No abstract

We report on the results of optical follow-up observations of the counterpart of GRB 970508, starting 7 hours after the event. Multi-color U, B, V, R c and I c band observations were obtained during the first three consecutive nights. The counterpart was monitored regularly in R c until ∼ 4 months after the burst. The light curve after the maximum follows a decline that can be fitted with a power law with exponent α = -1.141 ± 0.014. Deviations from a smooth power law decay are moderate (r.m.s. = 0.15 magnitude). We find no flattening of the light curve at late times. The optical afterglow fluence is a significant fraction, ∼ 5%, of the GRB fluence. The optical energy distribution can be well represented by a power law, the slope of which changed at the time of the maximum (the spectrum became redder).

A B S T R A C TWe present contemporary infrared (IR) and optical spectra of the plateau type II SN 1995V in NGC 1087 covering four epochs, approximately 22 to 84 d after shock break-out. The data show, for the first time, the IR spectroscopic evolution during the plateau phase of a typical type II event. In the optical region P Cygni lines of the Balmer series and of metals such as Sc II, Fe II, Sr II, Ca II and Ba II lines were identified. The IR spectra were largely dominated by the continuum, but P Cygni Paschen lines and Brackett g lines were also clearly seen. The other prominent IR features are confined to wavelengths blueward of 11 000 Å , and include Sr II 10327, Fe II 10547, C I 10695 and He I 10830 Å . Helium has never before been unambiguously identified in a type IIp supernova spectrum during the plateau phase. We demonstrate the presence of He I 10830 Å on days 69 and 85. The presence of this line at such late times implies reionization. A likely reionizing mechanism is g-ray deposition following the radioactive decay of 56 Ni. We examine this mechanism by constructing a spectral model for the He I 10830-Å line based on explosion model s15s7b2f of Weaver & Woosley. We find that this does not generate the observed line owing to the confinement of the 56 Ni to the central zones of the ejecta. In order to reproduce the He I line, it was necessary to introduce additional upward mixing or 'dredge-up' of the 56 Ni, with ϳ10 ¹5 of the total nickel mass reaching above the helium photosphere. In addition, we argue that the He I line formation region is likely to have been in the form of pure helium clumps in the hydrogen envelope. The study of He I 10830-Å emission during the photospheric phase of core-collapse supernovae provides a promising tool for the constraint of initial mixing conditions in explosion models.

We present a set of Roche tomography reconstructions of the secondary stars in the cataclysmic variables AM Her, QQ Vul, IP Peg and HU Aqr. The image reconstructions show distinct asymmetries in the irradiation pattern for all four systems that can be attributed to shielding of the secondary star by the accretion stream/column in AM Her, QQ Vul and HU Aqr, and increased irradiation by the bright‐spot in IP Peg. We use the entropy landscape technique to derive accurate system parameters (M1, M2, i and γ) for the four binaries. In principle, this technique should provide the most reliable mass determinations available, since the intensity distribution across the secondary star is known. We also find that the intensity distribution can systematically affect the value of γ derived from circular orbit fits to radial velocity variations.

We report on the results of R-band observations of the error box of the gray burst of 1997 August 28 made between 4 hr and 8 days after this burst occurred. No counterpart was found varying by more than 0.2 mag down to. We discuss the consequences of this nondetection for relativistic blast wave models of gray R 23.8 bursts and the possible effect of redshift on the relation between optical absorption and the low-energy cutoff in the X-ray afterglow spectrum.