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We compare the redshifts, host galaxy metallicities, and isotropic (E g,iso) and beaming-corrected (E g) gamma-ray energy release of 16 long-duration gamma-ray bursts (LGRBs) at z < 1. From this comparison, we find no statistically significant correlation between host metallicity and redshift, E g,iso, or E g. These results are at odds with previous theoretical and observational predictions of an inverse correlation between gamma-ray energy release and host metallicity, as well as the standard predictions of metallicity-driven wind effects in stellar evolutionary models. We consider the implications that these results have for LGRB progenitor scenarios, and discuss our current understanding of the role that metallicity plays in the production of LGRBs.

We present a comprehensive multiwavelength analysis of the bright, long-duration gamma-ray burst GRB 070125, comprised of observations in gamma-ray, X-ray, optical, millimeter, and centimeter wave bands. Simultaneous fits to the optical and X-ray light curves favor a break on day 3.78, which we interpret as the jet break from a collimated outflow. Independent fits to optical and X-ray bands give similar results in the optical bands but shift the jet break to around day 10 in the X-ray light curve. We show that for the physical parameters derived for GRB 070125, inverse Compton scattering effects are important throughout the afterglow evolution. While inverse Compton scattering does not affect radio and optical bands, it may be a promising candidate to delay the jet break in the X-ray band. Radio light curves show rapid flux variations, which are interpreted as due to interstellar scintillation and used to derive an upper limit of 2.4 x 1017 cm on the radius of the fireball in the lateral expansion phase of the jet. Radio light curves and spectra suggest a high synchrotron self-absorption frequency indicative of the afterglow shock wave moving in a dense medium. Our broadband modeling favors a constant density profile for the circumburst medium over a windlike profile ( R[?]2). However, keeping in mind the uncertainty of the parameters, it is difficult to unambiguously distinguish between the two density profiles. Our broadband fits suggest that GRB 070125 is a burst with high radiative efficiency (>60%).

We present the discovery and high signal-to-noise ratio spectroscopic observations of the optical afterglow of the long-duration gamma-ray burst GRB 070125. Unlike all previously observed long-duration afterglows in the redshift range 0.5 [?] z [?] 2.0, we find no strong (rest-frame equivalent width Wr[?] 1.0 A) absorption features in the wavelength range 4000-10000 A. The sole significant feature is a weak doublet that we identify as Mg II ll2796 ( Wr = 0.18 +- 0.02 A), 2803 ( Wr = 0.08 +- 0.01 A) at z = 1.5477 +- 0.0001. The low observed Mg II and inferred H I column densities are typically observed in galactic halos, far away from the bulk of massive star formation. Deep ground-based imaging reveals no host directly underneath the afterglow to a limit of 25.4"/> R > 25.4 mag. Either of the two nearest blue galaxies could host GRB 070125; the large offset ( d [?] 27 kpc) would naturally explain the low column densities. To remain consistent with the large local (i.e., parsec scale) circumburst density inferred from broadband afterglow observations, we speculate that GRB 070125 may have occurred far away from the disk of its host in a compact star-forming cluster. Such distant stellar clusters, typically formed by dynamical galaxy interactions, have been observed in the nearby universe and should be more prevalent at 1"/> z > 1, where galaxy mergers occur more frequently.

We have converted the Palomar 60 inch (1.52 m) telescope from a classic night-assistant-operated telescope to a fully robotic facility. The automated system, which has been operational since 2004 September, is designed for moderately fast (t?3 minutes) and sustained (R?23 mag) observations of gamma-ray burst afterglows and other transient events. Routine queue-scheduled observations can be interrupted in response to electronic notification of transient events. An automated pipeline reduces data in real time, which is then stored on a searchable Web-based archive for ease of distribution. We describe here the design requirements, hardware and software upgrades, and lessons learned from roboticization. We present an overview of the current system performance as well as plans for future upgrades.

We present the first full characterization of the transient radio sky via radio and optical follow-up observations of all the possible radio transients we have discovered in a survey covering [?]1/17 of the sky. The two confirmed radio transients turn out to be an optically obscured radio supernova (SN) in the nearby galaxy NGC 4216, the first such event to be discovered by a wide-field radio survey, and a source not associated with a bright host galaxy. We speculate that this second source may be a flare from a peculiar radio-loud AGN, or a burst from an unusual Galactic compact object, but its nature merits further study. We place an upper limit of 65 radio transients above 6 mJy over the entire sky (95% confidence level). The implications are as follows. First, we derive a limit on the typical beaming of GRBs; we find fimg1.gif [?] 60, ~5 times higher than our earlier results [fimg1.gif [?] (thimg2.gif/2)-1]. Second, our results impose an upper limit on the rate of events that eject [?]1051 ergs in unconfined relativistic ejecta, whether or not accompanied by detectable emission in wavebands other than the radio. Our estimated rate, img3.gif [?] 1000 yr-1 Gpc-1, is about 2 orders of magnitude smaller than the rate of core-collapse SNe (and Type Ib/c events in particular), indicating that only a minority of such events eject significant amounts of relativistic material, which are required by fireball models of long-soft GRBs. Finally, we show that wider and/or deeper radio variability surveys are expected to detect numerous orphan radio GRB afterglows and illustrate the great potential of new radio instruments to revolutionize the study of nearby SNe.

We present photometric typing analyses of three young supernovae observed with the robotic 60 inch (1.5 m) telescope at Palomar Observatory (P60). This represents the first time that such phototyping, conducted in a blind fashion, has been attempted on newly discovered supernovae. For one of the target supernovae, SN 2004cs, our photometry provided the first constraint on the supernova type, which we predicted would be Type Ia. Contrary to expectations, however, our subsequent Keck spectroscopy shows it to be an unusual Type II supernova. For each of the other two supernovae (SN 2004dh [Type II] and SN 2004dk [Type Ib]), our phototyping results are consistent with the known event type as determined from ground-based spectroscopy. However, the colors of SN 2004dk are also consistent with a Type Ic or Type II classification. We discuss our approach to the challenges of phototyping-contamination by host galaxy light and the unknown photometric quality of the data-for cases in which it is desirable to complete the analysis with just one night of observations. The growing interest in the properties and behavior of very young supernovae, and the increased discovery rate for such events, mean that prompt phototyping analyses can provide useful input for observational campaigns. Our results demonstrate the value and feasibility of such a project for P60, at the same time illustrating its chief inherent shortcoming: an inability to identify new and unusual events as such without later spectroscopic observations.

We present high-quality BVRICCD photometry of the variable star V716 Monocerotis (=NSV 03775). We confirm it to be an RR Lyrae star of variability type ab (i.e., a fundamental-mode pulsator) and determine its metallicity ([Fe/H] = -1.33 ± 0.25), luminosity (MV= 0.80 ± 0.06), and foreground reddening [E(B?V) = 0.05-0.17] from the Fourier components of its light curve. These parameters indicate a distance of 4.1 ± 0.3 kpc, placing V716 Mon near the plane of the Galaxy well outside the solar circle.

Despite both being outbursts of luminous blue variables (LBVs), SN 2009ip and UGC 2773 OT2009-1 have very different progenitors, spectra, circumstellar environments, and possibly physical mechanisms that generated the outbursts. From pre-eruption Hubble Space Telescope images, we determine that SN 2009ip and UGC 2773 OT2009-1 have initial masses of [?] 60 and [?] 25 M , respectively. Optical spectroscopy shows that at peak, SN 2009ip had a 10,000 K photosphere and its spectrum was dominated by narrow H Balmer emission, similar to classical LBV giant outbursts, also known as "supernova impostors." The spectra of UGC 2773 OT2009-1, which also have narrow Ha emission, are dominated by a forest of absorption lines, similar to an F-type supergiant. Blueshifted absorption lines corresponding to ejecta at a velocity of 2000-7000 km s-1 are present in later spectra of SN 2009ip--an unprecedented observation for LBV outbursts, indicating that the event was the result of a supersonic explosion rather than a subsonic outburst. The velocity of the absorption lines increases between two epochs, suggesting that there were two explosions in rapid succession. A rapid fading and rebrightening event concurrent with the onset of the high-velocity absorption lines is consistent with the double-explosion model. A near-infrared excess is present in the spectra and photometry of UGC 2773 OT2009-1 that is consistent with [?]2100 K dust emission. We compare the properties of these two events and place them in the context of other known massive star outbursts such as e Car, NGC 300 OT2008-1, and SN 2008S. This qualitative analysis suggests that massive star outbursts have many physical differences that can manifest as the different observables seen in these two interesting objects.