Context. Ultracompact X-ray binaries (UCXBs) typically consist of a white dwarf donor and a neutron star or black hole accretor. The evolution of UCXBs and very low mass ratio binaries in general is poorly understood. In particular, the dynamical behavior of an accretion disk extending to a large radius (relative to the orbit) is unclear. Aims. We investigate the evolution of UCXBs in order to learn for which mass ratios and accretor types these systems can exist, and if they do, what are their orbital and neutron star spin periods, mass transfer rates and evolutionary timescales. Methods. We compute tracks of a binary containing a Roche-lobe overflowing helium white dwarf in which mass transfer is driven by gravitational wave emission. For different assumptions concerning accretion disk behavior we calculate for which system parameters dynamical instability, thermal-viscous disk instability or the propeller effect emerge. The significance of these processes during the evolution of an UCXB is considered. Results. At the onset of mass transfer, the survival of the UCXB is determined by how efficiently the accretor can eject matter in the case of a super-Eddington mass transfer rate. At later times, the evolution of systems strongly depends on the binary's capacity to return angular momentum from the disk to the orbit. We find that this feedback mechanism most likely remains effective even at very low mass ratio. In the case of steady mass transfer, the propeller effect can stop accretion onto recycled neutron stars completely at a sufficiently low mass transfer rate, based on energy considerations. However, mass transfer will likely be non-steady because disk instability allows for accretion of some of the transferred matter. Together, the propeller effect and disk instability cause the low mass ratio UCXBs to be visible a small fraction of the time at most, thereby explaining the lack of observations of such systems. Conclusions. Most likely UCXBs avoid late-time dynamically unstable mass loss from the donor and continue to evolve as the age of the Universe allows. This implies the existence of a large population of low mass ratio binaries with orbital periods ∼70-80 min, unless some other mechanism has destroyed these binaries. Even though none have been discovered yet, black hole UCXBs could also exist, at orbital periods of typically 100-110 min.

We introduce the Galactic Bulge Survey (GBS) and we provide the Chandra source list for the region that has been observed to date. Among the goals of the GBS are constraining the neutron star equation of state and the black hole mass distribution via the identification of eclipsing neutron star and black hole low-mass X-ray binaries. The latter goal will, in addition, be obtained by significantly enlarging the number of black hole systems for which a black hole mass can be derived. Further goals include constraining X-ray binary formation scenarios, in particular the common envelope phase and the occurrence of kicks, via source-type number counts and an investigation of the spatial distribution of X-ray binaries, respectively. The GBS targets two strips of 6 • × 1 • (12 square degrees in total), one above (1 • < b < 2 • ) and one below (−2 • < b < −1 • ) the Galactic plane in the direction of the Galactic Center at both X-ray and optical wavelengths. By avoiding the Galactic plane (−1 • < b < 1 • ) we limit the influence of extinction on the X-ray and optical emission but still sample relatively large number densities of sources. The survey is designed such that a large fraction of the X-ray sources can be identified from their optical spectra. The X-ray survey, by design, covers a large area on the sky while the depth is shallow using 2 ks per Chandra pointing. In this way we maximize the predicted number ratio of (quiescent) low-mass X-ray binaries to Cataclysmic Variables. The survey is approximately homogeneous in depth to an 0.5-10 keV flux of 7.7×10 −14 erg cm −2 s −1 . So far, we have covered about two-thirds (8.3 square degrees) of the projected survey area with Chandra providing over 1200 unique X-ray sources. We discuss the characteristics and the variability of the brightest of these sources.

Context. X-ray binaries are usually divided in persistent and transient sources. For ultracompact X-ray binaries (UCXBs), the mass transfer rate is expected to be a strong function of orbital period, predicting persistent sources at short periods and transients at long periods. Aims. For 14 UCXBs including two candidates, we investigate the long-term variability and average bolometric luminosity with the purpose of learning how often a source can be expected to be visible above a given luminosity, and we compare the derived luminosities with the theoretical predictions. Methods. We use data from the RXTE All-Sky Monitor because of its long-term, unbiased observations. Many UCXBs are faint, i.e., they have a count rate at the noise level for most of the time. Still, information can be extracted from the data, either by using only reliable data points or by combining the bright-end variability behavior with the time-averaged luminosity.Results. Luminosity probability distributions show the fraction of time that a source emits above a given luminosity. All UCXBs show significant variability and relatively similar behavior, though the time-averaged luminosity implies higher variability in systems with an orbital period longer than 40 min. Conclusions. There is no large difference in the statistical luminosity behavior of what we usually call persistent and transient sources. UCXBs with an orbital period below ∼30 min have a time-averaged bolometric luminosity that is in reasonable agreement with estimates based on the theoretical mass transfer rate. Around 40 min the lower bound on the time-averaged luminosity is similar to the luminosity based on the theoretical mass transfer rate, suggesting these sources are indeed faint when not detected. Above 50 min some systems are much brighter than the theoretical mass transfer rate predicts, unless these systems have helium burning donors or lose additional angular momentum.

The INT Galactic Plane Survey (IGAPS) is the merger of the optical photometric surveys, IPHAS and UVEX, based on data from the Isaac Newton Telescope (INT) obtained between 2003 and 2018. Here, we present the IGAPS point source catalogue. It contains 295.4 million rows providing photometry in the filters, i, r, narrow-band Hα, g and U RGO . The IGAPS footprint fills the Galactic coordinate range, |b| < 5 • and 30 • < < 215 • . A uniform calibration, referred to the Pan-STARRS system, is applied to g, r and i, while the Hα calibration is linked to r and then is reconciled via field overlaps. The astrometry in all 5 bands has been recalculated on the Gaia DR2 frame. Down to i ∼ 20 mag. (Vega system), most stars are also detected in g, r and Hα. As exposures in the r band were obtained within the IPHAS and UVEX surveys a few years apart, typically, the catalogue includes two distinct r measures, r I and r U . The r 10σ limiting magnitude is ∼21, with median seeing 1.1 arcsec. Between ∼13th and ∼19th magnitudes in all bands, the photometry is internally reproducible to within 0.02 magnitudes. Stars brighter than r = 19.5 have been tested for narrow-band Hα excess signalling line emission, and for variation exceeding |r I − r U | = 0.2 mag. We find and flag 8292 candidate emission line stars and over 53000 variables (both at > 5σ confidence). The 174-column catalogue will be available via CDS Strasbourg.Article number, page 3 of 28 A&A proofs: manuscript no. main filters at each pointing should be observed consecutivelyusually within an elapsed time of ∼5 min. All included exposure sets meet this criterion. Article number, page 20 of 28 M. Monguió et al.: IGAPS