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

The Rapidly Flaring Afterglow of the Very Bright and Energetic GRB 070125

We report on multiwavelength observations, ranging from X-ray to radio wave bands, of the IPN-localized gamma-ray burst GRB 070125. Spectroscopic observations reveal the presence of absorption lines due to O i,Si ii,and C iv, implying a likely redshift of z ¼1:547. The well-sampled light curves, in particular from 0.5 to 4 days after the burst, suggest a jet break at 3.7 days, corresponding to a jet opening angle of $7.0, and implying an intrinsic GRB energy in the 1Y10,000 keV band of around E ¼(6:3Y6:9) ; 1051 ergs (based on the fluences measured by the gamma-ray de-tectorsof the IPN).GRB070125 is among the brightest afterglows observed to date.The SEDimplies ahostextinction of AV \u3c 0:9 mag. Two rebrightening episodes are observed, one with excellent time coverage, showing an increase in fluxof 56$8000 s.The evolution of the afterglow light curve is achromatic at all times.Late-time observationsof the afterglow do not show evidence for emission from an underlying host galaxy or supernova. Any host galaxy would be subluminous, consistent with current GRB host galaxy samples. Evidence for strong Mg ii absorption features is not found, which is perhaps surprising in view of the relatively high redshift of this burst and the high likelihood for such features along GRB-selected lines of sight

The Rapidly Flaring Afterglow of the Very Bright and Energetic GRB 070125

We report on multiwavelength observations, ranging from X-ray to radio wave bands, of the IPN-localized gamma-ray burst GRB 070125. Spectroscopic observations reveal the presence of absorption lines due to O I, Si II, and C IV, implying a likely redshift of z = 1.547. The well-sampled light curves, in particular from 0.5 to 4 days after the burst, suggest a jet break at 3.7 days, corresponding to a jet opening angle of ~7.0°, and implying an intrinsic GRB energy in the 1-10,000 keV band of around Eγ = (6.3–6.9) × 1051 ergs (based on the fluences measured by the gamma-ray detectors of the IPN). GRB 070125 is among the brightest afterglows observed to date. The SED implies a host extinction of AV \u3c 0.9 mag . Two rebrightening episodes are observed, one with excellent time coverage, showing an increase in flux of 56% in ~8000 s. The evolution of the afterglow light curve is achromatic at all times. Late-time observations of the afterglow do not show evidence for emission from an underlying host galaxy or supernova. Any host galaxy would be subluminous, consistent with current GRB host galaxy samples. Evidence for strong Mg II absorption features is not found, which is perhaps surprising in view of the relatively high redshift of this burst and the high likelihood for such features along GRB-selected lines of sight

The Rapidly Flaring Afterglow of the Very Bright and Energetic GRB 070125

We report on multi-wavelength observations, ranging from the X-ray to radio wave bands, of the IPN-localized gamma-ray burst GRB 070125. Spectroscopic observations reveal the presence of absorption lines due to O I, Si II, and C IV, implying a likely redshift of z = 1.547. The well-sampled light curves, in particular from 0.5 to 4 days after the burst, suggest a jet break at 3.7 days, corresponding to a jet opening angle of ~7.0 degrees, and implying an intrinsic GRB energy in the 1 - 10,000 keV band of around E = (6.3 - 6.9)x 10^(51) erg (based on the fluences measured by the gamma-ray detectors of the IPN network). GRB 070125 is among the brightest afterglows observed to date. The spectral energy distribution implies a host extinction of Av < 0.9 mag. Two rebrightening episodes are observed, one with excellent time coverage, showing an increase in flux of 56% in ~8000 seconds. The evolution of the afterglow light curve is achromatic at all times. Late-time observations of the afterglow do not show evidence for emission from an underlying host galaxy or supernova. Any host galaxy would be subluminous, consistent with current GRB host-galaxy samples. Evidence for strong Mg II absorption features is not found, which is perhaps surprising in view of the relatively high redshift of this burst and the high likelihood for such features along GRB-selected lines of sight.Comment: 50 pages, 9 figures, 5 tables Accepted to the Astrophysical Journa

Spitzer 24 um Survey for Dust Disks around Hot White Dwarfs

Two types of dust disks around white dwarfs (WDs) have been reported: small dust disks around cool metal-rich WDs consisting of tidally disrupted asteroids, and a large dust disk around the hot central WD of the Helix planetary nebula (PN) possibly produced by collisions among Kuiper Belt-like objects. To search for more dust disks of the latter type, we have conducted a Spitzer MIPS 24 um survey of 71 hot WDs or pre-WDs, among which 35 are central stars of PNe (CSPNs). Nine of these evolved stars are detected and their 24 um flux densities are at least two orders of magnitude higher than their expected photospheric emission. Considering the bias against detection of distant objects, the 24 um detection rate for the sample is >~15%. It is striking that seven, or ~20%, of the WD and pre-WDs in known PNe exhibit 24 um excesses, while two, or 5-6%, of the WDs not in PNe show 24 um excesses and they have the lowest 24 um flux densities. We have obtained follow-up Spitzer IRS spectra for five objects. Four show clear continuum emission at 24 um, and one is overwhelmed by a bright neighboring star but still show a hint of continuum emission. In the cases of WD 0950+139 and CSPN K1-22, a late-type companion is present, making it difficult to determine whether the excess 24 um emission is associated with the WD or its red companion. High-resolution images in the mid-IR are needed to establish unambiguously the stars responsible for the 24 um excesses.Comment: 45 pages, 18 figures, 6 tables, accepted for publication in the September 2011 edition of the Astronomical Journa

A very luminous magnetar-powered supernova associated with an ultra-long gamma-ray burst

A new class of ultra-long duration (>10,000 s) gamma-ray bursts has recently been suggested1,2,3. They may originate in the explosion of stars with much larger radii than normal long gamma-ray bursts3,4 or in the tidal disruptions of a star3. No clear supernova had yet been associated with an ultra-long gamma-ray burst. Here we report that a supernova (2011kl) was associated with the ultra-long duration burst 111209A, at z=0.677. This supernova is more than 3 times more luminous than type Ic supernovae associated with long gamma-ray bursts5,6,7, and its spectrum is distinctly different. The continuum slope resembles those of super-luminous supernovae8,9, but extends farther down into the rest-frame ultra-violet implying a low metal content. The light curve evolves much more rapidly than super-luminous supernovae. The combination of high luminosity and low metal-line opacity cannot be reconciled with typical type Ic supernovae, but can be reproduced by a model where extra energy is injected by a strongly magnetized neutron star (a magnetar), which has also been proposed as the explanation for super-luminous supernovae20,20a

Determining the dust composition of GRB host galaxies

Gamma ray burst (GRB) spectral energy distributions (SEDs) are typically fit with a dust extinction template with pre-determined graphite-to-silicate ratios (see Figure 1) to extract the approximate dust composition. These templates are an oversimplification of composition and often rely on non-physical fitting parameters. We instead propose a model that allows for the ratio of graphite to silicate to vary, enabling us to detect smaller amounts of graphite in GRB host galaxies than previously determined with physical model parameters relying on fixed ratios of graphite to silicate column densities. Data from the literature was used to construct GRB SEDs. The optical depths of astronomical silicates and graphite were calculated for a typical size distribution of dust particles and fit to the SEDs using our fitting function. A linear regression produced the best-fit and errors to the dust column densities