MAXI J1305-704: Swift follow-up observations
ATel #4034; J. A. Kennea (PSU), D. Altamirano (UvA), P. A. Evans (U Leicester), H. A. Krimm (CRESST/GSFC/USRA), P. Romano, V. Mangano (INAF-IASFPA), P. Curran (CEA-Saclay), K. Yamaoka (AGU) and H. Negoro (Nihon U.)
on 12 Apr 2012; 03:04 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Jamie A. Kennea (kennea@astro.psu.edu)
Subjects: X-ray, Transient
Referred to by ATel #: 4067
We present analysis of Swift follow-up observations of MAXI J1305-704 (Sato et al., ATEL #4024), which has been proposed to be a newly discovered Galactic black-hole binary (BHB; Greiner et al., ATEL #4030). Starting 16:07UT on April 11th, 2012, Swift observed MAXI J1305-704 for 1ks as a Target of Opportunity, with the Swift/XRT in Windowed Timing (WT) mode, to avoid pile-up. This follows initial observations made by Swift in a 4-pointing Photon Counting (PC) mode tiling observation on April 10th, initial results of which have been reported by Greiner et al. (ATEL #4030). In order to compare the latest observation, we have re-analyzed the data taken starting 16:04UT on April 10th, and first present those results:
Correcting the PC mode data for pile-up, we estimate that the XRT count rate on April 10th was 56 +/- 3 XRT count/s. Fitting the pile-up corrected spectrum with an absorbed power-law model, we find a photon index of 2.0 +/- 0.2, and an absorption column density of 2.3 +/- 0.4 x 10^21 cm^-2. We note that this photon index value is softer than that quoted by Greiner et al. (ATEL #4030), and believe this discrepancy likely due to the correction for pile-up in the PC mode data. The 0.3-10 keV flux is 2.0 x 10^-9 erg/s/cm^2 (uncorrected for absorption). A PSF fitted localization of the XRT source, utilizing UVOT to correct for astrometric errors finds a localization of RA,Dec (J2000) = 196.73138, -70.45144, equivalent to:
RA(J2000) = 13h 06m 55.53s,
Dec(J2000) = -70d 27' 01.8'',
with an estimated uncertainty of 2.0 arc-seconds radius (90% confidence). The GROND/UVOT counterpart lies 0.9 arc-seconds from the center of the XRT error circle.
The WT mode data shows that the source is at a similar brightness to the observation 24 hours earlier, the average WT count rate is 56 +/- 1 XRT count/s. Analysis of the power-spectrum of the WT mode data shows no significant detection of periodicities or QPOs.
The WT spectrum can be fit by a power-law model, with a similar photon index (2.2 +/- 0.1), however this fit shows significant residuals (reduced chi^2 = 3). An absorbed thermal disk model (diskbb) gives a significantly improved fit (reduced chi^2=1.48), with kT_in = 1.21 +/- 0.04 keV, suggesting that if this source is a BHB it is in the Soft/Thermal State. The addition of a power-law component to the diskbb model does not improve the fit. The non-detection of QPOs is also consistent with a BHB in the Soft/Thermal State. We note that fitting this thermal disk model to the earlier PC mode data gives a consistent fitted value of inner disk temperature: kT_in = 1.4 +/ 0.1 keV, although the lower statistics mean that this model is not significantly favored over the power-law model for the PC mode data. The WT mode flux is 1.8 x 10^-9 erg/s/cm^2 (0.3-10 keV), suggesting that the source flux and spectrum is not significantly evolving between the two observations.
MAXI J1305-704 is detected in the Swift/BAT hard X-ray transient monitor in the 15-50 keV band. The rate on April 10, 2012 was 0.0049 +/- 0.0006 (~25 mCrab). ÃÂ It is not clear at present whether the flux is rising.
Long term monitoring of MAXI J1305-704 with Swift is planned. We encourage follow-up with other instrumentation to confirm the nature of this source.
This work made use of data supplied by the UK Swift Science Data Centre at the University of Leicester.