arXiv subjects

Michael R. Garcia

Publications and source records attributed to Michael R. Garcia.

23 records · Page 2Linked to original sources

arXiv2000

A First Look at the Nuclear Region of M31 with Chandra

We report on the first observation of the nuclear region of M31 with the Chandra X-ray Observatory. The nuclear source seen with the Einstein and ROSAT HRIs is resolved into five point sources. One of these sources is within 1'' of the M31 central super-massive black hole. As compared to the other point sources in M31, this nuclear source has an unusual x-ray spectrum. Based on the spatial coincidence we identify this source with the central black hole, and note that the unusual spectrum is a challenge to current theories. A bright transient is detected ~26'' to the west of the nucleus, which may be associated with a stellar mass black hole.

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arXiv1999

Two Neutron Star SXT in Quiescence: 4U 2129+47 and EXO 0748-676

We report on x-ray observations of two soft x-ray transients containing neutron stars, 4U 2129+47 and EXO 0748-676. Our emphasis is on the quiescent properties of these sources. The x-ray spectrum and lightcurve of the eclipsing Low-Mass X-ray Binary (LMXB) 4U 2129+47 was measured with the ROSAT PSPC during its current quiescent state. The quiescent x-ray luminosity of ~ 10**32.7 ergs/s and blackbody temperature of kT ~ 0.21 keV are similar to other quiescent LMXB. The quiescent x-ray light curve appears to show orbital modulation, but the statistics are insufficient to distinguish between a v-shaped partial eclipse (as seen in the high state) or a total, square wave eclipse. The similarity in the luminosity and temperature to other (non-eclipsing) quiescent LMXB implies that the vertical structure in the disk which blocked our direct view of the neutron star in the high state has collapsed, and the neutron star is seen directly. EXO 0748-676 was serendipitously observed with the Einstein IPC in quiescence before it was discovered as a bright transient with EXOSAT. Our re-analysis of this quiescent observation finds L_x ~ 10**34.0 ergs/s, and blackbody temperature of kT ~ 0.22 keV, again similar to other LMXBs in quiescence.

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arXiv1999

Black Hole and Neutron Star Transients in Quiescence

We consider the X-ray luminosity difference between neutron star and black hole soft X-ray transients (NS and BH SXTs) in quiescence. The current observational data suggest that BH SXTs are significantly fainter than NS SXTs. The luminosities of quiescent BH SXTs are consistent with the predictions of binary evolution models for the mass transfer rate if (1) accretion occurs via an ADAF in these systems and (2) the accreting compact objects have event horizons. The luminosities of quiescent NS SXTs are not consistent with the predictions of ADAF models when combined with binary evolution models, unless most of the mass accreted in the ADAF is prevented from reaching the neutron star surface. We consider the possibility that mass accretion is reduced in quiescent NS SXTs because of an efficient propeller and develop a model of the propeller effect that accounts for the observed luminosities. We argue that modest winds from ADAFs are consistent with the observations while strong winds are probably not.

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arXiv1997

Advection-Dominated Accretion and Black Hole Event Horizons

The defining characteristic of a black hole is that it possesses an event horizon through which matter and energy can fall in but from which nothing escapes. Soft X-ray transients (SXTs), a class of X-ray binaries, appear to confirm this fundamental property of black holes. SXTs that are thought to contain accreting black holes display a large variation of luminosity between their bright and faint states, while SXTs with accreting neutron stars have a smaller variation. This difference is predicted if the former stars have horizons and the latter have normal surfaces.

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arXiv1995

Observations of the X-ray Nova GRO~J0422+32: II: Optical Spectra Approaching Quiescence

We present results obtained from a series of 5~Å resolution spectra of the X-ray Nova GRO~J0422+32 obtained in 1993~October, when the system was approximately 2 magnitudes above quiescence, with ${\rm R \sim 19}$. The data were obtained in an effort to measure the orbital radial velocity curve of the secondary, but detection of the narrow photospheric absorption lines needed to do this proved elusive. Instead we found wide absorption bands reminiscent of M~star photospheric features. The parameters determined by fitting accretion disk line profiles (Smak profiles) to the H$α$ line are similar to those found in several strong black-hole candidates. Measurements of the velocity of the H$α$ line are consistent with an orbital period of 5.1~hours and a velocity semi-amplitude of the primary of $34 \pm 6$~\kms. These measurements, when combined with measurements of the velocity semi-amplitude of the secondary made by others, indicate that the mass ratio $q \sim 0.09$. If the secondary follows the empirical mass-radius relation found for CVs, the low $q$ implies a primary mass of $M_x \sim 5.6$\mo, and a rather low (face-on) inclination. The H$α$ EW is found to be modulated on the orbital period with a phasing that implies a partial eclipse of the disk by the secondary, but simultaneous R~band photometry shows no evidence for such an eclipse.

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