CV1 in the globular cluster M 22: confirming its nature through X-ray observations and optical spectroscopy

Abstract

Context. Observations of cataclysmic variables in globular clusters appear to show a dearth of outbursts compared to those observed in the field. A number of explanations have been proposed, including low mass-transfer rates and/or moderate magnetic fields implying higher mass white dwarfs than the average observed in the field. Alternatively this apparent dearth may be simply a selection bias. Aims. We examine multi-wavelength data of a new cataclysmic variable, CV1, in the globular cluster M 22 to try to constrain its period and magnetic nature, with an aim at understanding whether globular cluster cataclysmic variables are intrinsically different from those observed in the field. Methods. We use the sub-arcsecond resolution of the Chandra ACIS-S to identify the X-ray counterpart to CV1 and analyse the X-ray spectrum to determine the spectral model that best describes this source. We also examine the low resolution optical spectrum for emission lines typical of cataclysmic variables. Cross correlating the H α line in each individual spectrum also allows us to search for orbital motion. Results. The X-ray spectrum reveals a source best-fitted with a high-temperature bremsstrahlung model and an X-ray unabsorbed luminosity of 1.8 × 10 32 erg s −1 (0.3-8.0 keV), which are typical of cataclysmic variables. Optical spectra reveal Balmer emission lines, which are indicative of an accretion disc. Potential radial velocity in the H α emission line is detected and a period for CV1 is proposed. Conclusions. These observations support the CV identification. The radial velocity measurements suggest that CV1 may have an orbital period of ∼7 h, but further higher resolution optical spectroscopy of CV1 is needed to unequivocally establish the nature of this CV and its orbital period.

A stellar census in globular clusters with MUSE: A spectral catalogue of emission-line sources

“…As described above, cataclysmic variables (CVs) are binary systems consisting of a hot, compact white dwarf and a dwarf star in a close orbit. Only ten CVs have been confirmed by spectroscopy in the whole globular cluster system of the Milky Way (Knigge 2012; Webb & Servillat 2013…”

“…However, the cluster with the most CV candidates, as determined by UV and optical photometry and X-ray data, is NGC 104 with 43 CVs (Sandoval et al 2018). In contrast, the number of spectroscopically confirmed CVs is much lower: only ten CVs have been confirmed by spectroscopy in the whole globular cluster system of the Milky Way (Knigge 2012; Webb & Servillat 2013). We add seven CVs to this list, including two newly detected CVs.…”

Two new magnetic cataclysmic variables discovered in the 3XMM catalogue

“…In the case of low quality spectra, this simple model is often appropriate (e.g. Homer et al 2005;Webb & Servillat 2013;Servillat et al 2008). However, whilst the X-ray lightcurve strongly resembles that of polars, the low signal to noise makes it difficult to completely rule out the possibility that the source is an X-ray binary.…”

Search for multiwavelength emission from the binary millisecond pulsar PSR J1836-2354A in the globular cluster M22

“…Among the nine sources, eight are within the cluster core and one within the half-mass radius. One of them corresponds to the source labelled CV1 by Webb & Servillat (2013), classified as a cataclysmic variable through the study of its the X-ray emission and optical spectrum. Its position matches the star R0047833 in the HUGS catalogue.…”

“…More recently, XMM-Newton observed the cluster in 2000 (Webb et al 2002(Webb et al , 2004 while Chandra in 2005 (Webb & Servillat 2013) and in 2014. Webb & Servillat (2013) analysed the Chandra observation made in 2005 and reported a faint X-ray source (Source 3 in their Table 1) as the possible X-ray counterpart of M22A. We use all the available archival data from Chandra and XMM-Newton, focusing especially on the longest Chandra observation (2014).…”