Discovery of the Very Red Near-Infrared and Optical Afterglow of the Short-Duration GRB 070724A

[Abridged] We report the discovery of the near-infrared and optical afterglow of the short-duration gamma-ray burst GRB070724A. The afterglow is detected in i,J,H,K observations starting 2.3 hr after the burst with K=19.59+/-0.16 mag and i=23.79+/-0.07 mag, but is absent in images obtained 1.3 years later. Fading is also detected in the K-band between 2.8 and 3.7 hr at a 4-sigma significance level. The optical/near-IR spectral index, beta_{O,NIR}=-2, is much redder than expected in the standard afterglow model, pointing to either significant dust extinction, A_{V,host}~2 mag, or a non-afterglow origin for the near-IR emission. The case for extinction is supported by a shallow optical to X-ray spectral index, consistent with the definition for ``dark bursts'', and a normal near-IR to X-ray spectral index. Moreover, a comparison to the optical discovery magnitudes of all short GRBs with optical afterglows indicates that the near-IR counterpart of GRB070724A is one of the brightest to date, while its observed optical emission is one of the faintest. In the context of a non-afterglow origin, the near-IR emission may be dominated by a mini-supernova, leading to an estimated ejected mass of M~10^-4 Msun and a radioactive energy release efficiency of f~0.005 (for v~0.3c). However, the mini-SN model predicts a spectral peak in the UV rather than near-IR, suggesting that this is either not the correct interpretation or that the mini-SN models need to be revised. Finally, the afterglow coincides with a star forming galaxy at z=0.457, previously identified as the host based on its coincidence with the X-ray afterglow position (~2" radius). Our discovery of the optical/near-IR afterglow makes this association secure.Comment: Submitted to ApJ; 10 pages, 5 figures, 1 tabl

X-ray Brightening and UV Fading of Tidal Disruption Event ASASSN-15oi

We present late-time observations by Swift and XMM-Newton of the tidal disruption event (TDE) ASASSN-15oi that reveal that the source brightened in the X-rays by a factor of $\sim10$ one year after its discovery, while it faded in the UV/optical by a factor of $\sim 100$. The XMM-Newton observations measure a soft X-ray blackbody component with $kT_{\rm bb} \sim 45$ eV, corresponding to radiation from several gravitational radii of a central $\sim 10^6 M_\odot$ black hole. The last Swift epoch taken almost 600 days after discovery shows that the X-ray source has faded back to its levels during the UV/optical peak. The timescale of the X-ray brightening suggests that the X-ray emission could be coming from delayed accretion through a newly forming debris disk, and that the prompt UV/optical emission is from the prior circularization of the disk through stream-stream collisions. The lack of spectral evolution during the X-ray brightening disfavors ionization breakout of a TDE "veiled" by obscuring material. This is the first time a TDE has been shown to have a delayed peak in soft X-rays relative to the UV/optical peak, which may be the first clear signature of the real-time assembly of a nascent accretion disk, and provides strong evidence for the origin of the UV/optical emission from circularization, as opposed to reprocessed emission of accretion radiation.Comment: Accepted for publication in ApJ Letter

The physical origin of optical flares following GRB 110205A and the nature of the outflow

The optical emission of GRB 110205A is distinguished by two flares. In this work we examine two possible scenarios for the optical afterglow emission. In the first scenario, the first optical flare is the reverse shock emission of the main outflow and the second one is powered by the prolonged activity of central engine. We however find out that it is rather hard to interpret the late ($t>0.1$ day) afterglow data reasonably unless the GRB efficiency is very high ($\sim 0.95$). In the second scenario, the first optical flare is the low energy prompt emission and the second one is the reverse shock of the initial outflow. Within this scenario we can interpret the late afterglow emission self-consistently. The reverse shock region may be weakly magnetized and the decline of the second optical flare may be dominated by the high latitude emission, for which strong polarization evolution accompanying the quick decline is possible, as suggested by Fan et al. in 2008. Time-resolved polarimetry by RINGO2-like polarimeters will test our prediction directly.Comment: Accepted for publication in RAA,8 pages, 2 figure

Establishing a Place at the Table: The Growing Influence of Non-Governmental Organizations in International Negotiations

Honors (Bachelor's)Political ScienceUniversity of Michiganhttp://deepblue.lib.umich.edu/bitstream/2027.42/91812/1/cenkobry.pd

The No-Reflow Phenomenon: Clinical and Angiographic Correlates

No-reflow occurring during PCI has been associated with poor outcomes. The objectives of this study were to evaluate the incidence of no-reflow as independent predictor of adverse events and to assess whether baseline pre-procedural treatment options may affect clinical outcomes. Data were derived from the ISACS-TC (NCT01218776) registry, a prospective survey of patients presenting with ACS over a 5-year period. Data were prospectively collected from 5997 patients undergoing PCI, identifying those with no-reflow, and analyzed their treatments and outcomes. No-reflow was defined as post-PCI TIMI flow grade 0-1, in the absence of post-procedural significant (≥25%) residual stenosis, abrupt vessel closure, dissection, perforation, thrombus of the original target lesion, or epicardial spasm. The outcome measure was in-hospital mortality. No-reflow was identified in 128 (2.1%) patients. On multivariate analysis, patients with no-reflow were more likely to be older (OR:1.20, 95%CI:1.01–1.44) and to be admitted with a diagnosis of ST-elevation myocardial infarction (OR:2.96, 95%CI:1.85–4.72). No-reflow was highly predictive of in-hospital mortality (17.2% vs. 4.2%, P<0.001) and remained a significant independent predictor of death after adjustment for demographic and clinical variables (OR:4.60,95%CI:2.61–8.09). Multivariable regression analysis was also performed to identify independent relationship between pre-procedural treatment regimens, angiographic characteristics and no-reflow phenomenon. Administration of pre-procedural unfractioned heparin, showed a strong inverse predictive value in terms of post-PCI TIMI flow and no-reflow phenomenon (OR: 0.65, 95%CI:0.43–0.99). Similarly, a 600-mg loading dose of clopidogrel showed a trend associated with a reduction in the likehood of no-reflow (OR:0.61,95%CI:0.37–1.00). Angiographic characteristics associated with no-reflow phenomenon were stenosis≥50% of the right coronary artery, presence of multivessel coronary disease and pre-procedural TIMI blood flow grade 0-1. In conclusion, no-reflow during PCI is a strong independent predictor of mortality. Pre-procedural administration of 600-mg loading dose of clopidogrel and/or unfractioned heparin is associated with reduced incidence of no-reflow

Gamma-Ray Bursts as a Threat to Life on Earth

Gamma-ray bursts (GRBs) are likely to have made a number of significant impacts on the Earth during the last billion years. The gamma radiation from a burst within a few kiloparsecs would quickly deplete much of the Earth's protective ozone layer, allowing an increase in solar ultraviolet radiation reaching the surface. This radiation is harmful to life, damaging DNA and causing sunburn. In addition, NO2 produced in the atmosphere would cause a decrease in visible sunlight reaching the surface and could cause global cooling. Nitric acid rain could stress portions of the biosphere, but the increased nitrate deposition could be helpful to land plants. We have used a two-dimensional atmospheric model to investigate the effects on the Earth's atmosphere of GRBs delivering a range of fluences, at various latitudes, at the equinoxes and solstices, and at different times of day. We have estimated DNA damage levels caused by increased solar UVB radiation, reduction in solar visible light due to NO2 opacity, and deposition of nitrates through rainout of HNO3. In this paper I give a concise review of this work and discuss current and future work on extending and improving our estimates of the terrestrial impact of a GRB.Comment: 8 pages; submitted as part of proceedings of the ESLAB Symposium on 'Cosmic Cataclysms and Life', November 2008; to be published in a special issue of the International Journal of Astrobiolog

The Anticipated Supernova Associated with GRB090618

We use the cannonball model of gamma ray bursts (GRBs) and public data from the first day of observations of GRB 090618 to predict its X-ray and optical lightcurves until very late times, and, in particular, the emergence of a photometric and spectroscopic signature of an SN akin to SN1998bw in its optical afterglow with an anticipated peak brightness of magnitude 23 in the R band around July 10, 2009, if extinction in the host galaxy can be neglected.Comment: 10 pages, 2 Figure

Observational implications of gamma-ray burst afterglow jet simulations and numerical light curve calculations

We discuss jet dynamics for narrow and wide gamma-ray burst (GRB) afterglow jets and the observational implications of numerical simulations of relativistic jets in two dimensions. We confirm earlier numerical results that sideways expansion of relativistic jets during the bulk of the afterglow emission phase is logarithmic in time and find that this also applies to narrow jets with half opening angle of 0.05 radians. As a result, afterglow jets remain highly nonspherical until after they have become nonrelativistic. Although sideways expansion steepens the afterglow light curve after the jet break, the jet edges becoming visible dominates the jet break, which means that the jet break is sensitive to the observer angle even for narrow jets. Failure to take the observer angle into account can lead to an overestimation of the jet energy by up to a factor 4. This weakens the challenge posed to the magneter energy limit by extreme events such as GRB090926A. Late time radio calorimetry based on a spherical nonrelativistic outflow model remains relevant when the observer is approximately on-axis and where differences of a few in flux level between the model and the simulation are acceptable. However, this does not imply sphericity of the outflow and therefore does not translate to high observer angles relevant to orphan afterglows. For more accurate calorimetry and in order to model significant late time features such as the rise of the counterjet, detailed jet simulations remain indispensable.Comment: 7 Figures. Replaced with accepted version. Significantly expanded, including additional discussion of time scale