely to be a slightly reddened, subluminous, extreme starburst compact galaxy, with luminosity about 0.1 L*, an age of about 0.5 Gyr and a specific Star Formation Rate (SFR) of approximately 30 Msol yr-1 (L/L*)-1. This is the highest specific SFR value for a GRB host inferred from optical/NIR data.

astro-ph↗

arXiv2005

The GRB 030328 host: another case of a blue starburst galaxy

We present for the first time the detection of the GRB 030328 host galaxy in four optical bands equivalent to UBRI. The host galaxy spectral energy distribution is consistent with a low extinction (E(B-V) < 0.21) starburst galaxy. The restframe B-band magnitude of the host is M_B ~ -20.4

astro-ph↗