Robert R. Uglesich

GRB 991216 Joins the Jet Set: Discovery and Monitoring of Its Optical Afterglow

The optical light curve of the energetic γ-ray burst GRB 991216 is consistent with jetlike behavior in which a power-law decay steepens from t-1.22±0.04 at early times to t-1.53±0.05 in a gradual transition at around 2 days. The derivation of the late-time decay slope takes into account the constant contribution of a host or intervening galaxy, which was measured 110 days after the event at R = 24.56 ± 0.14, although the light curve deviates from a single power law whether or not a constant term is included. The early-time spectral energy distribution of the afterglow can be described as Fν ∝ ν-0.74±0.05 or flatter between optical and X-ray, which, together with the slow initial decay, is characteristic of standard adiabatic evolution in a uniformly dense medium. Assuming that a reported absorption-line redshift of 1.02 is correct, the apparent isotropic energy of 6.7 × 1053 ergs is reduced by a factor of ≈200 in the jet model, and the initial half-opening angle is ≈6°. GRB 991216 is the third good example of a jetlike afterglow (following GRB 990123 and GRB 990510), supporting a trend in which the apparently most energetic γ-ray events have the narrowest collimation and a uniform interstellar medium environment. This, plus the absence of evidence for supernovae associated with jetlike afterglows, suggests that these events may originate from a progenitor in which angular momentum plays an important role but a massive stellar envelope or wind does not, e.g., in the coalescence of a compact binary.

Evidence of Halo Microlensing in M31

We have completed an intensive monitoring program of two fields on either side of the center of M31 and report here on the results concerning microlensing of stars in M31. These results stem from a 3 yr study ( the Vatican Advanced Technology Telescope [VATT]/Columbia survey) of microlensing and variability of M31 stars, emphasizing microlensing events of 3 day to 2 month timescales and likely due to masses in M31. These observations were conducted intensively from 1997 to 1999, with baselines 1995 to present, at the VATT and the 1.3 m telescope at MDM Observatory, with additional data from the Isaac Newton Telescope, including about 200 epochs total. The two fields monitored cover 560 arcmin(2) total, positioned along the minor axis on either side of M31. Candidate microlensing events are subject to a number of tests discussed here with the purpose of distinguishing microlensing from variable star activity. A total of four probable microlensing events, when compared with carefully computed event rate and efficiency models, indicate a marginally significant microlensing activity above that expected for the stars alone in M31 ( and the Galaxy) acting as lenses. A maximum likelihood analysis of the distribution of events in timescale and across the face of M31 indicates a microlensing dark matter halo fraction consistent with that seen by MACHO in our Galaxy toward the Large Magellanic Cloud. Specifically, for a nearly singular isothermal sphere model, we find a microlensing halo mass fraction fb = 0.29(-0.13)(+0.30) of the total dark matter and a poorly constrained lensing component mass (0.02 - 1.5 M-.; 1 sigma limits). This study serves as the prototype for a larger study approaching completion; between the two there is significant evidence for an asymmetry in the distribution of microlensing events across the face of M31 and therefore a large population of halo microlensing dark matter objects.

Formaldehyde emission in galaxies

Nine detections of extragalactic emission in the 4.83 GHz 1 10 -1 11 formaldehyde transition are presented in addition to the only previously known extragalactic emission source in IC 4553. Formaldehyde emission occurs among both OH absorbers and emitters. In contrast with the high-detection rate of extragalactic sources in this survey, there are only a small number of Galactic sources with 4.83 GHz emission. Formaldehyde absorption has been found in one new source. A comparison has been made between the H 2 CO and OH properties of the galaxies. In analogy with the OH megamasers and as proposed for the IC 4553 formaldehyde megamaser emission line, low-gain amplification of radio continuum by foreground clouds in a molecular disk is also suggested for the new formaldehyde megamasers

First Microlensing Events From The MEGA Survey Of M31

We present the first M31 candidate microlensing events from the Microlensing Exploration of the Galaxy and Andromeda (MEGA) survey. MEGA uses several telescopes to detect microlensing towards the nearby Andromeda galaxy, M31, in order to establish whether massive compact objects are a significant contribution to the mass budget of the dark halo of M31. The results presented here are based on observations with the Isaac Newton Telescope on La Palma, during the 1999/00 and 2000/01 observing seasons. In this data set, 14 variable sources consistent with microlensing have been detected, 12 of which are new and 2 have been reported previously by the POINT-AGAPE group. A preliminary analysis of the spatial and timescale distributions of the candidate events support their microlensing nature. We compare the spatial distributions of the candidate events and of long-period variable stars, assuming the chances of finding a long-period variable and a microlensing event are comparable. The spatial distribution of our candidate microlensing events is more far/near side asymmetric than expected from the detected long-period variable distribution. The current analysis is preliminary and the asymmetry not highly significant, but the spatial distribution of candidate microlenses is suggestive of the presence of a microlensing halo.

First microlensing candidates from the MEGA survey of M31

We present the first M31 candidate microlensing events from the Microlensing Exploration of the Galaxy and Andromeda (MEGA) survey. MEGA uses several telescopes to detect microlensing towards the nearby Andromeda galaxy, M31, in order to establish whether massive compact objects are a significant contribution to the mass budget of the dark halo of M31. The results presented here are based on observations with the Isaac Newton Telescope on La Palma, during the 1999/00 and 2000/01 observing seasons. In this data set, 14 variable sources consistent with microlensing have been detected, 12 of which are new and 2 have been reported previously by the POINT-AGAPE group. A preliminary analysis of the spatial and timescale distributions of the candidate events support their microlensing nature. We compare the spatial distributions of the candidate events and of long-period variable stars, assuming the chances of finding a long-period variable and a microlensing event are comparable. The spatial distribution of our candidate microlensing events is more far/near side asymmetric than expected from the detected long-period variable distribution. The current analysis is preliminary and the asymmetry not highly significant, but the spatial distribution of candidate microlenses is suggestive of the presence of a microlensing halo.