TITLE:   GCN CIRCULAR
NUMBER:  16414
SUBJECT: Swift trigger 601928 is V490 Cep: BAT+XRT refined analysis.
DATE:    14/06/17 15:58:58 GMT
FROM:    Phil Evans at U of Leicester  <pae9@leicester.ac.uk>

P. A. Evans (U. Leicester), A.P. Beardmore (U. Leicester), H. A. Krimm
(CRESST/GSFC/USRA) and A. Y. Lien (GSFC/CRESST/UMBC) report on behalf of 
the Swift team:

At 06:37:31 UT on 2014 June 17 Swift-BAT triggered on a source 
consistent with the location of V490 Cep (= Cep X-4 = Ginga 2138+56)
which the MAXI team reported to be in outburst in ATEL #6212.
This trigger was reported in GCN Circ. 16413. The refined BAT position 
is RA, Dec = 324.784, 56.949 deg which is equivalent to:
    RA(J2000)  =  21h 39m 08.2s
    Dec(J2000) = +56d 56' 56.9"
with an uncertainty of 4.1 arcmin, (radius, 90% confidence).
The partial coding was 89%.

XRT finds a single X-ray source in the BAT error circle, with an 
enhanced XRT position of RA,Dec= 324.87743, 56.98608 which is equivalent to:

   RA (J2000)  = 21h 39m 30.58s
   Dec (J2000) = +56d 59' 09.9"

with an uncertainty of 1.7 arcsec (radius, 90% confidence). This is 1 
arcsec away from the catalogued position of V490 Cep in SIMBAD.

This source has been regularly monitored by the BAT Transient Monitor
(Krimm et al., 2013, ApJSS 209,14) and has been seen to be steadily 
rising, with an average flux at present of 80 mCrab. The observation
which triggered BAT was that with the highest significance detection of
this source by the BAT during the current outburst.

The time-averaged BAT spectrum from T+0.00 to T+303.04 sec is best fit 
by a simple power-law model.  The power law index of the time-averaged 
spectrum is 3.52 +/- 0.63.  The fluence in the 15-150 keV band is
3.9 (+/- 1.1) e-7 erg cm^-2

The Windowed Timing mode XRT spectrum can be modelled with an absorbed
power-law, with a photon index of 1.01 (+/- 0.05) and an absorption 
column of 1.32 (+0.10, -0.09) e22 cm^-2. The mean 0.3-10 keV count-rate 
is ~15 ct/sec, which is consistent with the MAXI-GSC count-rate of 0.11
ct/sec reported in ATEL #6212, assuming the WT mode spectrum.

The X-ray data show clear signs of modulation, with a period of 66.58 
+/- 0.22 s (90% confidence). Compared to the measurements reported in 
Wilson, Finger & Scott (1999, ApJ, 511, 367), this implies a spin-down 
rate of 1.3e-13 Hz/s; a factor of 3 higher than measured in that paper 
between 1993 and 1997.

This circular is an official product of the Swift team.