Spectroscopic classification of X-ray sources in the Galactic Bulge Survey

Abstract

We present the classification of 26 optical counterparts to X-ray sources discovered in the Galactic Bulge Survey. We use (time-resolved) photometric and spectroscopic observations to classify the X-ray sources based on their multi-wavelength properties. We find a variety of source classes, spanning different phases of stellar/binary evolution. We classify CX21 as a quiescent cataclysmic variable (CV) below the period gap, and CX118 as a high accretion rate (nova-like) CV. CXB12 displays excess UV emission, and could contain a compact object with a giant star companion, making it a candidate symbiotic binary or quiescent low mass X-ray binary (although other scenarios cannot be ruled out). CXB34 is a magnetic CV (polar) that shows photometric evidence for a change in accretion state. The magnetic classification is based on the detection of X-ray pulsations with a period of 81 ± 2 min. CXB42 is identified as a young stellar object, namely a weak-lined T Tauri star exhibiting (to date unexplained) UX Ori-like photometric variability. The optical spectrum of CXB43 contains two (resolved) unidentified double-peaked emission lines. No known scenario, such as an AGN or symbiotic binary, can easily explain its characteristics. We additionally classify 20 objects as likely active stars based on optical spectroscopy, their X-ray to optical flux ratios and photometric variability. In 4 cases we identify the sources as binary stars.

The Swift Bulge Survey: Motivation, Strategy, and First X-ray Results

“…Swift J174142.0−283321 (SBS X92) matches 2RXP J174141.9−283324 and 2CXO J174141.7-283324, associated with a V=13 star at 111 pc, with T eff = 4139 K, L = 0.13L and R = 0.69R (Gaia Collaboration et al 2018) 2018) gives R P =10.5, T eff =4900 K, R=3.4 R , and d=466±15 pc. Wevers et al (2017) find that this star shows an optical spectrum consistent with a normal star. The peak SBS flux is 17±9 times the Chandra flux, though it is detected in 5 of 21 epochs.…”

“…SBS X29 matches CXOGBS J174133.7−284033 (CX21, Jonker et al 2011), which was classified as a quiescent CV below the period gap by Britt et al (2014) and Wevers et al (2017) on the basis of strong optical flickering and an optical spectrum, and argued to be closer than 500 pc. SBS X35 matches CXOGBS J174009.1−284725 (CX5), and AX 1740.1−2847, which shows an X-ray periodicity of 729 s (Sakano et al 2000).…”

“…The UV detection implies L UV = 2.5 × 10 34 (d/2 kpc) 2 erg s −1 , while the Chandra and XMM-Newton flux values imply L X ∼ 1 − 2 × 10 32 (d/2 kpc) 2 erg s −1 . Wevers et al (2017) suggest this is likely a symbiotic system, with either a WD or NS primary. Our Swift measurement of L X ∼ 3 × 10 33 (d/2 kpc) 2 erg s −1 strongly favors the symbiotic explanation over the alternative RS CVn interpretation considered by Wevers et al (2017).…”

“…Wevers et al (2017) suggest this is likely a symbiotic system, with either a WD or NS primary. Our Swift measurement of L X ∼ 3 × 10 33 (d/2 kpc) 2 erg s −1 strongly favors the symbiotic explanation over the alternative RS CVn interpretation considered by Wevers et al (2017). We also favor a WD, rather than NS, nature for the accretor, based on the high UV luminosity, naturally produced in boundary layers on WDs accreting at high rates (e.g.…”

Low-Mass X-ray Binaries

“…It is worth noting that Peak L X is the reported peak luminosity; SyXBs can have short lived flares (minutes long) reaching higher luminosities that may have been missed in pointed observations too short to be noticed in X-ray monitors. SyXB candidates not listed in this table include 4U 1954+31 (proposed to be a SyXB by [140,132], recent works indicate it is likely an HMXB [170]), IRXS J180431.1−273932 (suggested as SyXB by [171], disputed by [172]), IGR J16393−4643 (suggested as a SyXB by [149], disputed by [173]), 2XMM J174016.0−290337 (suggested as a SyXB by [134] and disputed by [174]), Swift J175233.3−293944 (suggested as symbiotic by [175], a white dwarf compact object is favored by [136]). a -indicates the distance (and luminosity) has been updated based on Gaia EDR3 [176].…”