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TITLE: GCN CIRCULAR | |
NUMBER: 16332 | |
SUBJECT: Swift detection of a bright X-ray source in M31 | |
DATE: 14/05/27 22:15:08 GMT | |
FROM: Scott Barthelmy at NASA/GSFC <scott@milkyway.gsfc.nasa.gov> | |
S. D. Barthelmy (GSFC), W. H. Baumgartner (GSFC/UMBC), | |
S. B. Cenko (GSFC), V. D'Elia (ASDC), D. Malesani (DARK/NBI), | |
V. Mangano (PSU), F. E. Marshall (NASA/GSFC) and | |
B. Sbarufatti (INAF-OAB/PSU) report on behalf of the Swift Team: | |
At 21:24:27 UT, the Swift Burst Alert Telescope (BAT) triggered and | |
located a source in M31 (NGC224) (trigger=600114). Swift slewed immediately | |
to the target. The BAT on-board calculated location is | |
RA, Dec 10.402, +41.556, which is | |
RA(J2000) = 00h 41m 37s | |
Dec(J2000) = +41d 33' 23" | |
with an uncertainty of 3 arcmin (radius, 90% containment, including | |
systematic uncertainty). This is an image trigger of 64 second duration, | |
with significance 6.18 sigma, which is lower than typical blind detection | |
significance. The BAT light curve does not show obvious structure, which | |
is typical for image triggers. | |
The XRT began observing the field at 21:27:05.4 UT, 157.5 seconds after | |
the BAT trigger. Using promptly downlinked data we find an X-ray source | |
with an enhanced position: RA, Dec 10.4292, 41.5734 which is equivalent to: | |
RA(J2000) = 00h 41m 43.02s | |
Dec(J2000) = +41d 34' 24.3" | |
with an uncertainty of 2.9 arcseconds (radius, 90% containment). This | |
location is 96 arcseconds from the BAT onboard position, within the BAT | |
error circle. The XRT position may be improved as | |
more data are received; the latest position is available at | |
http://www.swift.ac.uk/sper. This position is 4.0 arcseconds from that | |
of a known X-ray source: 3XMM J004143.0+413420 in the XMM-NEWTON XMMSSC | |
catalogue. | |
A power-law fit to a spectrum formed from promptly downlinked event | |
data gives a column density consistent with the Galactic value of 1.81 | |
x 10^21 cm^-2 (Willingale et al. 2013). | |
The associated source in the 3rd XMM catalog has a spectrum and variability | |
which is consistent with a low-mass X-ray binary (Stiele 2010). However, | |
the target is also spatially consistent with a globular cluster FMZ2005-7 of M31. | |
There may be multiple X-ray binaries within the same globular cluster. | |
The XRT count rate is approximately 30 counts per second. Assuming a typical | |
conversion factor to flux, the 0.3-10 keV flux is approximately 1e-9 erg/cm2/s | |
(uncertain to a factor of 2x). At the distance of Andromeda, this would be a | |
luminosity of 7e40 erg/s (uncertain to the same factor). | |
This luminosity is clearly super-Eddington for a ~solar mass compact object, | |
so either the object is of ULX class, or it is significantly beamed. | |
UVOT took a finding chart exposure of 150 seconds with the White filter | |
starting 162 seconds after the BAT trigger. UVOT detects a 16.9 mag source | |
at the position | |
RA (J2000) = 00:41:43.10 = 10.42957 | |
Dec(J2000) =+41:34:20.4 = 41.57232 | |
with a 90% confidence error radius of 0.61 arc sec. | |
This position is 4 arc seconds from the center of the XRT error circle | |
and consistent with the position of the globular cluster. | |
No correction has been made for the expected extinction corresponding to E(B-V) of 0.06. |
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