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AAVSO 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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