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We have discovered 10 periodic X-ray sources from the l Ms Chandra ACIS observation of the Limiting Window (LW), a low-extinction region (Av ~ 3.9) at 1°.4 south of the Galactic center. The observed periods (~1.3-3.4 hr) and the X-ray luminosities (1031.8-32.9 erg s-1 at 8 kpc) of the 10 periodic sources, combined with the lack of bright optical counterparts and thus high X-ray-to-optical flux ratios, suggest that they are likely accreting binaries, in particular, magnetic cataclysmic variables (MCVs). All of them exhibit a relatively hard X-ray spectrum (Γ < 2 for a power-law model), and relatively high extinction observed in the X-ray spectra of at least five sources indicates some intrinsic absorption in the system, which is also a typical sign of MCVs, On close inspection, the period distribution of these sources resembles those of polars, whereas the relatively hard spectra suggest that they could be intermediate polars (IPs). These puzzling properties can be explained by unusual polars with buried magnetic fields or a rare sub-class of MCVs, nearly synchronous MCVs. These unusual MCVs may provide important clues in the evolutionary path of MCVs from IPs to polars. The completeness simulation indicates that ≥40% of the hard X-ray sources in the LW are periodic. Therefore, this discovery provides a first direct evidence of a large MCV population in the bulge and further supports the current view that MCVs constitute the majority of low-luminosity hard X-ray sources (~ 1030-33 erg s-1) in the bulge. [ABSTRACT FROM AUTHOR]

KU Cyg is an eclipsing binary consisting of an F-type star accreting through a large accretion disk from a K5III red giant. Here we present the discovery of a 5 year dip around 1900 found from its 100 year DASCH light curve. It showed a ~0.5 mag slow fading from 1899 to 1903 and brightened back around 1904 on a relatively shorter timescale. The light curve shape of the 1899-1904 fading-brightening event differs from the dust production and dispersion process observed in R Coronae Borealis stars, which usually has a faster fading and slower recovery, and for KU Cyg is probably related to the accretion disk surrounding the F star. The slow fading in KU Cyg is probably caused by increases in dust extinction in the disk, and the subsequent quick brightening may be due to the evaporation of dust transported inward through the disk. The extinction excess which caused the fading may arise from increased mass transfer rate in the system or from dust clump ejections from the K giant. [ABSTRACT FROM AUTHOR]

We report the results of a joint ChandraHubble Space Telescope study of the X-ray binary (XRB) population in the massive, high-density globular cluster NGC 6388. NGC 6388 has one of the highest predicted XRB production rates of any Galactic cluster. We detected a large population of 61 Chandra sources within the half-mass radius with LX > 5 × 1030 erg s–1. From the X-ray colors, luminosities, (lack of) variability, and spectral fitting, we identify five as likely quiescent low-mass X-ray binaries. Due to the extremely crowded nature of the core of NGC 6388, finding optical identifications to Chandra sources is challenging. We have identified four blue, optically variable counterparts to spectrally hard X-ray sources, evidence that these are bright cataclysmic variables (CVs). One showed variability of 2 mag in V, indicative of a dwarf nova eruption. One other likely CV is identified by its X-ray spectrum (partial covering with high NH) and strong variability, making five likely CVs identified in this cluster. The relatively bright optical magnitudes of these sources put them in the same class as CV1 in M15 and the brightest CVs in 47 Tuc. [ABSTRACT FROM AUTHOR]

We present photometric and spectroscopic observations of a peculiar variable (designated DASCH J075731.1+201735 or J0757) discovered from our DASCH project using the digitized Harvard College Observatory archival photographic plates. It brightened by about 1.5 mag in B within a year starting in 1942, and then slowly faded back to its pre-outburst brightness from 1943 to 1950s. The mean brightness level was stable before and after the outburst, and ellipsoidal variations with a period of P = 119.18 ± 0.07 days are seen, suggesting that the star is tidally distorted. Radial-velocity measurements indicate that the orbit is nearly circular (e = 0.02 ± 0.01) with a spectroscopic period that is the same as the photometric period. The binary consists of a 1.1 ± 0.3 M☼ M0III star, and a 0.6 ± 0.2 M☼ companion, very likely a white dwarf (WD). Unlike other symbiotic binaries, there is no sign of emission lines or a stellar wind in the spectra. With an outburst timescale of ∼10 years and estimated B-band peak luminosity MB ∼ 0.7, J0757 is different from any other known classic or symbiotic novae. The most probable explanation of the outburst is hydrogen shell burning on the WD, although an accretion-powered flare cannot be ruled out. [ABSTRACT FROM AUTHOR]