Mansur A. Ibrahimov

THE AFTERGLOWS OF SWIFT-ERA GAMMA-RAY BURSTS. I. COMPARING PRE-SWIFT AND SWIFT-ERA LONG/SOFT (TYPE II) GRB OPTICAL AFTERGLOWS

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.

Magnetospheric accretion-ejection processes in the classical T Tauri star AA Tauri

Context. Accretion and ejection are complex and related processes that vary on various timescales in young stars. Aims. We intend to investigate the accretion and outflow dynamics and their interaction from observations of the classical T Tauri star AA Tau. Methods. From a long time series of high resolution (R = 115 000) HARPS spectra and simultaneous broad-band photometry, we report new evidence for magnetospheric accretion as well as ejection processes in the nearly edge-on classical T Tauri star AA Tau. Results. AA Tau’s light curve is modulated with a period of 8.22 d. The recurrent luminosity dips are due to the periodic occultation of the central star by the magnetically-warped inner disk edge located at about 9 R� . Balmer line profiles exhibit a clear rotational modulation of high-velocity redshifted absorption components with a period of 8.22 days as well, with a maximum strength when the main accretion funnel flow passes through the line of sight. At the same time, the luminosity of the system decreases by about 1 mag, indicative of circumstellar absorption of the stellar photosphere by the magnetically-warped, corotating inner disk edge. The photospheric and He I radial velocities also exhibit periodic variations, and the veiling is modulated by the appearance of the accretion shock at the bottom of the accretion funnel. Diagnostics of hot winds and their temporal behaviour are also presented. Conclusions. The peculiar geometry of the young AA Tau system (nearly edge-on) allows us to uniquely probe the acretion-ejection region close to the star. We find that most spectral and photometric diagnostics vary as expected from models of magneticallychannelled accretion in young stars, with a large scale magnetosphere tilted by 20 ◦ onto the star’s spin axis. We also find evidence for time variability of the magnetospheric accretion flow on a timescale of a few rotational periods.

Disk-Jet Connection in the Radio Galaxy 3C 120

We present the results of extensive multi-frequency monitoring of the radio galaxy 3C 120 between 2002 and 2007 at X-ray (2-10 keV), optical (R and V bands), and radio (14.5 and 37 GHz) wave bands, as well as imaging with the Very Long Baseline Array (VLBA) at 43 GHz. Over the 5 yr of observation, significant dips in the X-ray light curve are followed by ejections of bright superluminal knots in the VLBA images. Consistent with this, the X-ray flux and 37 GHz flux are anti-correlated with X-ray leading the radio variations. Furthermore, the total radiative output of a radio flare is related to the equivalent width of the corresponding X-ray dip. This implies that, in this radio galaxy, the radiative state of accretion disk plus corona system, where the X-rays are produced, has a direct effect on the events in the jet, where the radio emission originates. The X-ray power spectral density of 3C 120 shows a break, with steeper slope at shorter timescale and the break timescale is commensurate with the mass of the central black hole (BH) based on observations of Seyfert galaxies and black hole X-ray binaries (BHXRBs). These findings provide support for the paradigm that BHXRBs and both radio-loud and radio-quiet active galactic nuclei are fundamentally similar systems, with characteristic time and size scales linearly proportional to the mass of the central BH. The X-ray and optical variations are strongly correlated in 3C 120, which implies that the optical emission in this object arises from the same general region as the X-rays, i.e., in the accretion disk-corona system. We numerically model multi-wavelength light curves of 3C 120 from such a system with the optical-UV emission produced in the disk and the X-rays generated by scattering of thermal photons by hot electrons in the corona. From the comparison of the temporal properties of the model light curves to that of the observed variability, we constrain the physical size of the corona and the distances of the emitting regions from the central BH. In addition, we discuss physical scenarios for the disk-jet connection that are consistent with our observations.

Multiwavelength Analysis of the Intriguing GRB 061126: The Reverse Shock Scenario and Magnetization

We present a detailed study of the prompt and afterglow emission from Swift GRB 061126 using BAT, XRT, UVOT data and multicolor optical imaging from 10 ground-based telescopes. GRB 061126 was a long burst (T90 = 191 s) with four overlapping peaks in its γ-ray light curve. The X-ray afterglow, observed from 26 minutes to 20 days after the burst, shows a simple power-law decay with αX = 1.290 ± 0.008. Optical observations presented here cover the time range from 258 s (Faulkes Telescope North) to 15 days (Gemini North) after the burst; the decay rate of the optical afterglow shows a steep-to-shallow transition (from α1 = 1.48 ± 0.06 to α2 = 0.88 ± 0.03) approximately 13 minutes after the burst. We suggest the early, steep component is due to a reverse shock and show that the magnetic energy density in the ejecta, expressed as a fraction of the equipartition value, is a few 10 times larger than in the forward shock in the early afterglow phase. The ejecta might be endowed with primordial magnetic fields at the central engine. The optical light curve implies a late-time break at about 1.5 days after the burst, while there is no evidence of the simultaneous break in the X-ray light curve. We model the broadband emission and show that some afterglow characteristics (the steeper decay in X-ray and the shallow spectral index from optical to X-ray) are difficult to explain in the framework of the standard fireball model. This might imply that the X-ray afterglow is due to an additional emission process, such as late-time central engine activity rather than blast-wave shock emission. The possible chromatic break at 1.5 days after the burst would give support to the additional emission scenario.

A tale of two GRB-SNe at a common redshift of z=0.54

We present ground-based and Hubble Space Telescope optical observations of the optical transients (OTs) of long-duration Gamma Ray Bursts (GRBs) 060729 and 090618, both at a redshift of z= 0.54. For GRB 060729, bumps are seen in the optical light curves (LCs), and the late-time broad-band spectral energy distributions (SEDs) of the OT resemble those of local Type Ic supernovae (SNe). For GRB 090618, the dense sampling of our optical observations has allowed us to detect well-defined bumps in the optical LCs, as well as a change in colour, that are indicative of light coming from a core-collapse SN. The accompanying SNe for both events are individually compared with SN1998bw, a known GRB supernova, and SN1994I, a typical Type Ic supernova without a known GRB counterpart, and in both cases the brightness and temporal evolution more closely resemble SN1998bw. We also exploit our extensive optical and radio data for GRB 090618, as well as the publicly available Swift-XRT data, and discuss the properties of the afterglow at early times. In the context of a simple jet-like model, the afterglow of GRB 090618 is best explained by the presence of a jet-break at t-to > 0.5 d. We then compare the rest-frame, peak V-band absolute magnitudes of all of the GRB and X-Ray Flash (XRF)-associated SNe with a large sample of local Type Ibc SNe, concluding that, when host extinction is considered, the peak magnitudes of the GRB/XRF-SNe cannot be distinguished from the peak magnitudes of non-GRB/XRF SNe. --------------------------------------------------------------------------------

A tale of two GRB-SNe at a common redshift of z = 0.54

We present ground-based and Hubble Space Telescope optical observations of the optical transients (OTs) of long-duration Gamma Ray Bursts (GRBs) 060729 and 090618, both at a redshift of z= 0.54. For GRB 060729, bumps are seen in the optical light curves (LCs), and the late-time broad-band spectral energy distributions (SEDs) of the OT resemble those of local Type Ic supernovae (SNe). For GRB 090618, the dense sampling of our optical observations has allowed us to detect well-defined bumps in the optical LCs, as well as a change in colour, that are indicative of light coming from a core-collapse SN. The accompanying SNe for both events are individually compared with SN1998bw, a known GRB supernova, and SN1994I, a typical Type Ic supernova without a known GRB counterpart, and in both cases the brightness and temporal evolution more closely resemble SN1998bw. We also exploit our extensive optical and radio data for GRB 090618, as well as the publicly available Swift-XRT data, and discuss the properties of the afterglow at early times. In the context of a simple jet-like model, the afterglow of GRB 090618 is best explained by the presence of a jet-break at t-to > 0.5 d. We then compare the rest-frame, peak V-band absolute magnitudes of all of the GRB and X-Ray Flash (XRF)-associated SNe with a large sample of local Type Ibc SNe, concluding that, when host extinction is considered, the peak magnitudes of the GRB/XRF-SNe cannot be distinguished from the peak magnitudes of non-GRB/XRF SNe. --------------------------------------------------------------------------------

The Initial Mass Function and Young Brown Dwarf Candidates in NGC 2264. III. Photometric Data

We have performed deep wide-field CCD photometry of the young open cluster NGC 2264 to study the extent of star-forming regions (SFRs) and the shape of the initial mass function. In this paper, we present VRI and Hα photometry for more than 67,000 stars. From the spatial distribution of the selected Hα emission stars, we identify two active SFRs and a less active halo region surrounding these two SFRs. There are several Hα emission stars in the field region outside the halo region, and these may be newly formed stars in the Mon OB1 association surrounding the cluster. The locus of pre-main-sequence (PMS) stars in the IC versus V − IC diagram is revised from the distribution of Hα and X-ray emission stars in the diagram. The mean reddening of late-type PMS stars is estimated to be E(B − V) 0.2 mag using the distribution of X-ray emission stars in the 2MASS color-color diagram. We can confirm that the Hα emission stars below the PMS locus (so-called BMS stars) are bona-fide members of NGC 2264 from their spatial distribution as well as from their near-IR excess in the 2MASS color-color diagram. In addition, four objects around IRS-2 detected with the Spitzer IRAC are also classified as BMS stars.

Multi-frequency monitoring of γ-ray loud blazars I. Light curves and spectral energy distributions

Context. Being dominated by non-thermal emission from aligned relativistic jets, blazars allow us to elucidate the physics of extragalactic jets, and, ultimately, how the energy is extracted from the central black hole in radio-loud active galactic nuclei. Aims. Crucial information is provided by broad-band spectral energy distributions (SEDs), their trends with luminosity and correlated multi-frequency variability. With this study we plan to obtain a database of contemporaneous radio-to-optical spectra of a sample of blazars, which are and will be observed by current and future high-energy satellites. Methods. Since December 2004 we are performing a monthly multi-frequency radio monitoring of a sample of 35 blazars at the antennas in Medicina and Noto. Contemporaneous near-IR and optical observations for all our observing epochs are organised. Results. Until June 2006 about 4000 radio measurements and 5500 near-IR and optical measurements were obtained. Most of the sources show significant variability in all observing bands. Here we present the multi-frequency data acquired during the first eighteen months of the project, and construct the SEDs for the best-sampled sources.

SEOUL NATIONAL UNIVERSITY 4K × 4K CAMERA (SNUCAM) FOR MAIDANAK OBSERVATORY

We present the characteristics of the Seoul National University 4k Camera (SNUCAM) and report its performance on the 1.5m telescope at the Maidanak observatory in Uzbekistan. SNUCAM is a CCD camera with a pixel scale of 0.266” in 4096 × 4096 format, covering 18.1’×18.1’ field of view on the 1.5m. The camera is currently equipped with Bessell UBVRI, Hα, SDSS ugriz, and Y-band filters, allowing us to carry out a variety of scientific programs ranging from exoplanet studies to survey of quasars at high redshift. We examine properties of SNUCAM such as the bias level and its temporal variation, the dark current, the readout noise, the gain, the linearity, the fringe patterns, the amplifier bias, and the bad pixels. From our observations, we also constructed the master fringe frames in I-, z-, and Y-band. We outline some of the current scientific programs being carried out with SNUCAM, and demonstrate that SNUCAM on the 1.5m can deliver excellent images that reach to the 5-σ detection limits of R ~ 25.5 mag and z ~ 22.7 mag in 1 hour total integration.

Fine Structure in the Circumstellar Environment of a Young, Solar-like Star: The Unique Eclipses of KH 15D

ABSTRACT Results of an international campaign to photometrically monitor the unique pre–main‐sequence eclipsing object KH 15D are reported. An updated ephemeris for the eclipse is derived that incorporates a slightly revised period of 48.36 days. There is some evidence that the orbital period is actually twice that value, with two eclipses occurring per cycle. The extraordinary depth (∼3.5 mag) and duration (∼18 days) of the eclipse indicate that it is caused by circumstellar matter, presumably the inner portion of a disk. The eclipse has continued to lengthen with time, and the central brightness reversals are not as extreme as they once were. V−R and V−I colors indicate that the system is slightly bluer near minimum light. Ingress and egress are remarkably well modeled by the passage of a knife edge across a limb‐darkened star. Possible models for the system are briefly discussed.