GCN Circular 22405

D. N. Burrows (PSU), A. D'Ai (INAF-IASFPA), V. D'Elia (ASDC),
A. Deich (PSU), J.D. Gropp (PSU), J. A. Kennea (PSU),
D. M. Palmer (LANL), B. Sbarufatti (PSU) and A. Tohuvavohu (PSU)
report on behalf of the Neil Gehrels Swift Observatory Team:

At 22:58:59 UT, the Swift Burst Alert Telescope (BAT) detected
the known RS CVn star HD 8357 (trigger=809217) in a 320s image. 
Swift slewed immediately to the location.  The BAT on-board 
calculated location is 
RA, Dec 20.786, +7.444 which is 
   RA(J2000) = 01h 23m 09s
   Dec(J2000) = +07d 26' 38"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  As is usual for an image trigger,
no obvious variation is detectable in the immediately available
BAT rates lightcurve. 

The XRT began observing the field at 23:05:26.8 UT, 387.4 seconds after
the BAT trigger. XRT found a bright X-ray source located at RA, Dec
20.7371, 7.4200 which is equivalent to:
   RA(J2000)  = +01h 22m 56.90s
   Dec(J2000) = +07d 25' 12.0"
with an uncertainty of 4.8 arcseconds (radius, 90% containment). This
location is 194 arcseconds from the BAT onboard position, outside the
BAT error circle. This position is 1.9 arcseconds from that of a known
X-ray source: 3XMM J012256.8+072510 in the XMM-NEWTON XMMSSC catalogue
and is consistent with the position of HD 8357. 

Assuming a typical GRB power-law spectrum (NH=3e21 cm^-2, gamma=2), the
XMM catalogue entry for this object is equivalent to a 0.2-10 keV flux of
6.2e-11 erg cm^-2 s^-1; the flux in the 2.5 s XRT image was 7.63e-09
erg cm^-2 s^-1. No event data are yet available to determine the column
density using X-ray spectroscopy. 

Because HD 8357 is a known bright star (V=7.31) UVOT did not
take data. 

This appears to be the first time that this source has triggered
BAT in the on-board imaging.