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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 (kpa@star.le.ac.uk)

Subjects: Optical, X-ray, Neutron Star, Transient

Referred to by ATel #: 7852, 7949

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 to:

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 12827814717.43 +/- 0.05
u28640111317.46 +/- 0.10
u43946101168118.22 +/- 0.05
u101131039327617.97 +/- 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).