MAXI J1659-152: Swift localization and monitoring

ATel #2877; J. A. Kennea (PSU), H. Krimm (CRESST/GSFC/USRA), V. Mangano (INAF-IASFPA), P. Curran (CEA-Saclay), P. Romano (INAF-IASFPA) and P. Evans (U. Leicester), D. N. Burrows (PSU)
on 28 Sep 2010; 13:04 UT
Distributed as an Instant Email Notice Request For Observations
Credential Certification: Jamie A. Kennea (kennea@astro.psu.edu)

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We report on Swift monitoring of the newly discovered Galactic Transient source MAXI J1659-152 (Negoro et al, 2010, ATEL #2873; Mangano et al., 2010, GCN #11296). MAXI J1659-152 was first detected in the Swift/BAT hard X-ray transient monitor on September 24th, 2010 at a low significance.  The 15-50 keV flux then rose sharply to 0.033 +/- 0.012 ct/s/cm^2 (150 mCrab) on September 25th, 2010.  The count rate continues to rise, although more slowly, and is now 0.046 +/- 0.003 ct/s/cm^2 (~200 mCrab).

Swift XRT has performed monitoring observations in WT (timing) mode of this transient since it triggered the Swift/BAT at 08:05UT on September 25th, 2010. The XRT light-curve of MAXI J1659-152 shows a steady rise since this time, rising from approximately 30 XRT counts/s at the time of the BAT detection, to ~200 XRT counts/s during on September 27th, 2010. The XRT light-curve can be roughly modeled as a constant+power-law, with the best fit value of the rising power law slope being 0.805+/-0.002.

The light-curve also shows frequent dips, which may be an indication of eclipses. At this time it is not possible to determine any periodicity from the XRT data, as it appears that if periodicity exists, it is close to being a multiple of the Swift orbit timescale, making measurement difficult. Further Swift observations may allow an eclipse period to be pinned down if present.

The spectrum of MAXI J1659-152 is well fit by an absorbed power-law model, with N_H = 2 x 10^21 cm^-2, and a photon index of ~1.7. However plotting the hardness ratio as a function of time shows that the source shows some evidence for softening as the source gets brighter.

In order to obtain an accurate localization of the transient, we uploaded an ~1ks imaging observation starting at 14:57UT on September 27th, 2010. Using 975 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 = 254.75650, -15.25848 which is equivalent to:

RA (J2000):  16 59 1.56
Dec (J2000): -15 15 30.5

with an uncertainty of 1.7 arcsec (radius, 90% confidence). This position lies 2.8 arcseconds from the proposed UVOT counterpart (Marshal 2010, GCN #11298), given the errors on the XRT and UVOT positions, this equates to a 2.6 sigma difference, we therefore suggest that the XRT position is consistent with the UVOT counterpart to MAXI J1659-152.

Swift monitoring observations of this source are on-going. Multi-wavelength observations of this source are encouraged in order to determine the nature of MAXI J1659-152.

This work made use of data supplied by the UK Swift Science Data Centre at the University of Leicester (Evans et al. 2009, MNRAS, 397, 1177).