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INITIAL FOLLOW-UP OF OPTICAL TRANSIENTS WITH COLORES USING THE BOOTES NETWORK
摘要： The Burst Observer and Optical Transient Exploring System (BOOTES) is a network of telescopes that allows the continuous monitoring of transient astrophysical sources. It was originally devoted to the study of the optical emission from gamma-ray bursts (GRBs) that occur in the Universe. In this paper we show the initial results obtained using the spectrograph COLORES (mounted on BOOTES-2), when observing optical transients (OTs) of diverse nature.
关键词： stars: variables: general,gamma-ray burst: general,Telescopes
更新于2025-09-11 14:15:04
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The optical/NIR afterglow of GRB 111209A: Complex yet not unprecedented
摘要： Context. Afterglows of gamma-ray bursts (GRBs) are simple in the most basic model, but can show many complex features. The ultra-long duration GRB 111209A, one of the longest GRBs ever detected, also has the best-monitored afterglow in this rare class of GRBs. Aims. We want to address the question whether GRB 111209A was a special event beyond its extreme duration alone, and whether it is a classical GRB or another kind of high-energy transient. The afterglow may yield significant clues. Methods. We present afterglow photometry obtained in seven bands with the GROND imager as well as in further seven bands with the Ultraviolet/Optical Telescope (UVOT) on-board the Neil Gehrels Swift Observatory. The light curve is analysed by multi-band modelling and joint fitting with power-laws and broken power-laws, and we use the contemporaneous GROND data to study the evolution of the spectral energy distribution. We compare the optical afterglow to a large ensemble we have analysed in earlier works, and especially to that of another ultra-long event, GRB 130925A. We furthermore undertake a photometric study of the host galaxy. Results. We find a strong, chromatic rebrightening event at ≈0.8 days after the GRB, during which the spectral slope becomes redder. After this, the light curve decays achromatically, with evidence for a break at about 9 days after the trigger. The afterglow luminosity is found to not be exceptional. We find that a double-jet model is able to explain the chromatic rebrightening. The afterglow features have been detected in other events and are not unique. Conclusions. The duration aside, the GRB prompt emission and afterglow parameters of GRB 111209A are in agreement with the known distributions for these parameters. While the central engine of this event may differ from that of classical GRBs, there are multiple lines of evidence pointing to GRB 111209A resulting from the core-collapse of a massive star with a stripped envelope.
关键词： gamma-ray burst: general,gamma-ray burst: individual: GRB 111209A,gamma-ray burst: individual: GRB 130925A
更新于2025-09-10 09:29:36
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GRB 110530A: PECULIAR BROAD BUMP AND DELAYED PLATEAU IN EARLY OPTICAL AFTERGLOWS
摘要： We report our very early optical observations of GRB 110530A and investigate its jet properties together with its X-ray afterglow data. A peculiar broad onset bump followed by a plateau is observed in its early R band afterglow light curve. The optical data in the other bands and the X-ray data are well consistent with the temporal feature of the R band light curve. Our joint spectral ?ts of the optical and X-ray data show that they are in the same regime, with a photon index of ～1.70. The optical and X-ray afterglow light curves are well ?tted with the standard external shock model by considering a delayed energy injection component. Based on our modeling results, we ?nd that the radiative ef?ciency of the gamma-ray burst jet is ~1% and the magnetization parameter of the afterglow jet is <0.04 with a derived extremely low ?B (the ratio of shock energy to the magnetic ?eld) of . These results indicate that the jet may be matter dominated. A discussion on delayed energy injection from the accretion of the late fall-back material of its pre-supernova star is also presented.
关键词： gamma-ray burst: general
更新于2025-09-10 09:29:36
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Bulk Lorentz factors of gamma-ray bursts
摘要： Knowledge of the bulk Lorentz factor Γ0 of gamma-ray bursts (GRBs) allows us to compute their comoving frame properties shedding light on their physics. Upon collisions with the circumburst matter, the ?reball of a GRB starts to decelerate, producing a peak or a break (depending on the circumburst density pro?le) in the light curve of the afterglow. Considering all bursts with known redshift and with an early coverage of their emission, we ?nd 67 GRBs (including one short event) with a peak in their optical or GeV light curves p . The measure of tp provides the bulk Lorentz factor Γ0 of the ?reball at a time tp. For another 106 GRBs we set an upper limit tUL before deceleration. We show that tp is due to the dynamics of the ?reball deceleration and not to the passage of a characteristic frequency of the synchrotron spectrum across the optical band. Considering the tp of 66 long GRBs and the 85 most constraining upper limits, we estimate Γ0 or a lower limit ΓLL 0 . Using censored data analysis methods, we reconstruct the most likely distribution of tp. All tp are larger than the time Tp,γ when the prompt γ-ray emission peaks, and are much larger than the time Tph when the ?reball becomes transparent, that is, tp > Tp,γ > Tph. The reconstructed distribution of Γ0 has median value ～300 (150) for a uniform (wind) circumburst density pro?le. In the comoving frame, long GRBs have typical isotropic energy, luminosity, and peak energy (cid:104)Eiso(cid:105) = 3(8) × 1050 erg, (cid:104)Liso(cid:105) = 3(15) × 1047 erg s?1, and (cid:104)Epeak(cid:105) = 1(2) keV in the homogeneous (wind) case. We con?rm that the signi?cant correlations between Γ0 and the rest frame isotropic energy (Eiso), luminosity (Liso), and peak energy (Ep) are not due to selection e?ects. When combined, they lead to the observed Ep ? Eiso and Ep ? Liso correlations. Finally, assuming a typical opening angle of 5 degrees, we derive the distribution of the jet baryon loading which is centered around a few 10?6 M(cid:12).
关键词： radiation mechanisms: non-thermal,relativistic processes,gamma-ray burst: general
更新于2025-09-10 09:29:36
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A search for Galactic transients disguised as gamma-ray bursts
摘要： Context. A signi?cant fraction of cosmological gamma-ray bursts (GRBs) are characterised by a fast rise and exponential decay (FRED) temporal structure. This is not a distinctive feature of this class, since it is observed in many Galactic transients and is likely descriptive of a sudden release of energy followed by a di?usion process. Possible evidence has recently been reported for a Galactic contamination in the sample of FRED GRBs discovered with Swift. Aims. We searched for possible Galactic intruders disguised as FRED GRBs in the Swift catalogue up to September 2014. Methods. We selected 181 FRED GRBs (2/3 with unknown redshift) and considered di?erent subsamples. We tested the degree of isotropy through the dipole and quadrupole moment distributions, both with reference to the Galaxy and independent of any coordinate system as well as with the two-point angular autocovariance function. In addition, we searched for possible indicators of a Galactic origin among the spectral and temporal properties of individual GRBs. Results. We found marginal (～3σ) evidence for an excess of FREDs with unknown redshift towards the Galactic plane compared with what is expected for an isotropic distribution corrected for the non-uniform sky exposure. However, when we account for the observational bias against optical follow-up observations of low-Galactic latitude GRBs, the evidence for anisotropy decreases to ～2σ. In addition, we found no statistical evidence for di?erent spectral or temporal properties from the bulk of cosmological GRBs. Conclusions. We found marginal evidence for the presence of a disguised Galactic population among Swift GRBs with unknown redshift. The estimated fraction is f = (19 ± 11)%, with an upper limit of 34% (90% con?dence).
关键词： gamma-ray burst: general,methods: statistical
更新于2025-09-04 15:30:14