Swift follow-up observations of unclassified ASCA sources

¹ University of Michigan, Department of Astronomy, 500 Church St, Ann Arbor, MI 48109, USA
e-mail: degenaar@umich.edu
² Astronomical Institute “Anton Pannekoek”, University of Amsterdam, Postbus 94249, 1090 GE, Amsterdam, The Netherlands
³ Department of Physics and Astronomy, University of Leicester, University Road, Leicester LE1 7RH, UK
⁴ Department of Astronomy and Astrophysics, 525 Davey Lab, Pennsylvania State University, University Park, PA 16802, USA
⁵ Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
⁶ INAF – Osservatorio Astronomico di Brera, via Bianchi 46, 23807 Merate (Lc), Italy
⁷ INAF – IASF Palermo, via Ugo La Malfa 153, 90146 Palermo, Italy

Received: 11 December 2011
Accepted: 29 January 2012

Abstract

We present Swift follow-up observations of a sample of 35 unclassified faint X-ray sources drawn from the ASCA Galactic centre and Galactic plane surveys. Our short, pointed XRT observations allow detections down to a limiting 0.3–10 keV flux of F_X ~ 10^-13 erg   cm^-2   s^-1, which translates into a luminosity of L_X ~ 10³³ erg   s^-1 for an assumed distance of D = 8 kpc. The brightest source in our sample reaches a maximum 0.3–10 keV luminosity of L_X ~ 2 × 10³⁶   (D/8   kpc)²   erg   s^-1 during our Swift observations. We detect 16 (46%) of the ASCA sources with the XRT, while 19 were not detected during our program. Since we are probing the faint end of the ASCA source populations, we expect a large fraction of the non-detections to be due to the Eddington bias. This is strengthened by the fact that we find the observed XRT count rates to be predominantly lower than expected based on the reported ASCA intensities. Nevertheless, investigation of the ASCA properties and any possible long-term X-ray variability leads us to conclude that the non-detections likely include two spurious ASCA detections and three objects that are highly variable or transient X-ray sources. For the 16 XRT-detected sources we obtain positional accuracies of  ~2−4′′, which significantly improves upon their ASCA uncertainties of  ~1′. We use the X-ray spectra and variability to characterise these objects. Most appear to be faint, persistent X-ray emitters that have highly absorbed spectra. Based on their X-ray properties we identify three accreting compact objects: one confirmed (AX J1754.2–2754) and one candidate (AX J1538.3–5541) X-ray binary, and one possible magnetically accreting white dwarf (AX J1620.1–5002). Furthermore, we use the improved positions of XRT-detected sources to search for counterparts in simultaneously obtained Swift/UVOT images and possible associations with catalogued sources at various wavelengths. This reveals three possible main sequence stars amongst our sample. The other sources remain unclassified, but our improved XRT positions provide good prospects for dedicated follow-up observations that have the potential to elucidate the nature of these faint, unclassified ASCA sources.