Umit Kiziloglu

The Inclination Angle and Mass of the Black Hole in XTE J1118+480

We have obtained optical and infrared photometry of the quiescent soft X-ray transient XTE J1118+480. In addition to optical and J-band variations, we present the first observed H- and Ks-band ellipsoidal variations for this system. We model the variations in all bands simultaneously with the WD98 light curve modeling code. The infrared colors of the secondary star in this system are consistent with a spectral type of K7 V, while there is evidence for light from the accretion disk in the optical. Combining the models with the observed spectral energy distribution of the system, the most likely value for the orbital inclination angle is 68° ± 2°. This inclination angle corresponds to a primary black hole mass of 8.53 ± 0.60 M☉. Based on the derived physical parameters and infrared colors of the system, we determine a distance of 1.72 ± 0.10 kpc to XTE J1118+480.

Looking Into the Fireball: ROTSE-III and Swift Observations of Early GRB Afterglows

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 γ-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 ≲ Γ_0 ≲ 1000, consistent with expectations.

Detection of GRB 060927 at z = 5.47: Implications for the Use of Gamma-Ray Bursts as Probes of the End of the Dark Ages

We report on follow-up observations of the gamma-ray burst GRB 060927 using the robotic ROTSE-IIIa telescope and a suite of larger aperture ground-based telescopes. An optical afterglow was detected 20 s after the burst, the earliest rest-frame detection of optical emission from any GRB. Spectroscopy performed with the VLT about 13 hr after the trigger shows a continuum break at lambda~8070 A, produced by neutral hydrogen absorption at z~5.6. We also detect an absorption line at 8158 A, which we interpret as Si II lambda1260 at z=5.467. Hence, GRB 060927 is the second most distant GRB with a spectroscopically measured redshift. The shape of the red wing of the spectral break can be fitted by a damped Lyalpha profile with a column density with log(NH/cm-2)=22.50+/-0.15. We discuss the implications of this work for the use of GRBs as probes of the end of the dark ages and draw three main conclusions: (1) GRB afterglows originating from z>~6 should be relatively easy to detect from the ground, but rapid near-infrared monitoring is necessary to ensure that they are found; (2) the presence of large H I column densities in some GRB host galaxies at z>5 makes the use of GRBs to probe the reionization epoch via spectroscopy of the red damping wing challenging; and (3) GRBs appear crucial to locate typical star-forming galaxies at z>5, and therefore the type of galaxies responsible for the reionization of the universe. Partly based on observations carried out with the ESO telescopes under programs 077.D-0661, 077.A-0667, 078.D-0416, and the large program 177.A-f0591.

Exploring Broadband GRB Behavior during γ-Ray Emission

S. A. Yost, H. F. Swan, E. S. Rykoff, F. Aharonian, C. W. Akerlof, A. Alday, M. C. B. Ashley, S. Barthelmy, D. Burrows, D. L. Depoy, R. J. Dufour, J. D. Eastman, R. D. Forgey, N. Gehrels, E. Gogus, T. Guver, J. P. Halpern, L. C. Hardin, D. Horns, U. Kizilolu, H. A. Krimm, S. Lepine, E. P. Liang, J. L. Marshall, T. A. McKay, T. Mineo, N. Mirabal, M. Ozel, A. Phillips, J. L. Prieto, R. M. Quimby, P. Romano, G. Rowell, W. Rujopakarn, B. E. Schaefer, J. M. Silverman, R. Siverd, M. Skinner, D. A. Smith, I. A. Smith, S. Tonnesen, E. Troja, W. T. Vestrand, J. C. Wheeler, J. Wren, F. Yuan, and B. Zhang

The Anomalous Early Afterglow of GRB 050801

The ROTSE-IIIc telescope at the H.E.S.S. site, Namibia, obtained the earliest detection of optical emission from a Gamma-Ray Burst (GRB), beginning only 21.8 s from the onset of Swift GRB 050801. The optical lightcurve does not fade or brighten significantly over the first � 250 s, after which there is an achromatic break and the lightcurve declines in typical power-law fashion. The Swift/XRT also obtained early observations starting at 69 s after the burst onset. The X-ray lightcurve shows the same features as the optical lightcurve. These correlated variations in the early optical and X-ray emission imply a common origin in space and time. This behavior is difficult to reconcile with the standard models of early afterglow emission. Subject headings: gamma rays:bursts

The dark side of ROTSE-III prompt GRB observations

Wepresent severalcasesofopticalobservationsduring � -raybursts(GRBs) whichresulted in promptlimitsbutno detection of optical emission. These limits constrain the prompt optical flux densities and the optical brightness relative to the � -ray emission. The derived constraints fall within the range of properties observed in GRBs with prompt optical detections, although at the faint end of optical/� -rayflux ratios. The currently accessible prompt optical limits do not require a different set of intrinsic or environmental GRB properties, relative to the events with prompt optical detections. Subject headingg gamma rays: bursts Online material: color figure

Extensive multiband study of the X-ray rich GRB 050408: a likely off-axis event with an intense energy injection

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 γ-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.

ROTSE Observations of the Young Cluster IC 348

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 δ 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.

Optical and X-ray outbursts of Be/X-ray binary system SAX J2103.5+4545

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.

The first hours of the optical afterglow from the cosmic gamma-ray burst 030329

We describe the first results of our observations of the exceptionally bright optical afterglow from the cosmic gamma-ray burst (GRB) of March 29, 2003 (030329), with the 1.5-m Russian-Turkish telescope (RTT150) installed at the TUBITAK National Observatory (Turkey) at Mount Bakyrlytepe. RTT150 was one of the first medium-class telescopes pointed at the afterglow. The observations began as early as about six hours after the GRB. During the first five hours of our observations, the BV RI flux fell off exactly as a power law with the same slope −1.19±0.01. Subsequently, in all of the BV RI bands, we observed the same increase in the power-law slope of the light curve to a value that was later recorded during the observations at observatories in the western hemisphere. The break in the power-law light curve occurs at t − t0 ≈ 0.57 days (13.5 h) and lasts for about 0.2 days. Apart from this smooth decrease in the flux, the afterglow exhibited no flux variability. The upper limits on the variability are 10–1% on time scales of 0.1–1000 s, respectively. The BV RI spectral flux distribution during the first night of our observations closely corresponds to a power-law spectrum with a spectral index α=0.66±0.01. The change in the power-law slope of the light curve at the end of our observations is probably attributable to the deceleration of the ultrarelativistic jet to a gamma factor when its structural features begin to show up in the light curve. The radio, optical, and X-ray broadband spectrum is consistent with the assumption about the synchrotron radiation of the ultrarelativistic jet. This unique object continues to be observed with RTT150.We present the first results of the observations of the extrem ely bright optical afterglow of gamma-ray burst (GRB) 030329 with the 1.5m Russian-Turkish telescope RTT150 (T ¨ UBITAK National Observatory, Bakyrlytepe, Turkey). RTT150 was one of the first 1.5m-class telescopes pointed to the aftergl ow. Observations were started approximately 6 hours after the burst. During the first 5 hours of our observations the afterg low faded exactly as a power law with index −1.19 ± 0.01 in each of the BVRI Bessel filters. After that, in all BVRI filters simultaneously we observe a steepening of the power law light curve. The power law decay index smoothly approaches the value ≈ −1.9, observed by other observatories later. This power law break occurs at t − t0 ≈ 0.57 days and lasts for ≈ ±0.1 days. We observe no variability above the gradual fading with the upper limits 10‐1% on time scales 0.1‐1000 s. Spectral flux distribution in four BVRI filters corr esponds to the power law spectrum with spectral index α = 0.66 ± 0.01. The change of the power law decay index in the end of our observations can be interpreted as a signature of coll imated ultrarelativistic jet. The afterglow flux distribut ion in radio, optical and x-rays is consistent with synchrotron sp ectrum. We continue our observations of this unique object with RTT150.