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SWIFT J195456.7+261301: a flaring coronal X-ray emitting star

ATEL # 1103; J. A. Kennea (PSU), C. Pagani (PSU), C. B. Markwardt (CRESST/U.Md./GSFC), P. Evans (U. Leicester), G. Tagliaferri (INAF-OABr) on behalf of the Swift Team.
on 12 Jun 2007; 17:48 UT
Distributed as an Instant Email Notice (Transients)
Password Certification: Jamie A. Kennea (kennea@astro.psu.edu)

Subjects: X-ray, Transients

On June 10th, 2007 at 20:52:26 UT, the Swift BAT instrument triggered on the X-ray transient source SWIFT J195509.6+261406, which was initially thought to a GRB (Pagani et al., GCN #6489), but is more likely to be be a galactic X-ray transient (Markwardt et al., ATEL #1102). In the field of this transient, we detect a previously unknown variable X-ray source at the following location:

RA (J2000) = 19h 54m 56.78s,
Dec (J2000) = +26d 13' 01.13",

with error radius of 3.7 arcsec (90% uncertainty), designated SWIFT J195456.7+261301. This position lies 2.2" from a B=14.6 USNO-B1.0 star and is outside of the BAT error circle for GRB 070610/Swift J195509.6+261406.

This X-ray source has a quiescent flux of ~3x10-13 erg/s/cm2 (0.2-10 keV), however during an observation starting at June 12th, 2007 at 18:35 UT, the source brightened and appeared to be in the process of decaying from a bright flare. The X-ray flux from the source decays from 3x10-12 erg/s/cm2 to ~4.5x10-13 erg/s/cm2 during the 43 minute observation window. Due to the small observing window, it is not clear when the flare began or how long it lasted, however in a follow up observation 6.3 hours later the source was back to its quiescent level.

Analysis of the combined spectrum of the source shows the presence of blended lines, suggestive of coronal X-ray source. However, the best model fit to this source is a thermal plasma model combined with a warmer thermal continuum. Fitting a 2 component mekal model with the abundance fixed at 0.1 of the solar value (a typical value for a coronal X-ray emitter), we obtain the following temperatures for the combined data set.

kT1 = 0.56 (+0.11/-0.22) keV
kT2 = 4.9 (+7.7/-2.4) keV

These temperatures are highly consistent with the values expected for a coronal X-ray emitter with a hot thermal component from a flare. Although statistics are poor, comparing the combined spectrum with the non- flare spectrum suggests that the hotter thermal component is not required during quiescence,  supporting the idea that the origin of the hot thermal component is the flare event.

We therefore suggest that SWIFT J19546.7+261301 is a previously undetected flaring coronal X-ray emitting star. We note that SWIFT J195509.6+261406 (ATEL #1102) is well fit by a power law spectrum, so although both X-ray sources are seen to flare, they are not likely to be similar objects.  Observations of this field are ongoing.


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