Swift observations of Cep X-4 in outburst
ATel #6243; P. A. Evans (U. Leicester), A. P. Beardmore (U. Leicester), H. A. Krimm (CRESST/GSFC/USRA) and A. Y. Lien (GSFC/CRESST/UMBC) on behalf of the Swift team
on 17 Jun 2014; 15:59 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Phil Evans (pae9@star.le.ac.uk)
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 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) × 10-7 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) × 1022 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 1.3× 10-13 Hz/s; a factor of 3 higher than measured in that paper between 1993 and 1997.
