ALBERT

Description: We report on the discovery of a hydrogen-deficient compact binary (CXOGBS J175107.6-294037) belonging to the AM CVn class in the Galactic Bulge Survey. Deep archival X-ray observations constrain the X-ray positional uncertainty of the source to 0.57 arcsec, and allow us to uniquely identify the optical and UV counterpart. Optical spectroscopic observations reveal the presence of broad, shallow He i absorption lines while no sign of hydrogen is present, consistent with a high state system. We present the optical light curve from Optical Gravitational Lensing Experiment monitoring, spanning 15 yr. It shows no evidence for outbursts; variability is present at the 0.2 mag level on time-scales ranging from hours to weeks. A modulation on a time-scale of years is also observed. A Lomb–Scargle analysis of the optical light curves shows two significant periodicities at 22.90 and 23.22 min. Although the physical interpretation is uncertain, such time-scales are in line with expectations for the orbital and superhump periods. We estimate the distance to the source to be between 0.5 and 1.1 kpc. Spectroscopic follow-up observations are required to establish the orbital period, and to determine whether this source can serve as a verification binary for the eLISA gravitational wave mission.

Description: We report the detection of an infrared-selected transient which has lasted at least five years, first identified by a large mid-infrared and optical outburst from a faint X-ray source detected with the Chandra X-ray Observatory . In this paper we rule out several scenarios for the cause of this outburst, including a classical nova, a luminous red nova, AGN flaring, a stellar merger, and intermediate luminosity optical transients, and interpret this transient as the result of a young stellar object (YSO) of at least solar mass accreting material from the remains of the dusty envelope from which it formed, in isolation from either a dense complex of cold gas or massive star formation. This object does not fit neatly into other existing categories of large outbursts of YSOs (FU Orionis types) which may be a result of the object's mass, age, and environment. It is also possible that this object is a new type of transient unrelated to YSOs.

Description: We present the identification of optical counterparts to 23 GBS X-ray sources. All sources are classified as accreting binaries according to the emission-line characteristics inferred from medium-resolution spectroscopy. To distinguish accreting binaries from chromospherically active objects, we develop criteria based on Hα and He i 5786, 6678 emission-line properties available in the literature. The spectroscopic properties and photometric variability of each object is discussed and a classification is given where possible. At least 12 of the 23 systems show an accretion-dominated optical spectrum and another 6 show stellar absorption features in addition to emission lines indicating that they are probably accreting binaries in quiescence or in a low accretion rate state. Two sources are confirmed to be eclipsing: CX207 and CX794. CX207 is likely a magnetic cataclysmic variable (CV), while CX794 is a nova-like CV in the period gap. Finally, the large broadening (2100 km s –1 FWHM) of the Hα emission lines in CX446 and CX1004 suggests that they are also high-inclination or even eclipsing systems. Whether the compact object is a white dwarf in an eclipsing CV, a neutron star or a black hole in a high-inclination low-mass X-ray binary remains to be established.

Description: We measure the duty cycles for an existing sample of well-observed, nearby dwarf novae (DNe) using data from American Association of Variable Star Observers, and present a quantitative empirical relation between the duty cycle of DNe outbursts and the X-ray luminosity of the system in quiescence. We have found that log DC = 0.63(±0.21) (log L X (erg s –1 ) – 31.3) – 0.95(±0.1), where DC stands for duty cycle. We note that there is intrinsic scatter in this relation greater than what is expected from purely statistical errors. Using the DN X-ray luminosity functions from Pretorius & Knigge and Byckling et al., we compare this relation to the number of DNe in the Galactic Bulge Survey which were identified through optical outbursts during an 8-d-long monitoring campaign. We find a specific frequency of X-ray-bright ( L X 10 31 erg s – 1 ) cataclysmic variables (CVs) undergoing DNe outbursts in the direction of the Galactic bulge of $6.6\pm 4.7{\times} 10^{-5}\,\mathrm{M}_{{\odot }}^{-1}$ . Such a specific frequency would give a solar neighbourhood space density of long-period CVs of  = 5.6 ± 3.9 10 –6 pc –3 . We advocate the use of specific frequency in future work, given that projects like Large Synoptic Survey Telescope will detect DNe well outside the distance range over which   const.

Description: We present Gemini spectroscopy for 21 candidate optical counterparts to X-ray sources discovered in the Galactic Bulge Survey (GBS). For the majority of the 21 sources, the optical spectroscopy establishes that they are indeed the likely counterparts. One of the criteria we used for the identification was the presence of an Hα emission line. The spectra of several sources revealed an Hα emission line only after careful subtraction of the F or G stellar spectral absorption lines. In a subclass of three of these sources, the residual Hα emission line is broad ( 400 km s –1 ) which suggests that it is formed in an accretion disc, whereas in other cases the line width is such that we currently cannot determine whether the line emission is formed in an active star/binary or in an accretion disc. GBS source CX377 shows this hidden-accretion behaviour most dramatically. The previously identified broad Hα emission of this source is not present in its Gemini spectra taken ~1 yr later. However, broad emission is revealed after subtracting an F6 template star spectrum. The Gemini spectra of three sources (CX446, CX1004, and CXB2) as well as the presence of possible eclipses in light curves of these sources suggest that these sources are accreting binaries viewed under a high inclination.

Description: We report on the near-infrared matches, drawn from three surveys, to the 1640 unique X-ray sources detected by Chandra in the Galactic Bulge Survey (GBS). This survey targets faint X-ray sources in the bulge, with a particular focus on accreting compact objects. We present all viable counterpart candidates and associate a false alarm probability (FAP) to each near-infrared match in order to identify the most likely counterparts. The FAP takes into account a statistical study involving a chance alignment test, as well as considering the positional accuracy of the individual X-ray sources. We find that although the star density in the bulge is very high, ~90 per cent of our sources have an FAP 〈10 per cent, indicating that for most X-ray sources, viable near-infrared counterparts candidates can be identified. In addition to the FAP, we provide positional and photometric information for candidate counterparts to ~95 per cent of the GBS X-ray sources. This information in combination with optical photometry, spectroscopy and variability constraints will be crucial to characterize and classify secure counterparts.

Description: We present the results of a spectroscopic and photometric analysis of HD 314884, a slowly pulsating B star (SPB) in a binary system with detected soft-X-ray emission. We spectrally classify the B star as a B5V–B6V star with T eff = 15 490 ± 310 K, log g = 3.75 ± 0.25 dex, and a photometric period of P 0 = 0.889 521(12) d. A spectroscopic period search reveals an orbital period for the system of P orb = 1.3654(11) d. The discrepancy in the two periods and the identification of a second and third distinct frequency in the photometric Fourier transform at P 1 = 3.1347(56) and P 2 = 1.517(28) d provides evidence that HD 314884 is an SPB with at least three oscillation frequencies. These frequencies appear to originate from higher order, non-linear tidal pulsations. Using the dynamical parameters obtained from the radial-velocity curve, we find the most probable companion mass to be M 1 = ~0.8 M assuming a typical mass for the B star and most probable inclination. We conclude that the X-ray source companion to HD 314884 is most likely a coronally active G-type star or a white dwarf, with no apparent emission lines in the optical spectrum. The mass probability distribution of the companion star mass spans 0.6–2.3 M at 99 per cent confidence which allows the possibility of a neutron star companion. The X-ray source is unlikely to be a black hole unless it is of a very low mass or low binary inclination.

Description: We present phase-resolved spectroscopy and photometry of a source discovered with the Chandra Galactic Bulge Survey (GBS), CXOGBS J174444.7–260330 (aka CX93 and CX153 in the previously published GBS list). We find two possible values for the orbital period P , differing from each other by ~13 s. The most likely solution is P = 5.690 14(6) h. The optical lightcurves show ellipsoidal modulations, whose modelling provides an inclination of 32±1° for the most likely P . The spectra are dominated by a K5  V companion star (the disc veiling is 5 per cent). Broad and structured emission from the Balmer lines is also detected, as well as fainter emission from He  i . From the absorption lines we measure K 2  = 117 ± 8 km s – 1 and v sin i  = 69 ± 7 km s – 1 . By solving the system mass function we find M 1  = 0.8 ± 0.2 M for the favoured P and i , consistent with a white dwarf accretor, and M 2  = 0.6 ± 0.2 M . We estimate a distance in the range 400–700 pc. Although in a low accretion state, both spectroscopy and photometry provide evidence of variability on a time-scale of months or faster. Besides finding a new, long orbital period cataclysmic variable (CV) in a low accretion state, this work shows that the design of the GBS works efficiently to find accreting X-ray binaries in quiescence, highlighting that the spectra of CVs in a low accretion state can at times appear suggestive of a quiescent neutron star or a black hole system.

Description: Sco X-1 has been the subject of many multiwavelength studies in the past, being the brightest persistent extrasolar X-ray source ever observed. Here, we revisit Sco X-1 with simultaneous short cadence Kepler optical photometry and Monitor of All-sky X-ray Image X-ray photometry over a 78 d period, as well as optical spectroscopy obtained with High Efficiency and Resolution Mercator Echelle Spectrograph (HERMES). We find Sco X-1 to be highly variable in all our data sets. The optical fluxes are clearly bimodal, implying the system can be found in two distinct optical states. These states are generally associated with the known flaring/normal branch X-ray states, although the flux distributions associated with these states overlap. Furthermore, we find that the optical power spectrum of Sco X-1 differs substantially between optical luminosity states. Additionally we find rms–flux relations in both optical states, but only find a linear relation during periods of low optical luminosity. The full optical/X-ray discrete correlation function displays a broad 12.5 h optical lag. However, during the normal branch phase, the X-ray and optical fluxes are anticorrelated, whilst being correlated during the flaring branch. We also performed a Cepstrum analysis on the full Kepler light curve to determine the presence of any echoes within the optical light curve alone. We find significant echo signals, consistent with the optical lags found using the discrete cross-correlation. We speculate that whilst some of the driving X-ray emission is reflected by the disc, some is absorbed and re-processed on the thermal time-scale, giving rise to both the observed optical lags and optical echoes.

Description: We present the results of a photometric and spectroscopic analysis of the Galactic Bulge Survey X-ray source CXOGBS J174954.5–294335 (hereafter, referred to as CX19). CX19 is a long period, eclipsing intermediate polar-type cataclysmic variable with broad, single-peaked Balmer and Paschen emission lines along with He ii 4686 and Bowen blend emission features. With coverage of one full and two partial eclipses and archival photometry, we determine the ephemeris for CX19 to be HJD(eclipse) = 2455691.8581(5) + 0.358704(2) x N . We also recovered the white dwarf spin period of P spin = 503.32(3) s that gives a P spin / P orb = 0.016(6), comparable to several confirmed, long-period intermediate polars. CX19 also shows a clear X-ray eclipse in the 0.3–8.0 keV range observed with Chandra . Two optical outbursts were observed lasting between 6 and 8 h (lower limits) reaching ~1.3 mag in amplitude. The outbursts, both in duration and magnitude, the accretion disc-dominated spectra and hard X-ray emission are reminiscent of the intermediate polar V1223 Sgr sharing many of the same characteristics. If we assume a main-sequence companion, we estimate the donor to be an early G-type star and find a minimum distance of d 2.1 kpc and a 0.5–10.0 keV X-ray luminosity upper limit of 2.0 x 10 33  erg s –1 . Such an X-ray luminosity is consistent with a white dwarf accretor in a magnetic cataclysmic variable system. To date, CX19 is only the second deeply eclipsing intermediate polar with X-ray eclipses and the first that is optically accessible.