GRB 091127: The cooling break race on magnetic fuel

Using high-quality, broad-band afterglow data for GRB 091127, we investigate the validity of the synchrotron fireball model for gamma-ray bursts, and infer physical parameters of the ultra-relativistic outflow. We used multi-wavelength follow-up observations obtained with GROND and the XRT onboard the Swift satellite. The resulting afterglow light curve is of excellent accuracy, and the spectral energy distribution is well-sampled over 5 decades in energy. These data present one of the most comprehensive observing campaigns for a single GRB afterglow and allow us to test several proposed emission models and outflow characteristics in unprecedented detail. Both the multi-color light curve and the broad-band SED of the afterglow of GRB 091127 show evidence of a cooling break moving from high to lower energies. The early light curve is well described by a broken power-law, where the initial decay in the optical/NIR wavelength range is considerably flatter than at X-rays. Detailed fitting of the time-resolved SED shows that the break is very smooth with a sharpness index of 2.2 +- 0.2, and evolves towards lower frequencies as a power-law with index -1.23 +- 0.06. These are the first accurate and contemporaneous measurements of both the sharpness of the spectral break and its time evolution. The measured evolution of the cooling break (nu_c propto t^-1.2) is not consistent with the predictions of the standard model, wherein nu_c propto t^-0.5 is expected. A possible explanation for the observed behavior is a time dependence of the microphysical parameters, in particular the fraction of the total energy in the magnetic field epsilon_B. This conclusion provides further evidence that the standard fireball model is too simplistic, and time-dependent micro-physical parameters may be required to model the growing number of well-sampled afterglow light curves.Comment: accepted to A&A, 13 pages, 5 figure

BL Lacertae objects beyond redshift 1.3 - UV-to-NIR photometry and photometric redshift for Fermi/LAT blazars

Observations of the gamma-ray sky with Fermi led to significant advances towards understanding blazars, the most extreme class of Active Galactic Nuclei. A large fraction of the population detected by Fermi is formed by BL Lacertae (BL Lac) objects, whose sample has always suffered from a severe redshift incompleteness due to the quasi-featureless optical spectra. Our goal is to provide a significant increase of the number of confirmed high-redshift BL Lac objects contained in the 2 LAC Fermi/LAT catalog. For 103 Fermi/LAT blazars, photometric redshifts using spectral energy distribution fitting have been obtained. The photometry includes 13 broad-band filters from the far ultraviolet to the near-IR observed with Swift/UVOT and the multi-channel imager GROND at the MPG/ESO 2.2m telescope. Data have been taken quasi-simultaneously and the remaining source-intrinsic variability has been corrected for. We release the UV-to-near-IR 13-band photometry for all 103 sources and provide redshift constraints for 75 sources without previously known redshift. Out of those, eight have reliable photometric redshifts at z>1.3, while for the other 67 sources we provide upper limits. Six of the former eight are BL Lac objects, which quadruples the sample of confirmed high-redshift BL Lac. This includes three sources with redshifts higher than the previous record for BL Lac, including CRATES J0402-2615 with the best-fit solution at z~1.9.Comment: Uploaded correct Fig 4. Changed counterpart name for 2FGLJ0537.7-5716 from PKS 0541-834 (different source) to SUMSS J053748-57182

GROND multi-color imaging of the GRB 161219B / SN 2016jca field.

This folder contains optical/near-infrared imaging of the field of GRB 161219B (Swift trigger 727541; D'Ai, GCN #20296) collected simultaneously with the Gamma-Ray Burst Optical/Near Infrared imager (GROND; Greiner et al. 2008, PASP 120, 405) mounted at the 2.2m MPG telescope on ESO's LaSilla observatory. These data include a time-series of SN 2016jca following GRB 161219B with nearly daily monitoring around peak, and are published in Cano et al. 2017 (arXiv:1704.05401).<br><br>The data were obtained in the grizJHK photometric bands. They were collected over a period of several nights starting from 2016-12-20, and consisted of several dithered images that are stacked to yield one fits image per epoch per band. <br><br>Data were automatically processed with the GROND pipeline, which uses standard procedures in python/pyraf/IRAF (see e.g., Kruehler et al. 2008, ApJ, 685, 376). The data are sky-subtracted and astrometrically calibrated against the Pan-STARRS or GAIA catalogs.<br><br>Critical information can be found in the fits header, the following keywords are particularly useful:<br><br>- EXPTIME = Exposure time of a single image<br>- NIMGS = Total number of images combined<br>- COMBINE = How the images were combined<br>- IMGEXP = Summed exposure of the image<br>- MJD-MID = Midtime of the image<br>- FILTER = Filter band of the observation<br>- AIRMASS = Average airmass of the observation<br>- FWHM = FWHM of the stellar PSF<br>- ASTRRMS1 = Uncertainty of the astrometric solution in RA<br>- ASTRRMS2 = Uncertainty of the astrometric solution in DEC<br>- MAGLIM = Limiting magnitude of the image<br>- INTERPSM = Individual images are resampled during shifting and adding, this number describes an empirical estimate by what factor the noise is smoothed (i.e., too low as estimated by the rms in the image). In particular in the NIR bands, standard procedure will underestimate the photometric error by this factor.<br