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Register a new userGemini Spectroscopy of Galactic Bulge Sources: A Population of Hidden Accreting Binaries Revealed?
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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 Ha emission line. The spectra of several sources revealed an Ha emission line only after careful subtraction of the F or G stellar spectral absorption lines. In a sub-class of three of these sources the residual Halpha emission line is broad (> 400 km/s) which suggests that it is formed in an accretion disk, 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 disk. GBS source CX377 shows this hidden accretion behaviour most dramatically. The previously-identified broad Ha emission of this source is not present in its Gemini spectra taken about 1 year 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.
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(Abridged:) We present the identification of optical counterparts to 23 Galactic Bulge Survey X-ray sources. We report their accurate coordinates and optical spectra acquired at the VLT and Magellan. All sources are classified as accreting binaries according to their emission line characteristics. To distinguish accreting binaries from chromospherically active objects we develop and explain criteria based on Halpha and HeI 5786,6678 emission line properties available in the literature. The spectroscopic properties and photometric variability of all the objects are discussed and a classification of the source is given where possible. Among the 23 systems, at least 9 of them show an accretion-dominated optical spectrum (CX28, CX63, CX70, CX128, CX142, CX207, CX522, CX794, CX1011) and another 6 show photospheric lines from a late-type donor star in addition to accretion disc emission (CX44, CX93, CX137, CX154, CX377 and CX1004) 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 shows a broad asymmetric Halpha profile blue-shifted by >300 km/s. Such line profile characteristics are consistent with a magnetic (Polar) CV. CX794 is an eclipsing nova-like CV in the period gap. Time-resolved photometry and the large broadening of the Halpha emission lines in CX446 (2100 km/s FWHM) suggest that this is also an eclipsing or high-inclination accreting binary. Finally, the low-accretion rate source CX1004 shows a double-peaked Halpha profile with a FWHM of 2100 km/s. This supports a high inclination or even eclipsing system. Whether the compact object is a white dwarf in an eclipsing CV or a black hole primary in a high-inclination LMXB remains to be established.
The Galactic bulge is the central spheroid of our Galaxy, containing about one quarter of the total stellar mass of the Milky Way (M_bulge=1.8x10^10 M_sun; Sofue, Honma & Omodaka 2009). Being older than the disk, it is the first massive component of the Galaxy to have collapsed into stars. Understanding its structure, and the properties of its stellar population, is therefore of great relevance for galaxy formation models. I will review our current knowledge of the bulge properties, with special emphasis on chemical abundances, recently measured for several hundred stars.
Major galaxy mergers are thought to play an important part in fuelling the growth of supermassive black holes. However, observational support for this hypothesis is mixed, with some studies showing a correlation between merging galaxies and luminous quasars and others showing no such association. Recent observations have shown that a black hole is likely to become heavily obscured behind merger-driven gas and dust, even in the early stages of the merger, when the galaxies are well separated (5 to 40 kiloparsecs). Merger simulations further suggest that such obscuration and black-hole accretion peaks in the final merger stage, when the two galactic nuclei are closely separated (less than 3 kiloparsecs). Resolving this final stage requires a combination of high-spatial-resolution infrared imaging and high-sensitivity hard-X-ray observations to detect highly obscured sources. However, large numbers of obscured luminous accreting supermassive black holes have been recently detected nearby (distances below 250 megaparsecs) in X-ray observations. Here we report high-resolution infrared observations of hard-X-ray-selected black holes and the discovery of obscured nuclear mergers, the parent populations of supermassive-black-hole mergers. We find that obscured luminous black holes (bolometric luminosity higher than 2x10^44 ergs per second) show a significant (P<0.001) excess of late-stage nuclear mergers (17.6 per cent) compared to a sample of inactive galaxies with matching stellar masses and star formation rates (1.1 per cent), in agreement with theoretical predictions. Using hydrodynamic simulations, we confirm that the excess of nuclear mergers is indeed strongest for gas-rich major-merger hosts of obscured luminous black holes in this final stage.
The fastest-spinning neutron stars in low-mass X-ray binaries, despite having undergone millions of years of accretion, have been observed to spin well below the Keplerian break-up frequency. We simulate the spin evolution of synthetic populations of accreting neutron stars in order to assess whether gravitational waves can explain this behaviour and provide the distribution of spins that is observed. We model both persistent and transient accretion and consider two gravitational-wave-production mechanisms that could be present in these systems: thermal mountains and unstable $r$-modes. We consider the case of no gravitational-wave emission and observe that this does not match well with observation. We find evidence for gravitational waves being able to provide the observed spin distribution; the most promising mechanisms being a permanent quadrupole, thermal mountains and unstable $r$-modes. However, based on the resultant distributions alone it is difficult to distinguish between the competing mechanisms.
We explore the long-term evolution of mass-transferring white dwarf binaries undergoing both direct-impact and disk accretion and explore implications of such systems to gravitational wave astronomy. We cover a broad range of initial component masses and show that these systems, the majority of which lie within the LISA sensitivity range, exhibit prominent negative orbital frequency evolution (chirp) for a significant fraction of their lifetimes. Using a galactic population synthesis, we predict ~$2700$ double white dwarfs will be observable by LISA with negative chirps less than $-0.1 yr^{-2}$. We also show that detections of mass-transferring double white dwarf systems by LISA may provide astronomers with unique ways of probing the physics governing close compact object binaries.
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