Abstract Magnetic cataclysmic variables (CVs) are luminous Galactic X-ray sources, which have been difficult to find in purely optical surveys due to their lack of outburst behavior. The eROSITA telescope on board the Spektr-RG mission is conducting an all-sky X-ray survey and recently released the public eROSITA Final Equatorial Depth Survey (eFEDS) catalog. We crossmatched the eFEDS catalog with photometry from the Zwicky Transient Facility and discovered two new magnetic CVs. We obtained high-cadence optical photometry and phase-resolved spectroscopy for each magnetic CV candidate and found them both to be polars. Among the newly discovered magnetic CVs is eFEDS J085037.2+044359/ZTFJ0850+0443, an eclipsing polar with orbital period P orb = 1.72 hr and WD mass M WD = 0.81 ± 0.08 M ⊙ . We suggest that eFEDS J085037.2+044359/ZTFJ0850+0443 is a low magnetic field strength polar, with B WD ≲ 10 MG. We also discovered a non-eclipsing polar, eFEDS J092614.1+010558/ZTFJ0926+0105, with orbital period P orb = 1.47 hr and magnetic field strength B WD = 36–42 MG.
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Deep near-infrared imaging observation of the faint X-ray point sources constituting the Galactic bulge X-ray emission
The presence of the apparently extended hard (2–10 keV) X-ray emission along the Galactic plane has been known since the early 1980s. With a deep X-ray exposure using the Chandra X-ray Observatory of a slightly off-plane region in the Galactic bulge, most of the extended emission was resolved into faint discrete X-ray sources in the Fe K band (Revnivtsev et al. 2009, Nature, 458, 1142). The major constituents of these sources have long been considered to be X-ray active stars and magnetic cataclysmic variables (CVs). However, recent works including our near-infrared (NIR) imaging and spectroscopic studies (Morihana et al. 2013, ApJ, 766, 14; Morihana et al. 2016, PASJ, 68, 57) argue that other populations should be more dominant. To investigate this further, we conducted a much deeper NIR imaging observation at the center of the Chandra’s exposure field. We have used the MOIRCS on the Subaru telescope, reaching the limiting magnitude of ∼18 mag in the J, H, and Ks bands in this crowded region, and identified ${\sim}50\%$ of the X-ray sources with NIR candidate counterparts. We classified the X-ray sources into three groups (A, B, and C) based on their positions in the X-ray color–color diagram and characterized them based on the X-ray and NIR features. We argue that the major populations of the Group A and C sources are, respectively, CVs (binaries containing magnetic or non-magnetic white dwarfs with high accretion rates) and X-ray active stars. The major population of the Group B sources is presumably white dwarf (WD) binaries with low mass accretion rates. The Fe K equivalent width in the composite X-ray spectrum of the Group B sources is the largest among the three and comparable to that of the Galactic bulge X-ray emission. This leads us to speculate that there are numerous WD binaries with low mass accretion rates which are not recognized as CVs but are the major contributor of the apparently extended X-ray emission.
more » « less- NSF-PAR ID:
- 10365653
- Publisher / Repository:
- Oxford University Press
- Date Published:
- Journal Name:
- Publications of the Astronomical Society of Japan
- Volume:
- 74
- Issue:
- 2
- ISSN:
- 0004-6264
- Format(s):
- Medium: X Size: p. 283-297
- Size(s):
- ["p. 283-297"]
- Sponsoring Org:
- National Science Foundation
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ABSTRACT Recent observations demonstrate that the symbiotic X-ray binary (SyXB) IGR J17329−2731 contains a highly magnetized neutron star (NS), which accretes matter through the wind from its giant star companion, and suggest that 4U 1700+24 may also have a highly magnetized NS. Accretion-induced collapse (AIC) from oxygen–neon–magnesium white dwarf (ONeMg WD) + red giant (RG) star binaries is one promising channel to form these SyXBs, while other long standing formation channels have difficulties to produce these SyXBs. By considering non-magnetic and magnetic ONeMg WDs, I investigate the evolution of ONeMg WD + RG binaries with the mesa stellar evolution code for producing SyXBs with non-magnetic or magnetized NSs. In the pre-AIC evolution with magnetic confinement, the mass accumulation efficiency of the accreting WD is increased at low-mass transfer rate compared with the non-magnetic case. The newborn NSs formed via AIC of highly magnetized WDs could inherit the large magnetic field through conservation of magnetic flux, and the systems could have a long age compatible with that of the red giant companions. These young and highly magnetized NSs could accrete matters from the stellar wind of the giant companions to that shine as those observed SyXBs, and could preserve their high magnetic field during this time. The mesa calculation results show that the initial parameter (initial RG mass and orbital period) space for the AIC with magnetic confinement to form SyXBs with highly magnetized NSs shifts to be lower and narrower compared with that of the no magnetic confinement case.
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Abstract We explore the observational appearance of the merger of a low-mass star with a white dwarf (WD) binary companion. We are motivated by recent work finding that multiple tensions between the observed properties of cataclysmic variables (CVs) and standard evolution models are resolved if a large fraction of CV binaries merge as a result of unstable mass transfer. Tidal disruption of the secondary forms a geometrically thick disk around the WD, which subsequently accretes at highly super-Eddington rates. Analytic estimates and numerical hydrodynamical simulations reveal that outflows from the accretion flow unbind a large fraction ≳90% of the secondary at velocities ∼500–1000 km s −1 within days of the merger. Hydrogen recombination in the expanding ejecta powers optical transient emission lasting about a month with a luminosity ≳10 38 erg s −1 , similar to slow classical novae and luminous red novae from ordinary stellar mergers. Over longer timescales the mass accreted by the WD undergoes hydrogen shell burning, inflating the remnant into a giant of luminosity ∼300–5000 L ⊙ , effective temperature T eff ≈ 3000 K, and lifetime ∼10 4 –10 5 yr. We predict that ∼10 3 –10 4 Milky Way giants are CV merger products, potentially distinguishable by atypical surface abundances. We explore whether any Galactic historical slow classical novae are masquerading CV mergers by identifying four such post-nova systems with potential giant counterparts for which a CV merger origin cannot be ruled out. We address whether the historical transient CK Vul and its gaseous/dusty nebula resulted from a CV merger.more » « less
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Context. After about 16 years since its first outburst, the transient neutron star low-mass X-ray binary XTE J1701−462 turned on again in September 2022, allowing for the first study of its X-ray polarimetric characteristics by a dedicated observing program with the Imaging X-ray Polarimeter Explorer (IXPE). Aims. Polarimetric studies of XTE J1701−462 have been expected to improve our understanding of accreting weakly magnetized neutron stars, in particular, the physics and the geometry of the hot inner regions close to the compact object. Methods. The IXPE data of two triggered observations were analyzed using time-resolved spectroscopic and polarimetric techniques, following the source along its Z -track of the color–color diagram. Results. During the first pointing on 2022 September 29, an average 2–8 keV polarization degree of (4.6 ± 0.4)% was measured, the highest value found up to now for this class of sources. Conversely, only a ∼0.6% average degree was obtained during the second pointing ten days later. Conclusions. The polarimetric signal appears to be strictly related to the higher energy blackbody component associated with the boundary layer (BL) emission and its reflection from the inner accretion disk, and it is as strong as 6.1% and 1.2% (> 95% significant) above 3–4 keV for the two measurements, respectively. The variable polarimetric signal is apparently related to the spectral characteristics of XTE J1701−462, which is the strongest when the source was in the horizontal branch of its Z -track and the weakest in the normal branch. These IXPE results provide new important observational constraints on the physical models and geometry of the Z -sources. Here, we discuss the possible reasons for the presence of strong and variable polarization among these sources.more » « less
