307 research outputs found
Soft Gamma-Ray Repeaters in Nearby Galaxies: Rate, Luminosity Function, and Fraction among Short Gamma-Ray Bursts
It was suggested that some of the short-duration gamma-ray bursts (GRBs) are giant flares of soft gamma-ray repeaters (SGRs) in nearby galaxies. To test this hypothesis, I have constructed a sample of 47 short GRBs, detected by the Interplanetary Network (IPN), for which the position is constrained by at least one annulus on the celestial sphere. For each burst, I have checked whether its IPN 3 σ error region coincides with the apparent disk of one of 316 bright, star-forming galaxies found within 20 Mpc. I find a single match of GRB 000420B with M74, which could, however, be due to a chance coincidence. I estimate the IPN efficiency as a function of fluence and derive the galaxy sample completeness. I find that assuming there is a cutoff in the observed energy distribution of SGR flares at ≤10^(47) ergs, the fraction of SGRs among short GRBs with fluence above ~10^(-5) ergs cm^(-2) is <16% (95% confidence). I estimate the number of active SGRs in each one of the galaxies in the sample, and combine it with the distances to these galaxies, the IPN efficiency, and the SGR flare energy distribution, to derive the rate of giant flares with energy above 4 × 10^(46) ergs. I find that the rate of such giant flares is about (0.4-5) × 10^(-4) yr^(-1) per SGR. This rate is marginally consistent with the observed Galactic rate. Comparison of the Galactic rate with the inferred extragalactic rate implies a gradual cutoff (or steepening) of the flare energy distribution at ≾3 × 10^(46) ergs (95% confidence). Using the Galactic SGR flare rate, I set a lower limit of 1% on the fraction of SGR flares among short GRBs
Calibrated griz magnitudes of Tycho stars: All-sky photometric calibration using bright stars
Photometric calibration to 5% accuracy is frequently needed at arbitrary
celestial locations; however, existing all-sky astronomical catalogs do not
reach this accuracy and time consuming photometric calibration procedures are
required. I fit the Hipparcos B_T and V_T magnitudes along with the 2MASS J, H,
and K magnitudes of Tycho-2 catalog-stars with stellar spectral templates. From
the best fit spectral template derived for each star, I calculate the synthetic
SDSS griz magnitudes and constructed an all-sky catalog of griz magnitudes for
bright stars (V<12). Testing this method on SDSS photometric telescope
observations, I find that the photometric accuracy for a single star is usually
about 0.12, 0.12, 0.10 and 0.08 mag (1 sigma), for the g, r, i, and z-bands,
respectively. However, by using ~10 such stars, the typical errors per
calibrated field (systematic + statistical) can be reduced to about 0.04, 0.03,
0.02, and 0.02,mag, in the g, r, i, and z-bands, respectively. Therefore, in
cases for which several calibration stars can be observed in the field of view
of an instrument, accurate photometric calibration is possible.Comment: 3 pages, PASP, in pres
Evidence for a New Class of Extreme UV Sources
Most of the sources detected in the extreme ultraviolet (EUV; 100 Ang to 600
Ang) by the Rosat WFC and EUVE all-sky surveys have been identified with active
late-type stars and hot white dwarfs that are near enough to escape absorption
by interstellar gas. However, about 15% of EUV sources are as of yet
unidentified with any optical counterparts. We examine whether the unidentified
EUV sources may consist of the same population of late-type stars and white
dwarfs. We present B and R photometry of stars in the fields of seven of the
unidentified EUV sources. We detect in the optical the entire main-sequence and
white-dwarf population out to the greatest distances where they could still
avoid absorption. We use colour-magnitude diagrams to demonstrate that, in most
of the fields, none of the observed stars have the colours and magnitudes of
late-type dwarfs at distances less than 100 pc. Similarly, none are white
dwarfs within 500 pc that are hot enough to be EUV-emitters. The unidentified
EUV sources we study are not detected in X-rays, while cataclysmic variables,
X-ray binaries, and active galactic nuclei generally are. We conclude that some
of the EUV sources may be a new class of nearby objects, that are either very
faint at optical bands or which mimic the colours and magnitudes of distant
late-type stars or cool white dwarfs. One candidate for optically faint objects
is isolated old neutron stars, slowly accreting interstellar matter. Such
neutron stars are expected to be abundant in the Galaxy, and have not been
unambiguously detected.Comment: 8 pages, incl. figures, MNRAS, accepte
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