The Near Infrared and Multiwavelength Afterglow of GRB 000301c

We present near-infrared observations of the counterpart of GRB 000301c. The K' filter (2.1 micron) light curve shows a well-sampled break in the decay slope at t=3.5 days post-burst. The early time slope is very shallow (~ -0.1), while the late time slope is steep (-2.2). Comparison with the optical (R band) light curve shows marginally significant differences, especially in the early time decay slope (which is steeper in the optical) and the break time (which occurs later in the optical). This is contrary to the general expectation that light curve breaks should either be achromatic (e.g., for breaks due to collimation effects) or should occur later at longer wavelengths (for most other breaks). The observed color variations might be intrinsic to the afterglow, or might indicate systematic errors of > 0.08 magnitude in all fluxes. Even if the break is achromatic, we argue that its sharpness poses difficulties for explanations that depend on collimated ejecta. The R light curve shows further signs of fairly rapid variability (a bump, steep drop, and plateau) that are not apparent in the K' light curve. In addition, by combining the IR-optical-UV data with millimeter and radio fluxes, we are able to constrain the locations of the self-absorption break and cooling break and to infer the location of the spectral peak at t=3 days: f_nu = 3.4 mJy at nu=1e12 Hz. Using the multiwavelength spectral energy distribution, we are able to constrain the blast wave energy, which was E > 3e53 erg if the explosion was isotropic. This implies a maximum gamma ray production efficiency of ~ 0.15 for GRB 000301C.Comment: Accepted to The Astrophysical Journal. 24 pages, 4 figures, 3 tables; uses AASTeX 5 macros. This version includes a new figure (R-K' color vs. time), a better sampled R band light curve, and more extensive discussion of the optical data and error analysi

The prompt optical/near-infrared flare of GRB 050904: the most luminous transient ever detected

With a redshift of z=6.295, GRB 050904 is the most distant gamma-ray burst ever discovered. It was an energetic event at all wavelengths and the afterglow was observed in detail in the near-infrared bands. We gathered all available optical and NIR afterglow photometry of this GRB to construct a composite NIR light curve spanning several decades in time and flux density. Transforming the NIR light curve into the optical, we find that the afterglow of GRB 050904 was more luminous at early times than any other GRB afterglow in the pre-\emph{Swift} era, making it at these wavelengths the most luminous transient ever detected. Given the intrinsic properties of GRB 050904 and its afterglow, we discuss if this burst is markedly different from other GRBs at lower redshifts.Comment: The Astronomical Journal, in press; revised version, including the comments of the referee (one figure added, text restructured, all conclusions unchanged), 7 pages, 3 figure

Optical identification of X-ray source 1RXS J180431.1-273932 as a magnetic cataclysmic variable

The X-ray source 1RXS J180431.1-273932 has been proposed as a new member of the symbiotic X-ray binary (SyXB) class of systems, which are composed of a late-type giant that loses matter to an extremely compact object, most likely a neutron star. In this paper, we present an optical campaign of imaging plus spectroscopy on selected candidate counterparts of this object. We also reanalyzed the available archival X-ray data collected with XMM-Newton. We find that the brightest optical source inside the 90% X-ray positional error circle is spectroscopically identified as a magnetic cataclysmic variable (CV), most likely of intermediate polar type, through the detection of prominent Balmer, He I, He II, and Bowen blend emissions. On either spectroscopic or statistical grounds, we discard as counterparts of the X-ray source the other optical objects in the XMM-Newton error circle. A red giant star of spectral type M5 III is found lying just outside the X-ray position: we consider this latter object as a fore-/background one and likewise rule it out as a counterpart of 1RXS J180431.1-273932. The description of the X-ray spectrum of the source using a bremsstrahlung plus black-body model gives temperatures of around 40 keV and around 0.1 keV for these two components, respectively. We estimate a distance of about 450 pc and a 0.2-10 keV X-ray luminosity of about 1.7e32 erg/s for this system and, using the information obtained from the X-ray spectral analysis, a mass of about 0.8 solar masses for the accreting white dwarf (WD). We also confirm an X-ray periodicity of 494 s for this source, which we interpret as the spin period of the WD. In summary, 1RXS J180431.1-273932 is identified as a magnetic CV and its SyXB nature is excluded.Comment: 9 pages, 7 figures, 3 tables, accepted for publication on Astronomy & Astrophysics, main journal. Version 2 includes the A&A Language Editor's correction

Resolving Gamma-Ray Burst 000301C with a Gravitational Microlens

The afterglow of the Gamma-Ray Burst (GRB) 000301C exhibited achromatic, short time-scale variability that is difficult to reconcile with the standard relativistic shock model. We interpret the observed light curves as a microlensing event superimposed on power-law flux decays typical of afterglows. In general, a relativistic GRB shock appears on the sky as a thin ring expanding at a superluminal speed. Initially the ring is small relative to its angular separation from the lens and so its flux is magnified by a constant factor. As the ring grows and sweeps across the lens its magnification reaches a maximum. Subsequently, the flux gradually recovers its unlensed value. This behavior involves only three free parameters in its simplest formulation and was predicted theoretically by Loeb & Perna (1998). Fitting the available R-band photometric data of GRB 000301C to a simple model of the microlensing event and a broken power-law for the afterglow, we find reasonable values for all the parameters and a reduced chi^2/DOF parameter of 1.48 compared with 2.99 for the broken power-law fit alone. The peak magnification of ~2 occurred 3.8 days after the burst. The entire optical-IR data imply a width of the GRB ring of order 10% of its radius, similar to theoretical expectations. The angular resolution provided by microlensing is better than a micro-arcsecond. We infer a mass of approximately 0.5 M_Sun for a lens located half way to the source at z_s=2.04. A galaxy 2'' from GRB 000301C might be the host of the stellar lens, but current data provides only an upper-limit on its surface brightness at the GRB position.Comment: to appear in the ApJ Letters, 13 pages, 3 figures (one additional figure included); all data used for the fits available at ftp://cfa-ftp.harvard.edu/pub/kstanek/GRB000301C/ and through WWW at http://cfa-www.harvard.edu/cfa/oir/Research/GRB