N. Mirabal

GRB 060218/SN 2006aj: A Gamma-Ray Burst and Prompt Supernova at z = 0.0335

We report the imaging and spectroscopic localization of GRB 060218 to a low-metallicity dwarf starburst galaxy at z = 0.03345 ± 0.00006. In addition to making it the second nearest gamma-ray burst known, optical spectroscopy reveals the earliest detection of weak, supernova-like Si II near 5720 A (~0.1c), starting 1.95 days after the burst trigger. UBVRI photometry obtained between 1 and 26 days postburst confirms the early rise of supernova light, and suggests a short time delay between the gamma-ray burst and the onset of SN 2006aj if the early appearance of a soft component in the X-ray spectrum is understood as a shock breakout. Together, these results verify the long-hypothesized origin of soft gamma-ray bursts in the deaths of massive stars.

The WEBT Campaign on the Blazar 3C 279 in 2006

Thequasar3C279wasthetargetof anextensivemultiwavelengthmonitoringcampaignfrom2006Januarythrough April. An optical-IR-radio monitoring campaign by the Whole Earth Blazar Telescope (WEBT) collaboration was organized around target-of-opportunity X-ray and soft � -ray observations with Chandra and INTEGRAL in 2006 midJanuary, with additional X-ray coverage by RXTE and Swift XRT. In this paper we focus on the results of the WEBT campaign. Thesource exhibited substantial variability of opticalflux and spectralshape,witha characteristictimescale of a few days. The variability patterns throughout the optical BVRI bands were very closely correlated with each other, while there was no obvious correlation between the optical and radio variability. After the ToO trigger, the optical flux

Optical and X-Ray Observations of GRB 060526: A Complex Afterglow Consistent with an Achromatic Jet Break

We obtained 98 R-band and 18 B, r, i images of the optical afterglow of GRB 060526 (z = 3.21) with the MDM 1.3 m, 2.4 m, and the PROMPT telescopes at CTIO over the five nights following the burst trigger. Combining these data with other optical observations reported in GCN and the Swift XRT observations, we compare the optical and X-ray afterglow light curves of GRB 060526. Both the optical and X-ray afterglow light curves show rich features, such as flares and breaks. The densely sampled optical observations provide very good coverage at T > 104 s. We observed a break at 2.4 × 105 s in the optical afterglow light curve. Compared with the X-ray afterglow light curve, the break is consistent with an achromatic break supporting the beaming models of GRBs. However, the prebreak and postbreak temporal decay slopes are difficult to explain in simple afterglow models. We estimated a jet angle of θj ~ 7° and a prompt emission size of Rprompt ~ 2 × 1014 cm. In addition, we detected several optical flares with amplitudes of Δm ~ 0.2, 0.6, and 0.2 mag. The X-ray afterglows detected by Swift have shown complicated decay patterns. Recently, many well-sampled optical afterglows also show decays with flares and multiple breaks. GRB 060526 provides an additional case of such a complex, well-observed optical afterglow. The accumulated well-sampled afterglows indicate that most of the optical afterglows are complex.

Discovery of a Tidal Extension of the Sagittarius Dwarf Spheroidal Galaxy

We report the discovery of stars associated with the Sagittarius dwarf spheroidal galaxy (Sgr) located nearly 10° from the center of the main body of the galaxy, far beyond the outer boundary defined by the star counts of Ibata and coworkers. The newly discovered stars in Sgr include three RR Lyrae stars found serendipitously behind the Galactic globular cluster M55, red horizontal branch stars, and main-sequence stars identified both in the M55 field and in another field located away from this cluster but still about 10° from Sgr. The photometric properties of all of these stars are perfectly consistent with the previously determined distance, metallicity, and age of the Sgr galaxy. Our results indicate that Sgr is much larger than previously believed and that the stellar distribution in the outer parts of the galaxy is probably quite clumpy. We discuss the implications of our findings on the total luminosity and mass of Sgr and on the characteristics of its most recent past perigalacticon passages.

THE MERGER IN ABELL 576: A LINE OF SIGHT BULLET CLUSTER?

Using a combination of Chandra and XMM-Newton observations, we confirmed the presence of a significant velocity gradient along the northeast-southwest direction in the intracluster gas of the cluster Abell 576. The results are consistent with a previous ASCA SIS analysis of this cluster. The error-weighted average over the ACIS-S3 and EPIC MOS1 and MOS2 spectrometers for the maximum velocity difference is >3.3 × 103 km s-1 at the 90% confidence level, similar to the velocity limits estimated indirectly for the Bullet Cluster (1E 0657-56). The probability that the velocity gradient is generated by standard random gain fluctuations with Chandra and XMM-Newton is <0.1%. The regions of maximum velocity gradient are in CCD zones that have the lowest temporal gain variations. It is unlikely that the velocity gradient is due to Hubble distance differences between projected clusters (probability 0.01%). We mapped the distribution of elemental abundance ratios across the cluster and detected a strong chemical discontinuity using the abundance ratio of silicon to iron, equivalent to a variation from 100% SN Ia iron mass fraction in the west-northwest regions to 32% in the eastern region. The center of the cluster is located at the chemical discontinuity boundary, which is inconsistent with the radially symmetric chemical gradient found in some regular clusters, but consistent with a cluster merging scenario. We predict that the velocity gradient as measured will produce a variation of the cosmic microwave background (CMB) temperature toward the east of the core of the cluster that will be detectable by current and near-future bolometers. The measured velocity gradient opens up the possibility that this cluster is passing through a near line-of-sight merger stage where the cores have recently crossed.

Optical light curve and cooling break of GRB 050502A

We present light curves of the afterglow of GRB 050502A, including very early data at ttGRB <60 s. The light curve is composed of unfiltered ROTSE-IIIb optical observations from 44 s to 6 hr postburst, R-band MDM observations from 1.6 to 8.4 hr postburst, and PAIRITEL JHKs observations from 0.6 to 2.6 hr postburst. The optical lightcurveisfitbyabrokenpowerlaw,wheretsteepensfrom � ¼� 1:13 � 0:02to � 1:44 � 0:02at � 5700s.This steepening is consistent with the evolution expected for the passage of the cooling frequencyc through the optical band. Even in our earliest observation at 44 s postburst, there is no evidence that the optical flux is brighter than a backward extrapolation of the later power law would suggest. The observed decay indices and spectral index are consistentwitheitheranISMorawindfireballmodel,butslightlyfavortheISMinterpretation.Theexpectedspectral index in the ISM interpretation is consistent within 1 � with the observed spectral index � ¼� 0:8 � 0:1; the wind interpretation would imply a spectral index slightly (� 2 � ) shallower than observed. A small amount of dust extinc- tion at the source redshift could steepen an intrinsic spectrum sufficiently to account for the observed value of � .I n this picture, the early optical decay, with the peak at or below 4:7 ;10 14 Hz at 44 s, requires very small electron and magnetic energy partitions from the fireball. Subject headingg gamma rays: bursts

Millisecond Pulsar Origin of the Galactic Center Excess and Extended Gamma-Ray Emission from Andromeda: A Closer Look

A new measurement of a spatially extended gamma-ray signal from the center of the Andromeda galaxy (M31) has recently been published by the Fermi-LAT collaboration, reporting that the emission broadly resembles the so-called Galactic center excess (GCE) of the Milky Way (MW). The weight of the evidence is steadily accumulating on a millisecond pulsar (MSPs) origin for the GCE. These elements prompt us to compare these observations with what is, perhaps, the simplest model for an MSP population, which is solely obtained by rescaling of the MSP luminosity function that is determined in the local MW disk via the respective stellar mass of the systems. Remarkably, we find that without free fitting parameters, this model can account for both the energetics and the morphology of the GCE within uncertainties. For M31, the estimated luminosity due to primordial MSPs is expected to only contribute about a quarter of the detected emission, although a stronger contribution cannot be excluded given the large uncertainties. If correct, the model predicts that the M31 disk emission due to MSPs is not far below the present upper bound. We also discuss additional refinements of this simple model. Using the correlation between globular cluster gamma-ray luminosity and stellar encounter rate, we gauge the dynamical MSP formation in the bulge. This component is expected to contribute to the GCE only at a level of less than or similar to 5%, it could affect the signals morphology. We also comment on the limitations of our model and on future perspectives for improved diagnostics.

ROTSE-III performance in the Swift era

We report the successful performance of the Robotic Optical Transient Search Experiment (ROTSE) to promptly‐disseminated Swift GRB triggers. ROTSE‐III is a worldwide network of 4 unfiltered 0.45m optical telescopes. The telescopes operate robotically, automatically responding to GRB position triggers with a preset observations sequence. Including weather and other downtime, ROTSE‐III can immediately respond to ∼40% of Swift trigger positions, with shutter open within approximately 6–8 seconds from the trigger dissemination. We discuss improvements made possible in the automated response the the small, accurate Swift error boxes. We report ROTSE‐III’s general results, including OTs discovered or confirmed, the distribution of imaging start times relative to the GRB duration, and an overview of OT lightcurves.