R. Sagar

Optical and near-infrared observations of the GRB020405 afterglow

We report on photometric, spectroscopic and polarimetric monitoring of the optical and near-infrared (NIR) afterglow of GRB020405. Ground-based optical observations, performed with 8 different telescopes, started about 1 day after the high-energy prompt event and spanned a period of ∼10 days; the addition of archival HST data extended the coverage up to ∼150 days after the GRB. We report the first detection of the afterglow in NIR bands. The detection of Balmer and oxygen emission lines in the optical spectrum of the host galaxy indicates that the GRB is located at redshift z = 0.691. Fe II and Mg II absorption systems are detected at z = 0.691 and at z = 0.472 in the afterglow optical spectrum. The latter system is likely caused by absorbing clouds in the galaxy complex located ∼2 southwest of the GRB020405 host. Hence, for the first time, the galaxy responsible for an intervening absorption line system in the spectrum of a GRB afterglow is spectroscopically identified. Optical and NIR photometry of the afterglow indicates that, between 1 and 10 days after the GRB, the decay in all bands is consistent with a single power law of index a = 1.54 ′0.06. The late-epoch VLT J-band and HST optical points lie above the extrapolation of this power law, so that a plateau (or bump) is apparent in the VRIJ light curves at 10-20 days after the GRB. The light curves at epochs later than day ∼20 after the GRB are consistent with a power-law decay with index α = 1.85 ′ 0.15. While other authors have proposed to reproduce the bump with the template of the supernova (SN) 1998bw, considered the prototypical hypernova, we suggest that it can also be modeled with a SN having the same temporal profile as the other proposed hypernova SN2002ap, but 1.3 mag brighter at peak, and located at the GRB redshift. Alternatively, a shock re-energization may be responsible for the rebrightening. A single polarimetric R-band measurement shows that the afterglow is polarized, with P = 1.5 ′ 0.4% and polarization angle 0 = 172° ′8°. Broad-band optical-NIR spectral flux distributions show, in the first days after the GRB, a change of slope across the J band which we interpret as due to the presence of the electron cooling frequency v c . The analysis of the multiwavelength spectrum within the standard fireball model suggests that a population of relativistic electrons with index p ∼ 2.7 produces the optical-NIR emission via synchrotron radiation in an adiabatically expanding blastwave, with negligible host galaxy extinction, and the X-rays via Inverse Compton scattering off lower-frequency afterglow photons.

Identification of 13 Cepheids and 333 other variables in M 31

We present Cousins R and I band photometry of variable stars in a

GRB 030227: The first multiwavelength afterglow of an INTEGRAL GRB

{sim}13times 13

Photometric binary content in intermediate/old open clusters

region in the disk of Mu200931 galaxy, obtained during 141 nights. Of the 26 Cepheid variables present in the region, two are newly discovered, 11 are classified as Cepheids for the first time and 13 are confirmed as Cepheids. The extensive photometry of these Cepheids enabled us to determine precise phase and amplitude of pulsation which ranges from 0.11 to 0.48 mag in R band. The period of variability ranges from ~7.5 to 56 days. The period-luminosity diagram is used to derive a distance modulus of

Photometric study of two novae in M 31

24.49pm0.11

Optical and radio observations of the bright GRB 010222 afterglow: evidence for rapid synchrotron cooling?

xa0mag for Mu200931 galaxy. We also report variability in 333 other stars, of them, 115 stars appear to be long period variables, 2 suspected eclipsing binaries and remaining 216 are irregular variables.

Optical observations of the bright long duration peculiar GRB 021004 afterglow

We present multiwavelength observations of a gamma-ray burst detected byINTEGRAL (GRB 030227) between 5.3 hours and ~ 1.7days after the event. Here we report the discovery of a dim opticalafterglow (OA) that would not have been detected by many previoussearches due to its faintess (R ~ 23). This OA was seen to declinefollowing a power law decay with index alpha R = -0.95 +/-0.16. The spectral index beta_ opt/NIR yielded -1.25 +/- 0.14. Thesevalues may be explained by a relativistic expansion of a fireball (withp = 2.0) in the cooling regime. We also find evidence for inverseCompton scattering in X-rays.Based on observations with INTEGRAL, an ESA project with instruments andscience data centre funded by ESA member states (especially the PIcountries: Denmark, France, Germany, Italy, Switzerland, Spain), CzechRepublic and Poland, and with the participation of Russia and the USA.Also partially based on observations collected by the Gamma-Ray BurstCollaboration at ESO (GRACE) at the European Southern Observatory, Chile(ESO Large Programme 165.H-0464). (Less)

GRB 000301C with peculiar afterglow emission

The stellar evolutionary models by Bertelli et al. (1994) have been used to generate synthetic colour-magnitude diagrams (CMDs) of open clusters. A comparison of the synthetic integrated luminosity function (ILF) and synthetic colour distribution with the corresponding observed distributions is used to estimate the photometric binary content in three intermediate age open clusters, which is found to be about 30% in these clusters.

A photometric study of the open cluster M39

We report RcIc light curves of 2 novae in the M 31 galaxy which were detected in the four year Nainital Microlensing Survey. One of these novae has been tracked from the initial increase in flux while other has been observed during its descend- ing phase of brightness. The photometry of the first nova during the outburst phase suggests its peak Rc magnitude to be about 17.2 mag with a flux decline rate of 0.11 mag day −1 which indicates that it was a fast nova. A month after its outburst, it shows reddening followed by a plateau in Ic flux. The second nova exhibits a bump in Rc and Ic, possibly about three weeks after the outburst.