Swift/XRT confirmation of activity from H 1658-298, no detection of MAXI J1327-627
ATel #7957; A. Bahramian, C. O. Heinke (Alberta), R. Wijnands (Amsterdam)
on 26 Aug 2015; 21:22 UT
Credential Certification: Arash Bahramian (bahramia@ualberta.ca)
Subjects: X-ray, Binary, Neutron Star, Transient
Referred to by ATel #: 10169
We report our follow up Swift observations of reported enhanced activity by MAXI from tentative sources MAXI J1327-627 and MAXI J1702-301 (ATel #7946).
MAXI reported enhanced activity from the direction of H 1658-298 (or MXB 1659-298) on 2015, August 21st (ATel #
7943). This was later confirmed using INTEGRAL observations (ATel #
7946). We observed this region with Swift/XRT in PC mode on August 25th. H 1658-298 is clearly detected in our observation. Performing XRTCentroid gives the coordinates of this source as RA = 17:02:06.50 and Dec = -29:56:40.86 (with radial uncertainty of 4 arcsec). This is consistent with the published coordinates of H 1658-298 (Wijnands et al. 2003, ApJ, 594, 952).
H 1658-298 is an eclipsing X-ray transient which has exhibited type-I X-ray bursts in the past (Lewin et al. 1976, IAU Circ. 2994, Cominsky & Wood 1984, ApJ, 283, 765). Due to heavy pile-up in our observation, we extracted a spectrum following the Swift/XRT pile up thread (http://www.swift.ac.uk/analysis/xrt/pileup.php). We fit the spectrum with an absorbed power-law, assuming Wilms et al. (2000, ApJ, 542, 914) abundances. This resulted in an acceptable fit with reduced chi^2 of 0.7167 for 78 d.o.f. We find N_H = 4.1(+/-0.7)e21 cm^-2, photon index of 1.7 (+/-0.1) and an unabsorbed flux of 5.8(+/-0.3)e-10 erg/s/cm2 in 0.5-10 keV band, implying a luminosity (for a 10 kpc distance, e.g. Oosterbroek et al. 2001, A&A, 376, 532) of 7e36 erg/s.
To compare hydrogen column density with previous studies, we also performed a similar fit assuming Anders et al. 1989 (GeCoA, 53, 197) abundances and we found N_H = 3.3(+/-0.5)e21 cm^-2. This is higher than measured in quiescence by Cackett et al. (2008, ApJ, 687, L87), but consistent with suggested variations of N_H observed in later observations (Cackett et al. 2013 ApJ, 774, 131). We also looked for flares, type-I X-ray bursts and dips in the XRT/PC lightcurve, but did not find any.
We also observed the vicinity of MAXI J1327-627 with Swift/XRT. Due to large uncertainties in position of the initial detection (~20 arcmin around RA = 13:27:29, Dec=-62:47:39), we performed a 4-tile set of 500 second observations to cover the error circle (Obs.IDs: 34006-9). We found no sources in this region down to an X-ray flux (unabsorbed, assuming photon index of 1.7, N_H=1e22) of 2e-12 erg/s/cm2 in 0.5-10 keV band. It is unclear whether the MAXI J1327-627 detection was a very short outburst, a spurious detection, or produced by another, more distant source.
We thank the Swift team for rapidly scheduling our observations.