Galactic Bulge Sources Research Articles

Polarimetric microlensing of circumstellar discs

We study the benefits of polarimetry observations of microlensing events to detect and characterize circumstellar disks around the microlensed stars located at the Galactic bulge. These disks which are unresolvable from their host stars make a net polarization effect due to their projected elliptical shapes. Gravitational microlensing can magnify these signals and make them be resolved. The main aim of this work is to determine what extra information about these disks can be extracted from polarimetry observations of microlensing events in addition to those given by photometry ones. Hot disks which are closer to their host stars are more likely to be detected by microlensing, owing to more contributions in the total flux. By considering this kind of disks, we show that although the polarimetric efficiency for detecting disks is similar to the photometric observation, but polarimetry observations can help to constraint the disk geometrical parameters e.g. the disk inner radius and the lens trajectory with respect to the disk semimajor axis. On the other hand, the time scale of polarimetric curves of these microlensing events generally increases while their photometric time scale does not change. By performing a Monte Carlo simulation, we show that almost 4 optically-thin disks around the Galactic bulge sources are detected (or even characterized) through photometry (or polarimetry) observations of high-magnification microlensing events during 10 years monitoring of 150 million objects.

Detecting stellar spots through polarimetric observations of microlensing events in caustic-crossing

In this work, we investigate if gravitational microlensing can magnify the polarization signal of a stellar spot and make it be observable. A stellar spot on a source star of microlensing makes polarization signal through two channels of Zeeman effect and breaking circular symmetry of the source surface brightness due to its temperature contrast. We first explore the characteristics of perturbations in polarimetric microlensing during caustic-crossing of a binary lensing as follows: (a) The cooler spots over the Galactic bulge sources have the smaller contributions in the total flux, although they have stronger magnetic fields. (b) The maximum deviation in the polarimetry curve due to the spot happens when the spot is located near the source edge and the source spot is first entering the caustic whereas the maximum photometric deviation occurs for the spots located at the source center. (c) There is a (partial) degeneracy for indicating spot's size, its temperature contrast and its magnetic induction from the deviations in light or polarimetric curves. (d) If the time when the photometric deviation due to spot becomes zero (between positive and negative deviations) is inferred from microlensing light curves, we can indicate the magnification factor of the spot, characterizing the spot properties except its temperature contrast. The stellar spots alter the polarization degree as well as strongly change its orientation which gives some information about the spot position. Although, the photometry observations are more efficient in detecting stellar spots than the polarimetry ones, but polarimetry observations can specify the magnetic field of the source spots.

Gemini spectroscopy of Galactic Bulge Sources: a population of hidden accreting binaries revealed?★

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.

MOA-2010-BLG-523: “FAILED PLANET” = RS CVn STAR

The Galactic bulge source MOA-2010-BLG-523S exhibited short-term deviations from a standard microlensing lightcurve near the peak of an Amax ~ 265 high-magnification microlensing event. The deviations originally seemed consistent with expectations for a planetary companion to the principal lens. We combine long-term photometric monitoring with a previously published high-resolution spectrum taken near peak to demonstrate that this is an RS CVn variable, so that planetary microlensing is not required to explain the lightcurve deviations. This is the first spectroscopically confirmed RS CVn star discovered in the Galactic bulge.

A new eclipsing low-mass X-ray binary with a giant companion

We detected hard X-ray emission from the unidentified Galactic bulge source 1RXS J175721.2-304405 with ASCA. The observed absorption column, flux and power-law index led us to consider that 1RXS J175721.2-304405 may be a new low-mass X-ray binary located near the Galactic center. Furthermore, the X-ray light-curve shows a step-function-like time variability, which is likely due to the occultation of a companion star. Future follow-up observations by missions such as ASTROSAT may reveal a periodic eclipse from 1RXS J175721.2-304405 if it is covered long enough. Since the long orbital period suggests a giant companion, follow-up observations will give firm evidence that 1RXS J175721.2-304405 is a new and rare eclipsing low-mass X-ray binary with a giant companion.

Gravitational Microlensing near Caustics. I. Folds

We study the local behavior of gravitational lensing near fold catastrophes. Using a generic form for the lensing map near a fold, we determine the observable properties of the lensed images, focusing on the case in which the individual images are unresolved, i.e., microlensing. Allowing for images not associated with the fold, we derive analytic expressions for the photometric and astrometric behavior near a generic fold caustic. We show how this form reduces to the more familiar linear caustic, which lenses a nearby source into two images that have equal magnification, opposite parity, and are equidistant from the critical curve. In this case, the simplicity and high degree of symmetry allow for the derivation of semianalytic expressions for the photometric and astrometric deviations in the presence of finite sources with arbitrary surface brightness profiles. We use our results to derive some basic properties of astrometric microlensing near folds; in particular, we predict, for finite sources with uniform and limb-darkening profiles, the detailed shape of the astrometric curve as the source crosses a fold. We find that the astrometric effects of limb darkening will be difficult to detect with the currently planned accuracy of the Space Interferometry Mission for Galactic bulge sources; however, this also implies that astrometric measurements of other parameters, such as the size of the source, should not be compromised by an unknown amount of limb darkening. We verify our results by numerically calculating the expected astrometric shift for the photometrically well-covered Galactic binary lensing event OGLE-1999-BUL-23, finding excellent agreement with our analytic expressions. Our results can be applied to any lensing system with fold caustics, including Galactic binary lenses and quasar microlensing.

Can Jupiters be found by monitoring Galactic bulge microlensing events from northern sites?

In 1998 the EXPORT team monitored microlensing event light curves using a charge-coupled device (CCD) camera on the IAC 0.8-m telescope on Tenerife to evaluate the prospect of using northern telescopes to find microlens anomalies that reveal planets orbiting the lens stars. The high airmass and more limited time available for observations of Galactic bulge sources make a northern site less favourable for microlensing planet searches. However, there are potentially a large number of northern 1-m class telescopes that could devote a few hours per night to monitor ongoing microlensing events. Our IAC observations indicate that accuracies sufficient to detect planets can be achieved despite the higher airmass.

Limb Darkening of a K Giant in the Galactic Bulge: PLANET Photometry of MACHO 97‐BLG‐28

We present the PLANET photometric data set10 for the binary-lens microlensing event MACHO 97- BLG-28, consisting of 696 I- and V -band measurements, and analyze it to determine the radial surface brightness pro—le of the Galactic bulge source star. The microlensed source, demonstrated to be a K giant by our independent spectroscopy, crossed an isolated cusp of the central caustic of the lensing binary, generating a sharp peak in the light curve that was well-resolved by dense (3¨30 minute) and continuous monitoring from PLANET sites in Chile, South Africa, and Australia. This is the —rst time that such a cusp crossing has been observed. Analysis of the PLANET data set has produced a measure- ment of the square-root limb-darkening coefficients of the source star in the I and V bands; the resulting stellar pro—les are in excellent agreement with those predicted by stellar atmospheric models for K giants. The limb-darkening coefficients presented here are the —rst derived from microlensing. They are also among the —rst found for normal giants by any technique and the —rst for any star as distant as the Galactic bulge. Modeling of our light curve for MACHO 97-BLG-28 indicates that the lensing binary has a mass ratio q 0.23 and an (instantaneous) separation in units of the angular Einstein ring radius of d 0.69. For a lens in the Galactic bulge, this corresponds to a typical stellar binary with a projected separation between 1 and 2 AU. If the lens lies closer (i.e., in the Galactic disk), the separation is smaller, and one or both of the lens objects is in the brown dwarf regime. Assuming that the source is a bulge K2 giant at 8 kpc, the relative lens-source proper motion is k 19.4 ^ 2.6 km s~1 kpc~1, consistent with a disk or bulge lens. If the nonlensed blended light is due to a single star, it is likely to be a young white dwarf in the bulge, consistent with the blended light coming from the lens itself. Subject headings: binaries: visualgravitational lensingstars: fundamental parameters ¨ stars: late-type

X-Ray Timing in 1E 1740.7−2942 and GRS 1758−258

We report on deep observations of the Galactic bulge sources 1E 1740.7-2942 and GRS 1758-258 with the Rossi X-Ray Timing Explorer. These objects form a class of two, showing radio jets, very hard spectra, and persistent emission. Here we show that their power spectra are also similar: flat-topped noise, breaking to a power law, with a pair of weak, harmonically spaced quasi-periodic oscillations above the break and an integrated rms variability of ~20%. We discuss to what extent these results can be considered support for the identification of both as black hole candidates.

Observations of Low Mass X-Ray Binaries

AbstractIn this paper I review the properties and behavior of low-mass X-ray binary systems (LMXBs) that contain neutron stars (NS), concentrating on the Galactic bulge sources and bursters. I describe the observed characteristics of LMXBs including their light curves, spectra, eclipses, dips, bursts, flares, pulsations, QPO, long-term periodicities and orbital period changes, and explain how fast timing results and the distinction between ‘Z’ and ‘atoll’-type sources provide the key to a unified model of LMXB behavior.

A three-year record of the branch behaviour of the Z source GX 5 - 1 (4U 1758 - 25)

A three-year random sample of the X-ray flux variations of the bright Galactic Bulge source GX 5-1 obtained with the Ginga All-Sky Monitor is presented. The data allow, for the first time, to perform a statistical study of the branch behavior of a Z source. They cover 98 per cent of the branch pattern displayed by the source. The pattern remained remarkably stable over the three years of observation.

Quasi-Periodic Oscillations in Celestial X-Ray Sources

Much of what we know about x-ray sources in our galaxy derives from regularities in the way the intensity of their radiation varies with time. Cyclic increases and decreases in intensity can indicate that a star is orbiting the x-ray source, eclipsing it periodically. X-ray pulses uniformly spaced less than a few seconds apart, on the other hand, imply that the source is a high-density object, know as a neutron star, that happens to be strongly magnetized and spinning rapidly as well. Intermittent bursts of x rays also indicated the presence of a neutron star, but one on whose surface enormous thermonuclear explosions are taking place. In late 1984 and early 1985, the author and his colleagues realized that their observations of one of the brightest galactic-bulge sources, GX5-1, had revealed a peculiar type of regular variation in its x-ray intensity. The observations were made with a satellite of the European Space Agency, the x-ray observatory, EXOSAT. The variations observed in the intensity of GX5-1's x-ray emissions were particularly surprising because they were not strictly periodic: in any given observation the interval between x-ray intensity peaks was likely to be close to .03 second, but often it was a bitmore » longer or shorter. Such not quite periodic fluctuations are generally called quasi-periodic oscillations (QPO's). 6 figs.« less

X-ray observations of the Galactic center by SPARTAN 1

The results of Spartan 1 X-ray observations of the region within 1.25 deg of the Galactic center are presented. Four strong, hard X-ray point sources were detected: A1742-294, 1E 1740.7-2942, IE 1743.1-2843, and a new source SP 1744.2-2959 about 0.5 deg southeast of A1742-294. The new source had an intensity about 20 percent of that of A1742-294 and was relatively hard, having spectral parameters close to those of the diffuse source and of the Galactic bulge source GX 3+1. The Galactic nucleus X-ray source appears to have been a factor of four fainter during the 1985 Spartan 1 observations than during the 1979 Einstein observations. Hard, diffuse X-ray emission centered near the Galactic center, about 1 deg in extent, aligned to within 20 deg of the Galactic equator, and having a maximum 2-10 keV brightness of 1.5 x 10 to the -6th ergs/sq cm/s/sr was detected, as were two weaker point sources about 0.1 deg northeast and southeast of the Galactic nucleus, embedded in the diffuse emission.

X-ray spectral models of Galactic bulge sources - The emission-line factor

Current difficulties in finding unique and physically meaningful models for the X-ray spectra of Galactic bulge sources are exacerbated by the presence of strong, variable emission and absorption features that are not resolved by the instruments observing them. Nine Einstein solid state spectrometer (SSS) observations of five Galactic bulge sources are presented for which relatively high resolution objective grating spectrometer (OGS) data have been published. It is found that in every case the goodness of fit of simple models to SSS data is greatly improved by adding line features identified in the OGS that cannot be resolved by the SSS but nevertheless strongly influence the spectra observed by SSS.

Simulataneous X-ray and optical observations of LMC X-2

The X-ray source LMC X-2 was observed with EXOSAT for -8hours on February 2nd, 1984 with simultaneous photometric and spectroscopic optical coverage. The source was seen at a luminosity of L x ∼ 1.5 10 38erg.s −1 with a mean X-ray spectrum which can be described equally well by either a superposition of a thermal emission of a thin hot (T th∼5keV) plasma and a cooler blackbody radiation (T bb∼1.3keV) or by a comptonized thermal (T co∼3keV) radiation. There is no evidence of iron line. The mean source spectrum is found very alike those of the bright galactic bulge sources. The source is however only moderately variable in X-rays with spectral variations similar to those of Sco X-1 in quiescent state. No coherent periodic (0.016–2500s) pulsations are detected. Large photometric optical variations are present which suggest an orbital period of 6.4h though at a low level of significance. There is no clear correlation however between X-ray and optical variations.

The discovery of 7 and 24-28 hertz quasi-periodic oscillations in the X-ray flux of GX 17 + 2 (4U 1813-14)

view Abstract Citations (18) References (37) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Discovery of 7 and 24--28 Hertz Quasi-periodic Oscillations in the X-Ray Flux of GX 17+2 (4U 1812-14) Stella, L. ; Parmar, A. N. ; White, N. E. Abstract The discovery of quasi-periodic oscillations (QPOs) in the X-ray flux of the bright galactic bulge source GX 17 + 2 is reported. The relation between the QPO frequency and the source intensity of GX 17 + 2 is bimodal and is similar to that previously reported from Sco X-1. On August 20-21, 1985, the QPO frequency was about 7 Hz and showed no significant change associated with about 10 percent intensity variations. In another observation 25 days later, the frequency varied between 24 Hz and 28 Hz and was strongly correlated with 5 percent variations in intensity. During an observation on August 3-4, 1983, no QPOs were observed with an upper limit to the rms strength of about 1.4 percent. A search for a possible correlation between the QPOs and spectral properties was carried out by fitting to the spectra the disk plus black-body boundary layer model suggested by White et al. (1986). The QPO frequency was larger when the black-body luminosity was minimum, contrary to that observed from Sco X-1. Publication: The Astrophysical Journal Pub Date: October 1987 DOI: 10.1086/165640 Bibcode: 1987ApJ...321..418S Keywords: Galactic Nuclei; Stellar Oscillations; X Ray Stars; Astronomical Models; Black Body Radiation; Galactic Bulge; Temporal Resolution; Astrophysics; X-RAYS: SOURCES; X-RAYS: BINARIES; STARS: PULSATION full text sources ADS | data products SIMBAD (4)

An emission-line object in the core of M15

Optical identification of globular cluster X-ray sources is essential if their evolutionary state and relation to the galactic bulge sources is to be understood. The 10 high-luminosity (⩾1036 erg s−1) X-ray sources associated with globular clusters have very accurate (∼1 arc s) positions from the Einstein Observatory survey1,2. No globular cluster source has yet been optically identified, primarily because all of the bright X-ray sources are located close to the dynamical centre (implying total masses of ∼1.5M⊙ (ref. 2)), where crowding makes optical photometry and spectroscopy extremely difficult. The only northern cluster containing a bright X-ray source is M15, which has the extremely ultraviolet (U − B = −1.2) and variable star AC211 near the X-ray error circle3,4. The reported spectrum is that of an A star5,6, with no emission lines which are normally present in galactic low-mass X-ray binaries. We have now discovered7,8 strong He II 4,686-A emission from the region of AC211, and weaker Balmer emission. We conclude that it is an intrinsically high-luminosity (Lx⩾1038erg s−1) X-ray source on the basis of the optical brightness of the star and the He II flux. As the observed Lx is only 6 × 1036 erg s−1, this implies that the system is at a high inclination and that only scattered X-rays from an extended region are observed. The mass transfer rate required to account for this is very high and places important constraints on the evolutionary state of the mass-losing star which, for its optical brightness, must either be a red giant or horizontal branch star. The location of AC211 questions the accuracy of the Einstein Observatory positions.

X-ray emission lines from three Galactic bulge sources

X-ray spectroscopic observations obtained with the Einstein objective grating spectrometer (OGS) and monitor proportional counter (MPC) instruments of three Galactic bulge sources, the globular cluster burst source 1820-30, the burster Serpens X-1, and the GX 9+9 are presented. Joint spectral fits to the OGS and MPC data are consistent for all three sources with either a thermal bremsstrahlung model with temperature ranging from 6 to 10 keV or with a two-component blackbody model, where one component may be associated with the neutron star and one with the accretion disk. The spectra of Serpens X-1 and 1820-30 harden with increases in intensity. The implications of the results for recent models of low-mass X-ray binaries are discussed.

Quasiperiodic Oscillations Probe Galactic X-Ray Sources

“It's very frustrating,” says William Priedhorsky, an x-ray astronomer at Los Alamos, “that the brightest x-ray sources in the sky are the ones we have had to be most speculative about.” The sources of Priedhorsky's lament are the so-called galactic bulge sources, energetic enigmas found primarily in the galaxy's central bulge. Theorists had long speculated that the x-ray emissions of a bulge source originate from hot gas falling from a low-mass star onto the surface of its rapidly rotating neutron star companion. Unfortunately, evidence for this model had been indirect; the purported neutron star lies hidden beneath a plasma shroud. But recently the situation changed dramatically when a collaboration of researchers from the Max Planck Institute for Extraterrestrial Physics, the University of Amsterdam and MIT reported observations of rapid, quasiperiodic x-ray oscillations from the bright bulge source GX5—1. Subsequent observations of other bulge sources by Günter Hasinger (Max Planck Institute, Garching) and collaborators, and by Priedhorsky and John Middleditch (Los Alamos) have uncovered five more quasiperiodic oscillators. The existence of QPOs may provide astronomers with their first handle on the bulge source's neutron star. As a bonus they may also offer solutions to a few long-standing puzzles of galactic astronomy.

Quasi-periodic oscillations in bright galactic-bulge X-ray sources

Quasiperiodic oscillations with frequencies in the range 5-50 Hz have recently been discovered in X-rays from two bright galactic-bulge sources and Sco X-1. These sources are weakly magnetic neutron stars accreting from disks which the plasma is clumped. The interaction of the magnetosphere with clumps in the inner disk causes oscillations in the X-ray flux with many of the properties observed.