Swift follow-up observations of unclassified ASCA sources

Abstract

We present Swift follow-up observations of a sample of 35 unclassified faint X-ray sources drawn from the ASCA Galactic centre and plane surveys. Our short pointed XRT observations allow detections down to a limiting 0.3-10 keV flux of ~1E-13 erg cm-2 s-1, which translates into a luminosity of ~1E33 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 ~2E36 (D/8kpc)^2 erg s-1 during our 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. Investigation of the ASCA properties and long-term X-ray variability leads us to conclude that the non-detections likely include two spurious ASCA detections and three objects that are 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 cataloged sources at various wavelengths. This reveals three possible main sequence stars amongst our sample.

14 pages, 7 figures, 7 tables. Accepted for publication in A&A without revision. Updated a few references

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Cited by in corpus (12)

• Low-level accretion in neutron-star X-ray binaries
• A four-year XMM-Newton/Chandra monitoring campaign of the Galactic Centre: analysing the X-ray transients
• An in-depth study of a neutron star accreting at low Eddington rate: On the possibility of a truncated disk and an outflow
• Low-Mass X-ray Binaries
• The X-ray spectral properties of very-faint persistent neutron star X-ray binaries
• UltraCompCAT: a comprehensive Catalogue of Ultra-Compact and Short Orbital Period X-ray Binaries
• Extended Gamma-ray Emission from the G25.0+0.0 Region: A Star Forming Region Powered by the Newly Found OB Association?
• Chasing the Identification of ASCA Galactic Objects (ChIcAGO) - An X-ray Survey of Unidentified Sources in the Galactic Plane I: Source Sample and Initial Results
• The very faint hard state of the persistent neutron star X-ray binary SLX 1737-282 near the Galactic centre
• Near-infrared counterparts of three transient very faint neutron star X-ray binaries
• Determining the Nature of Faint X-Ray Sources from the ASCA Galactic Center Survey
• SRG/ART-XC all-sky X-ray survey: Catalog of sources detected during the first year