543 research outputs found
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
Angstroms (0.1c), starting 1.95 days after the burst trigger. UBVRI photometry
obtained between 1 and 26 days post-burst 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.Comment: 5 pages, 2 figure
The Smith Cloud and its dark matter halo: Survival of a Galactic disc passage
The current velocity of the Smith Cloud indicates that it has undergone at
least one passage of the Galactic disc. Using hydrodynamic simulations we
examine the present day structure of the Smith Cloud. We find that a dark
matter supported cloud is able to reproduce the observed present day neutral
hydrogen mass, column density distribution and morphology. In this case the
dark matter halo becomes elongated, owing to the tidal interaction with the
Galactic disc. Clouds in models neglecting dark matter confinement are
destroyed upon disc passage, unless the initial cloud mass is well in excess of
what is observed today. We then determine integrated flux upper limits to the
gamma-ray emission around such a hypothesised dark matter core in the Smith
Cloud. No statistically significant core or extended gamma-ray emission are
detected down to a 95% confidence level upper limit of ph
cm s in the 1-300 GeV energy range. For the derived distance of
12.4 kpc, the Fermi upper limits set the first tentative constraints on the
dark matter cross sections annihilating into and
for a high-velocity cloud.Comment: 10 pages, 8 figures. Submitted to MNRA
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