Continued activity of IGR J00291+5934 observed by Swift XRT and UVOT.
ATel #7849; Paul Kuin (MSSL/UCL), Kim Page (U. Leicester), Sergio Campana (INAF-Osservatorio Astronomico di Brera) and Silvia Zane (MSSL/UCL)
on 28 Jul 2015; 10:51 UT
Credential Certification: Kim Page (firstname.lastname@example.org)
Subjects: Optical, X-ray, Neutron Star, Transient
Swift BAT triggered on the accreting neutron star IGR J00291+5934 on
2015-07-25 02:12:4.94 UT (Swift trigger 650221; Barthelmy et al. GCN Circ. #18061, Sakamoto et al.,
GCN Circ. #18067). This follows an earlier report of its reactivation
by Sanna et al. (ATEL #7836).
The XRT began observing the field of IGR J00291+5934 (=V1037 Cas) 119.5
seconds after the BAT trigger. Using 1797 s of XRT Photon Counting mode
data and 2 UVOT images, we find an astrometrically corrected X-ray
position (using the XRT-UVOT alignment and matching UVOT field sources to
the USNO-B1 catalogue): RA, Dec = 7.26175, +59.57204, which is equivalent
RA (J2000): 00h 29m 02.82s
Dec (J2000): +59d 34' 19.3"
with an uncertainty of 2.1 arcsec (radius, 90% confidence).
The X-ray source was initially very bright, peaking around 300 count s^-1.
The first snapshot of data, entirely in Windowed Timing (WT) mode, between 123 and 400 s after the trigger, can be parameterised by an exponential decay with an e-folding time of t^-(115
+/- 4) s. The following two snapshots, between 4.4 and 10.4 ks (mainly in Photon Counting - PC - mode) are
consistent with a constant level of 8.8 (+/- 0.3) count s^-1.
A spectrum formed from the WT mode data can be fitted with an absorbed
black body with mean temperature 1.21 (+0.02, -0.02) keV. The
best-fitting absorption column is 2.3 (+0.4, -0.4) x 10^21 cm^-2. The absorbed BB fit is significantly better than an absorbed power-law,
with Δ χ2 = 26 for the same number of degrees of freedom.
This is consistent with being the cooling tail of a type I burst.
The PC mode spectrum can be either fitted with a power law (NH=1.0+/-0.2 10^22 cm^-2; Gamma=1.7+/-0.2) or a black body
(NH=2.1+/-1.0 10^21 cm^-2; kT=1.0+/-0.1 keV).
The 0.3-10 keV unabsorbed PC flux is
3.0-5.6 x 10^-11 erg cm^-2 s^-1, depending on the assumed spectral model.
This is the first type I X-ray burst observed from IGR J00291+5934.
A source consistent with the position given by the XRT and by Fox and Kulkarni (ATEL #354) was found in the UVOT data.
The UVOT position of the trigger is:
RA (J2000) = 00:29:03.06 = 7.26275 (deg.)
Dec (J2000) = +59:34:19.1 = 59.57198 (deg.)
with an estimated uncertainty of 0.43 arc sec. (radius, 90% confidence).
Preliminary magnitudes using the UVOT photometric system (Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the early exposures are:
|Filter|| T_start(s)||T_stop(s)||Exp(s)|| Mag|
|white ||128||278||147||17.43 +/- 0.05|
|u||286||401||113||17.46 +/- 0.10|
|u||4394||6101||1681||18.22 +/- 0.05|
|u||10113||10393||276||17.97 +/- 0.08|
The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.71 in the direction of the burst
(Schlegel et al. 1998).