We present results of our analysis of the optical emission of the unique source GRB 070610/SWIFT J195509+261406 during its 2007 outburst and discuss the implications of the non-detection of the very ...faint optical counterpart in quiescence. We analyse the statistical properties of the optical brightness and X-ray intensity during the outburst and determined an approximate value of the mean colour index R−I. The optical emission has the form of spikes whose duration decreases with the outburst's progress, and the spectral energy distribution from the X-ray to the optical bands undergoes variations during the spikes. We show that the optical emission can be explained by a synchrotron mechanism while any thermal component is below the detection limit. We discuss the similarities between synchrotron emission of J1955 and the optical afterglows (OAs) of the extragalactic long gamma-ray bursts. We conclude that while the emission mechanisms in J1955 and OAs are quite similar, the configurations of their light-emitting regions largely differ. The synchrotron emission of the spike must be primarily generated near (several times 1010 cm) the central object, and cannot be explained by density enhancements in the interstellar medium. We argue that the activity of a central engine plays a great role in generating the afterglow emission of J1955. We discuss the possibility that J1955 is an ultracompact binary with a very low time-averaged mass transfer rate on to the neutron star. This can fulfil the conditions required for J1955, with the 2007 outburst as the extreme case seen from the very close X-ray binary SAX J1808.4−3658 in 2005.
Aims.We have performed optical polarimetric observations of the SN 2006aj associated to the X-ray flash (XRF) of February 18, 2006, XRF 060218 that provide information on its expansion geometry. ...Methods.The data were acquired in the R-band with the 0.7 m telescope of Crimea, 2.5 m Nordic Optical Telescope and the 2.2 m of Calar Alto. Results.We report the detection of linear polarization between 3 and 39 days after the gamma-ray event ($t-t_0$). This represents the first polarization detection of a Ic supernova (SN) associated to an XRF. Our data exhibit a degree of linear polarization (P) around $P\sim4\%$ at $t-t_0 \sim 3{-}5$ days, followed by a constant polarization phase with $P \sim 1.4\%$ at $13.7 \la t-t_0 \la 39$ days. Our data suggest a decay in P, and more interestingly, show a position angle (θ) rotation of ~$100^{\circ}$ comparing data taken before and after the R-band lightcurve peak. Conclusions.The reported polarization measurements can be explained by the evolution of an asymmetric SN expansion. We discuss on several ingredients that could account for the observed θ rotation.
Context.GRB 050730 is a long duration high-redshift burst ($z=3.967$) that was discovered by Swift. The afterglow shows variability and was well monitored over a wide wavelength range. We present ...comprehensive temporal and spectral analysis of the afterglow of GRB 050730 including observations covering the wavelength range from the millimeter to X-rays. Aims.We use multi-wavelength afterglow data to understand the complex temporal and spectral decay properties of this high redshift burst. Methods.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 h post-burst in the $B, V, R, I, J$ and K bands. X-ray data from the satellites Swift and XMM-Newton were used to study the afterglow evolution at higher energies. Results.The early afterglow shows variability at early times and the slope steepens 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 a powerlaw slope index $\alpha_1 = -0.60\pm0.07$ and subsequently steepened to $\alpha_2 = -1.71\pm0.06$ based on the R and I band data. A millimeter detection of the afterglow around 3 days after the burst shows an excess in comparison to theoretical 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 decay with $\alpha_x = -2.5\pm0.15$ which is steeper than the optical light curve. A spectral energy distribution (SED) was constructed at ~2.9 h 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 implies that the synchrotron cooling frequency $\nu_{\rm c}$ is above the X-ray band.
Aims. Understand the shape and implications of the multiband ligth curve of GRB 050408, an X-ray rich (XRR) burst. Methods. We present a multiband optical light curve, covering the time from the ...onset of the \gamma-ray event to several months after, when we only detect the host galaxy. Together with X-ray, millimetre and radio observations we compile what, to our knowledge, is the most complete multiband coverage of an XRR burst afterglow to date. Results. The optical and X-ray light curve is characterised by an early flattening and an intense bump peaking around 6 days after the burst onset. We explain the former by an off-axis viewed jet, in agreement with the predictions made for XRR by some models, and the latter with an energy injection equivalent in intensity to the initial shock. The analysis of the spectral flux distribution reveals an extinction compatible with a low chemical enrichment surrounding the burst. Together with the detection of an underlying starburst host galaxy we can strengthen the link between XRR and classical long-duration bursts.
We describe the evolution of of Swift J1644+57, whose unique X-ray properties have led several groups to interpret its behavior as corresponding to an extraordinary event of tidal disruption of a ...star by a supermassive black hole in the nucleus of a (z = 0.3545) galaxy, as derived by GTC. Multiwavelength observations (X-rays, optical, millimetre, centimitre) are proving to be essential to reveal the long term nature of the emission in this source. In particular, we identify for the first time the properties of a forming relativistic jet. In our interpretation of the phenomenon, we leave the still open possibility that it may correspond to the onset of a dormant AGN, but this may only be tested with longer term X-ray, millimetre and centimetre monitoring.
We present multicolor optical observations of the nearby (z = 0.1685) gamma-ray burst 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 6 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 fraught 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% level puts stringent constraints on possible small-scale angular inhomogeneities within the jet or fluctuations in the external density.
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 ($A_{\rm v} \lesssim 0.4$ mag) so it cannot explain the faintness of the afterglow. This short GRB host galaxy is more massive (${\sim}5\times10^{10}\, M_{\odot}$) and luminous (${\sim}1.1~ L^{\star}$) 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.
Aims. We present multiwavelength observations of one of the most distant gamma-ray bursts detected so far, GRB 080913. Based on these observations, we consider whether it could be classified as a ...short-duration GRB and discuss the implications for the progenitor nature and energy extraction mechanisms. Methods. Multiwavelength X-ray, near IR and millimetre observations were made between 20.7 h and ~16.8 days after the event. Results. Whereas a very faint afterglow was seen at the 3.5m CAHA telescope in the nIR, the X-ray afterglow was clearly detected in both Swift and XMM-Newton observations. An upper limit is reported in the mm range. We have modeled the data assuming a collimated $\theta_0$ ≳ 3° blast wave with an energy injection at $\sim0.5$ days carrying $5\sim 10^{52}$ erg or approximately 12 times the initial energy of the blast wave. We find that GRB 080913 shares many of the gamma-ray diagnostics with the more recent burst GRB 090423 for being classified as short had they ocurred at low redshift. If the progenitor were a compact binary merger, it is likely composed by a NS and BH. The Blandford-Znajek (BZ) mechanism is the preferred one to extract energy from the central, maximally-rotating BH. Both the magnetic field close to the event horizon (B) and the BH mass ($M_{\rm bh}$) are restricted within a relatively narrow range, such that $(B / 3\times 10^{16}\,{\rm G}) (M_{\rm bh} / 7\,M_\odot ) \sim 1$. Similar constraints on the central BH hold for collapsar-like progenitor systems if the BZ-mechanism works for the system at hand.
Astronomical Hosting in Central Asia Pozanenko, A.; Volnova, A.; Guziy, S. ...
EAS Publications Series,
2013, 2013-00-00, 20130101, Letnik:
61
Journal Article
Networked projects e.g. Gamma-Ray Burst follow up optical observations require dense worldwide coverage. We are investigating potentially interesting sites for observatories in Central Asia where ...coverage by observatories is still poor. One of the most important parameter of a site is a number of clear night hours. We present first results of direct parameter measurements gathered with weather stations and our own observations in different sites of Mongolia.
Broad-band optical observations of the extraordinarily bright optical afterglow of the intense gamma-ray burst GRB 991208 started ~2.1 days after the event and continued until 4 Apr. 2000. The flux ...decay constant of the optical afterglow in the R-band is -2.30 ± 0.07 up to ~5 days, which is very likely due to the jet e ffect, and it is followed by a much steeper decay with constant -3.2 ± 0.2, the fastest one ever seen in a GRB optical afterglow. A negative detection in several all-sky films taken simultaneously with the event, that otherwise would have reached naked eye brightness, implies either a previous additional break prior to ~2 days after the occurrence of the GRB (as expected from the jet effect) or a maximum, as observed in GRB 970508. The existence of a se cond break might indicate a steepening in the electron spectrum or the superposition of two events, resembling GRB 000301C. Once the afterglow emission vanished, contribution of a bright underlying supernova was found on the basis of the late-time R-band measurements, but the light curve is not sufficiently well sampled to rule out a dust echo explanation. Our redshift determination of $z = 0.706$ indicates that GRB 991208 is at 3.7 Gpc (for $H_{0}= 60$ km s-1 Mpc-1, $\Omega_{0}= 1$ and $\Lambda_{0}= 0$), implying an isotropic energy release of 1.15 1053 erg which may be relaxed by beaming by a factor >102. Precise astrometry indicates that the GRB coincides within 0.2″ with the host galaxy, thus supporting a massive star origin. The absolute magnitude of the galaxy is MB = -18.2, well below the knee of the galaxy luminosity function and we derive a star-forming rate of ($11.5 \pm 7.1$) $M_{\odot}$ yr-1, which is much larger than the present-day rate in our Galaxy. The quasi-simultaneous broad-band photometric spectral energy distribution of the afterglow was determined ~3.5 day after the burst (Dec. 12.0) implying a cooling frequency $\nu_{\rm c}$ below the optical band, i.e. supporting a jet model with $p = -$2.30 as the index of the power-law electron distribution.
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