Special Notice #120: SGR 0501+4516

August 25, 2008

On August 22, 2008, the SWIFT satellite started detecting short bursts from a new source.  As reported in Holland, et al. (GCN 8112) and Barthelmy, et. al. (GCN 8113), this source is a new soft gamma-ray repeater.  We only know of a handful of these exotic sources, thought to be neutron stars with extremely high magnetic fields, or magnetars.

Barthelmy, et al. (ATEL 1676) indicate that the afterglow from the first four short (less than 128ms) bursts was detected by the onboard XRT system, and gave a precise position of

05:01:06.78  +45:16:34.0  J2000

with an error radius of 2.6 arcsec (90% containment).  They also mention that the ROSAT source, 1RXS J050107.9+451631, with a catalogued location 12.2 arcsec from the SWIFT source, but with at 13arcsec uncertainty, may have been a previous detection of quiescent flux from this object.

About a hundred bursts have followed; Palmer (ATEL 1678) gives the times of the first 32 of these bursts.  Some have been in the kiloCrab range.

In ATEL 1677, Gogus et al. report results from a ToO triggered SGR program on RXTE, giving the spin period of the new magnetar as 5.769 +/- 0.004 seconds.

What is enticing for the amateur community, however, is that there have been reports of an optical counterpart.  Tanvir and Varricatt (GCN 8126) report UKIRT observations of the field, showing a faint K-band source (K=18.6) in the error circle with position of

05:01:06.75 +45:16:34.0 (J2000)

with tentative evidence of variability at the 0.2mag between sub-exposures. Denissenko (GCN 8114) reports on archival Palomar Observatory Sky Survey plates, and finds that a star at

05:01:06.47 +45:16:28.0 (J2000)

(also known as USNO-B1 1352-0128683), appears to be variable, with B ~ 18 and R ~ 16.  This object is 7arcsec from the XRT position, and therefore formally outside of the error circle. Halpern (GCN 8129) reports that 2 hours of MDM monitoring found this star to be constant at R=17.16 for the first hour, but then monotonically declining to R=17.35 over the next hour.

At R=17, this variable source is within range of most amateur CCD systems.  If you have a 30cm or larger telescope, we recommend obtaining filtered observations, either V or Rc, with at least S/N=20 per image, and monitor the field as long per night as possible.  For smaller telescopes, go unfiltered.

Report observations of the possible variable star as: USNO-B1.0 1352-0128683 (AUID 000-BFT-697)

If you see a new object appear during any of your exposures in the actual SWIFT error box, report that observation directly to HQ.

This Special Notice was written by A. Henden


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