Massive White Dwarfs in the Galactic Center: A Chandra X-Ray Spectroscopy of Cataclysmic Variables

Abstract

Previous X-ray observations toward the Nuclear Star Cluster (NSC) at the Galactic center have discovered thousands of point sources, most of which were believed to be cataclysmic variables (CVs), i.e., a white dwarf (WD) accreting from a low-mass companion. However, the population properties of these CVs remain unclear, which otherwise contain important information about the evolutionary history of the NSC. In this work we utilize ultradeep archival Chandra observations to study the spectral properties of the NSC CVs, in close comparison with those in the Solar vicinity. We find that the NSC CVs have strong Fe XXV and Fe XXVI lines (both of which show equivalent widths ∼ 200 − 300 eV), indicating metal-rich companions. Moreover, their Fe XXVI to Fe XXV line flux ratio is used to diagnose the characteristic white dwarf mass (M WD ) of NSC CVs. The results show that the CVs with L 2−10keV > 6 × 10 31 erg s −1 have a mean M WD of ∼ 0.6/1.0 M ⊙ if they are magnetic/non-magnetic CVs; while those with L 2−10keV between 1 − 6 × 10 31 erg s −1 have a mean xuxj@nju.edu.cn M WD of ∼ 0.8/1.2 M ⊙ if they are magnetic/non-magnetic CVs. All these Chandra-detected CVs collectively contribute ∼30-50% of the unresolved 20-40 keV X-ray emission from the NSC. The CV population with massive (i.e., M WD ∼ 1.2M ⊙ ) WDs have not been observed in the Solar vicinity or the Galactic bulge, and they might have been formed via dynamical encounters in the NSC.

Chemical Abundances in Sgr A East: Evidence for a Type Iax Supernova Remnant

“…Type Iax SNe are preferentially found in a young environment and arise from more massive WDs with short evolution time (Lyman et al 2013). It is suggested that the population of WDs in cataclysmic variables in the Galactic center have a mean mass of 1.2M e (assuming nonmagnetic WDs), significantly heavier than the WDs in local and Galactic-bulge cataclysmic variables (∼0.8 M e , Xu et al 2019). Although many Type Iax SNe were found in the "outskirts" of late-type galaxies (Jha et al 2019), our study suggests that this peculiar class can also occur in the center of galaxies.…”

Constraining the White-dwarf Mass and Magnetic Field Strength of a New Intermediate Polar through X-Ray Observations

“…The only unambiguous way to explore CV populations in the GC, Bulge, and Ridge is by analyzing the detected X-ray point sources and determining their source types by analysis of their plasma temperatures or Fe emission lines. For example, in the GC region, which has been extensively observed in the X-ray band (Wang et al 2002;Muno et al 2009), Chandra ACIS analysis of X-ray point sources investigated the CV population by confirming that low-luminosity IPs (L X  10 33 erg s −1 ) make up a large fraction of the CHXE (Xu et al 2019a). However, in the Bulge region outside the GC region (r ∼ 100 pc from Sgr A * ), X-ray sources and their populations are poorly understood, partially due to the lack of extensive X-ray surveys and/or follow-up of deeper X-ray observations.…”

Chemical abundances in Sgr A East: evidence for a Type Iax supernova remnant

“…We uniformly reprocessed the archival data with CIAO v4.10 and calibration files CALDB v4.7.8, following the procedures detailed in Zhu et al (2019). We have examined the light curve of each ObsID, and when necessary, removed time intervals contaminated by particle flares.…”

“…Type Iax SNe are preferentially found in a young environment and arise from more massive WDs with short evolution time (Lyman et al 2013). It is suggested that the population of WDs in cataclysmic variables in the Galactic center have a mean mass of 1.2 M (assuming non-magnetic WDs), significantly heavier than the WDs in local and Galactic-bulge cataclysmic variables (∼ 0.8 M , Xu et al 2019). Although many type Iax SNe were found in the "outskirts" of late-type galaxies (Jha et al 2019), our study suggests that this peculiar class can also occur in the center of galaxies.…”