Aims. We present a multiwavelength study of the Be/X-ray binary system XTE J1946+274 with the main goal of better characterizing its behavior during X-ray quiescence. We also aim to shed light on the ...possible mechanisms which trigger the X-ray activity for this source. Methods. XTE J1946+274 was observed by Chandra-ACIS during quiescence in 2013 March 12. In addition, this source has been monitored from the ground-based astronomical observatories of El Teide (Tenerife, Spain), Roque de los Muchachos (La Palma, Spain) and Sierra Nevada (Granada, Spain) since 2011 September, and from the TÜBİTAK National Observatory (Antalya, Turkey) since 2005 April. We have performed spectral and photometric temporal analyses in order to investigate the quiescent state and transient behavior of this binary system. Results. Our optical study revealed that a long mass ejection event from the Be star took place in 2006, lasting for about seven years, and another one is currently ongoing. We also found that a large Be circumstellar disk is present during quiescence, although major X-ray activity is not observed. We made an attempt to explain this by assuming the permanently presence of a tilted (sometimes warped) Be decretion disk. The 0.3−10 keV X-ray spectrum of the neutron star during quiescence was well fitted with either an absorbed black-body or an absorbed power-law models. The main parameters obtained for these models were kT = 1.43 ± 0.17 and Γ = 0.9 ± 0.4 (with NH ~ 2−7 × 1022 cm-2). The 0.3−10 keV flux of the source was ~0.8−1 × 10-12 erg-1 cm-2 s-1. Pulsations were found with Ppulse = 15.757(1) s (epoch MJD 56 363.115) and an rms pulse fraction of 32.1(3)%. The observed X-ray luminosity during quiescent periods was close to that of expected in supersonic propeller regimen.
We present a multi-wavelength study of GRB 081008, at redshift 1.967, by Swift, ROTSE-III, and Gamma-Ray Burst Optical/NearInfrared Detector. Compared to other Swift GRBs, GRB 081008 has a typical ...gamma-ray isotropic equivalent energy output ({approx}10{sup 53} erg) during the prompt phase, and displayed two temporally separated clusters of pulses. The early X-ray emission seen by the Swift X-Ray Telescope was dominated by the softening tail of the prompt emission, producing multiple flares during and after the Swift Burst Alert Telescope detections. Optical observations that started shortly after the first active phase of gamma-ray emission showed two consecutive peaks. We interpret the first optical peak as the onset of the afterglow associated with the early burst activities. A second optical peak, coincident with the later gamma-ray pulses, imposes a small modification to the otherwise smooth light curve and thus suggests a minimal contribution from a probable internal component. We suggest the early optical variability may be from continuous energy injection into the forward shock front by later shells producing the second epoch of burst activities. These early observations thus provide a potential probe for the transition from the prompt phase to the afterglow phase. The later light curve of GRB 081008 displays a smooth steepening in all optical bands and X-ray. The temporal break is consistent with being achromatic at the observed wavelengths. Our broad energy coverage shortly after the break constrains a spectral break within optical. However, the evolution of the break frequency is not observed. We discuss the plausible interpretations of this behavior.
We present the optical luminosity function (LF) of gamma-ray bursts (GRBs) estimated from a uniform sample of 58 GRBs from observations with the Robotic Optical Transient Search Experiment III ...(ROTSE-III). Our GRB sample is divided into two sub-samples: detected afterglows (18 GRBs) and those with upper limits (40 GRBs). We derive R-band fluxes for these two sub-samples 100 s after the onset of the burst. The optical LFs at 100 s are fitted by assuming that the co-moving GRB rate traces the star formation rate. While fitting the optical LFs using Monte Carlo simulations, we take into account the detection function of ROTSE-III. We find that the cumulative distribution of optical emission at 100 s is well described by an exponential rise and power-law decay, a broken power law, and Schechter LFs. A single power-law (SPL) LF, on the other hand, is ruled out with high confidence.
We have gathered optical photometry data from the literature on a large sample of Swift-era gamma-ray burst (GRB) afterglows including GRBs up to 2009 September, for a total of 76 GRBs, and present ...an additional three pre-Swift GRBs not included in an earlier sample. Furthermore, we publish 840 additional new photometry data points on a total of 42 GRB afterglows, including large data sets for GRBs 050319, 050408, 050802, 050820A, 050922C, 060418, 080413A, and 080810. We analyzed the light curves of all GRBs in the sample and derived spectral energy distributions for the sample with the best data quality, allowing us to estimate the host-galaxy extinction. We transformed the afterglow light curves into an extinction-corrected z = 1 system and compared their luminosities with a sample of pre-Swift afterglows. The results of a former study, which showed that GRB afterglows clustered and exhibited a bimodal distribution in luminosity space, are weakened by the larger sample. We found that the luminosity distribution of the two afterglow samples (Swift-era and pre-Swift) is very similar, and that a subsample for which we were not able to estimate the extinction, which is fainter than the main sample, can be explained by assuming a moderate amount of line-of-sight host extinction. We derived bolometric isotropic energies for all GRBs in our sample, and found only a tentative correlation between the prompt energy release and the optical afterglow luminosity at 1 day after the GRB in the z = 1 system. A comparative study of the optical luminosities of GRB afterglows with echelle spectra (which show a high number of foreground absorbing systems) and those without, reveals no indication that the former are statistically significantly more luminous. Furthermore, we propose the existence of an upper ceiling on afterglow luminosities and study the luminosity distribution at early times, which was not accessible before the advent of the Swift satellite. Most GRBs feature afterglows that are dominated by the forward shock from early times on. Finally, we present the first indications of a class of long GRBs, which form a bridge between the typical high-luminosity, high-redshift events and nearby low-luminosity events (which are also associated with spectroscopic supernovae) in terms of energetics and observed redshift distribution, indicating a continuous distribution overall.
ROTSE IIId (The Robotic Optical Transient Experiment) observations of the X-ray binary system 4U 0115+634/V635 Cas obtained during 2004 June and 2005 January make it possible, for the first time, to ...study the correlation between optical and type II X-ray outbursts. The X-ray outburst sharply enhanced after periastron passage where the optical brightness was reduced by 0.3 mag for a few days. We interpret the sharp reduction of optical brightness as a sign of mass ejection from the outer parts of the disc of the Be star. After this sharp decrease, the optical brightness rose again and reached the pre-X-ray outburst level. Afterwards, a gradual decrease of the optical brightness followed until a minimum then a gradual increase started again. Qualitatively, the change of the optical lightcurve suggests a precession of the Be star disc of around a few hundred days. We also investigate the periodic signatures from the archival RXTE-ASM (Rossi X-ray Timing Explorer – All Sky Monitor) light curve covering a time span of ∼9 years. We find significant orbital modulation in the ASM light curve during the type I X-ray outburst.
We report on a complete set of early optical afterglows of gamma-ray bursts (GRBs) obtained with the Robotic Optical Transient Search Experiment (ROTSE-III) telescope network from 2005 March through ...2007 June. This set is comprised of 12 afterglows with early optical and Swift/X-Ray Telescope observations, with a median ROTSE-III response time of 45 s after the start of Delta *g-ray emission (8 s after the GCN notice time). These afterglows span 4 orders of magnitude in optical luminosity, and the contemporaneous X-ray detections allow multi-wavelength spectral analysis. Excluding X-ray flares, the broadband synchrotron spectra show that the optical and X-ray emission originate in a common region, consistent with predictions of the external forward shock in the fireball model. However, the fireball model is inadequate to predict the temporal decay indices of the early afterglows, even after accounting for possible long-duration continuous energy injection. We find that the optical afterglow is a clean tracer of the forward shock, and we use the peak time of the forward shock to estimate the initial bulk Lorentz factor of the GRB outflow, and find 100 Delta *G0 1000, consistent with expectations.
Aims. The main goal of this study is to investigate the relationship between the optical and X-ray behaviours of the Be/X-ray binary system SAX J2103.5+4545. Methods. We present the relations between ...the Hα equivalent width, optical brightness and X-ray flux of the system, by analysing the optical photometric and spectroscopic observations together with the X-ray observations . Results. The X-ray outburst of the system occurred just after the optical outburst. The nearly symmetric Hα emission line profiles observed during the beginning of the optical outburst turn into asymmetric profiles with increased EW values during the dissipation of the Be disc. A correlation between the Hα emission line strength and the X-ray flux was found. The Hα line indicates the existence of an equatorial disc around the Be star. The Hα line changed from emission to absorption during the observation period. The observed double peaked HeI emission lines might come from the accretion disc of neutron star which is temporarily formed at the time of X-ray outburst.
CCD observations of stars in the young cluster IC 348 were obtained from 2004 August to 2005 January with the 0.45 m ROTSE-IIId robotic reflecting telescope at the Turkish National Observatory site, ...Bakirlitepe, Turkey. The timing analysis of selected stars whose X-ray counterparts were detected by the Chandra X-Ray Observatory were studied. The time series of stars were searched for rotational periodicity by using different period-search methods. Thirty-five stars were found to be periodic with periods ranging from 0.74 to 32.3 days. Eighteen of the 35 periodic stars were new detections. Four of the new detections were classical T Tauri stars (CTTSs), and the others were weak-line T Tauri stars (WTTSs) or G-type (or unknown spectral class) stars. In this study, we confirmed the stability of rotation periods of TTSs. The periods obtained by Cohen et al. and us were different by 1%. We also confirmed the 3.24 hr pulsation period of H254, which is a d Scuti-type star as noted by Ripepi et al., but the other periods detected by them were not found. We examined the correlation between X-ray luminosity and rotation period for our sample of TTSs. There is a decline in the rotation period with X-ray luminosity for late-type TTSs.
Aims.Using the archival RXTE/ASM and SWIFT/BAT observations, the new orbital phases of type I outbursts of EXO 2030+375 are estimated. A possible correlation between the type II outburst and optical ...brightness variations is investigated. Methods.In order to estimate the phases of type I outbursts, we fitted Gaussian profiles to the RXTE/ASM and SWIFT/BAT light curves. The time corresponding to the maximum value of the profiles is treated as the arrival time of type I outburst. We used differential magnitudes in the time-series analysis of the optical light curve. MIDAS and its suitable packages were used to reduce and analyze the spectra. Results.Prior to the type II outburst, orbital phases of type I outbursts were delayed for ~6 days after the periastron passage, which is consistent with findings of Wilson et al. (2002, ApJ, 570, 287; 2005, ApJ, 620, L99). After the giant type II outburst, the phase of type I outbursts underwent a sudden shift of ~13 days after the periastron passage. The amplitudes of type I outbursts were increased between MJD ~ 52 500 and ~53 500. These amplitudes then decreased for 10 orbital cycles until the type II outburst was triggered. If the change of outburst amplitudes correlated with the mass accretion, then during the decrease of these amplitudes mass should be deposited in a disk around neutron star temporarily. The release of this stored mass may ignite the type II outburst. We report that the optical light curve became fainter by 0.4 mag during the decrease of amplitude of the type I outbursts. The observed Hα profiles and their equivalent widths during the decay and after the giant outburst are consistent with previous observations of the system.
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