A search for very active stars in the Galaxy

Abstract

Abstract. We report the first results of a systematic search near the plane of the Galaxy for so-called very active stars (VAS), which are characterized by a hard X-ray spectrum and activity in the radio domain. Candidates with hard X-ray binary-like spectra have been selected from the Bright ROSAT Source Catalogue in the Zone of Avoidance (|b| < 20• ) and were tentatively identified in GB6/PMM/NVSS radio surveys. Most of them were observed with the ATCA and VLA. Precise radio coordinates have led to unambiguous optical identification for 60 candidates, and a sub-sample of five of them has been observed with the VLT. Also some discovery and confirmatory spectra were obtained with the AAT (4-m) and BTA (6-m). Spectroscopy with moderate dispersion, made with the FORS1 spectrograph of the VLT has revealed two stellar objects (one of them, VASC J1628-41, is definitively a binary VAS), one new AGN and two featureless spectrum sources. One of these objects, VASC J1353-66, shows marginal evidence of proper motion, which, if confirmed, would imply the discovery of a new type of galactic source.

Using the ROSAT catalogue to find counterparts for unidentified objects in the first Fermi/LAT catalogue

“…This source also has a counterpart in the XMMSlew catalogue (XMMSL1 J135340.5−663958) which provides a restricted error box, a secure identification with VASC J1353−66 and a 0.2–12 keV flux of 3.9 × 10 −12 erg cm −2 s −1 . Tsarevsky et al (2005) report that this source has a featureless optical spectrum and a flat radio spectrum which together with the detected X‐ray and radio emission leads these authors to suggest that it is probably a new BL Lac type object behind our galaxy despite the marginal detection of a small proper motion.…”

“…Flat spectrum sources are those with a ≥−0.5 Only four (1FGL J0137.8+5814, 1FGL J2056.7+4938, 1FGL J2329.2+3755 and 1FGL J1926.8+6153) of the 30 ROSAT sources had information about the spectral slope in the Specfind data base and all were found to have a flat radio spectrum. Indication of a flat spectrum in other sources can be found in the literature (Reich et al 2000; Ribó et al 2002; Tsarevsky et al 2005; Jackson et al 2007; Mahony et al 2010a; Landi et al 2010) or from archival radio data [taken from HEASARC and/or NASA/IPAC Extragalactic Data base (NED)]; when available a radio loudness indication is also reported in the discussion of each individual source. The ROSAT flux, calculated in the 0.1–2.4 keV band, has also been estimated and listed in Table 2.…”

“…1FGL J1942.7+1033 . This ROSAT source is identified with a radio object having very similar flux at 20 and 6 cm (around 100 mJy) and so is likely to be a flat spectrum source (Tsarevsky et al 2005). The optical spectrum is featureless which, when combined with the detection at radio and X‐ray frequencies, suggests that it is probably a new BL Lac type object located behind the plane of the Galaxy, and it is classified as such in NED.…”

BL Lacertae identifications in a ROSAT-selected sample ofFermiunidentified objects

“…Thus, 1FGL J1910.9+0906 (associated with the SNR G043.3−00.2; Mezger et al 1967) was not considered, nor was 1FGL J1227.9−4852 (the possible γ-ray counterpart of the atypical low-mass X-ray binary XSS J12270−4859; de Martino et al 2010Martino et al , 2013, along with the confirmed BL Lacs 1FGL J1353.6−6640 and 1FGL J1942.7+1033 (both observed with the ESO 8-m Unit 2 "Kueyen" of the Very Large Telescope by Tsarevsky et al 2005).…”

“…To complete the information about the Fermi-ROSAT sample of Stephen et al (2010), we also retrieved from the ESO data archive 2 the relevant files concerning optical spectroscopy of the counterparts of sources 1FGL J1353.6−6640 and 1FGL J1942.7+1033 (also in this case, these two objects are not reported in Table 1; we refer the reader to Tsarevsky et al 2005, for details about the corresponding observational setup) and reduced them independently. In total we analyzed the spectra of 28 optical objects connected with 27 Fermi-ROSAT associations from Stephen et al (2010).…”