Swiftfollow-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 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 ∼ 10 33 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 × 10 36 (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 nondetections 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.

Extended Gamma-Ray Emission from the G25.0+0.0 Region: A Star-forming Region Powered by the Newly Found OB Association?

“…• 2 in diameter; it is difficult to determine precise spectral parameters of such a faint extended source with ASCA. Swift XRT also observed this source for 8 ks on 2007 March, but did not detect it; the upper limit of the flux is 2 × 10 −13 erg cm −2 s −1 in 0.3-10 keV (Degenaar et al 2012). The authors conclude that AX J1836.3−0647 is a strongly variable or transient source because the upper limit is much lower than the flux reported by ASCA.…”

The very faint hard state of the persistent neutron star X-ray binary SLX 1737–282 near the Galactic Centre

“…SLX 1737-282 is one of the few known LMXB persistently accreting at luminosities lower than L X ∼0.01L Edd (e.g. in 't Zand et al 2005;Del Santo et al 2007;Chelovekov & Grebenev 2007;Degenaar et al 2012;Armas Padilla et al 2013;Degenaar et al 2017). In order to study the spectral and timing properties in this regime, high-quality observations resulting from long exposures, such as those presented here, are essential.…”

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

“…ChI J153818-5541_1 also falls within Region vi in Figure 8 along with these same magnetars, further indicating a possible magnetar nature. However, during the writing of this paper, recent studies with Swift have indicated that ChI J153818-5541_1 (published under its AGPS name AX J1538.3-5541) may in fact be a low mass X-ray binary (LMXB; Degenaar et al 2012). If this is the case, then X-ray and infrared statistical analysis is not a foolproof diagnostic.…”

“…Several unidentified AGPS sources have been investigated by Degenaar et al (2012) using the X-ray telescope (XRT) on the Swift satellite (Gehrels et al 2004). Degenaar et al (2012) suggest that ChI J150436-5824_1 (which they publish under the AGPS name AX J1504.6-5824) could possibly be associated with a main sequence star that falls within the 90% XRT error circle. However, the ChIcAGO Chandra observation of ChI J150436-5824_1 demonstrates that there is no cataloged counterpart within the < 1 position error circle of this X-ray source, arguing against this tentative identification.…”