A Catalog of Diffuse X-ray-Emitting Features within 20 pc of Sgr A*: Twenty Pulsar Wind Nebulae?

We present a catalog of 34 diffuse features identified in X-ray images of the Galactic center taken with the Chandra X-ray Observatory. Several of the features have been discussed in the literature previously, including 7 that are associated with a complex of molecular clouds that exhibits fluorescent line emission, 4 that are superimposed on the supernova remnant Sgr A East, 2 that are coincident with radio features that are thought to be the shell of another supernova remnant, and one that is thought to be a pulsar wind nebula only a few arcseconds in projection from Sgr A*. However, this leaves 20 features that have not been reported previously. Based on the weakness of iron emission in their spectra, we propose that most of them are non-thermal. One long, narrow feature points toward Sgr A*, and so we propose that this feature is a jet of synchrotron-emitting particles ejected from the supermassive black hole. For the others, we show that their sizes (0.1-2 pc in length for D=8 kpc), X-ray luminosities (between 10^32 and 10^34 erg/s, 2-8 keV), and spectra (power laws with Gamma=1-3) are consistent with those of pulsar wind nebulae. Based on the star formation rate at the Galactic center, we expect that ~20 pulsars have formed in the last 300 kyr, and could be producing pulsar wind nebulae. Only one of the 19 candidate pulsar wind nebulae is securely detected in an archival radio image of the Galactic center; the remainder have upper limits corresponding to L_R<la10^31 erg/s. These radio limits do not strongly constrain their natures, which underscores the need for further multi- wavelength studies of this unprecedented sample of Galactic X-ray emitting structures.Comment: 14 pages, 8 figures, 5 in color. Submitted to Ap

Isolated, Massive Supergiants near the Galactic Center

We have carried out a pilot project to assess the feasibility of using radio, infrared, and X-ray emission to identify young, massive stars located between 1 and 25 pc from the Galactic center. We first compared catalogs compiled from the Very Large Array, the Chandra X-ray Observatory, and 2MASS. We identified two massive, young stars: the previously-identified star that is associated with the radio HII region H2, and a newly-identified star that we refer to as CXOGC J174516.1-290315. The infrared spectra of both stars exhibit very strong Br-gamma and He I lines, and resemble those of massive supergiants that have evolved off of the main sequence, but not yet reached the Wolf-Rayet phase. We estimate that each star has a bolometric luminosity >10^6 L_sun. The detection of these two sources in X-rays is surprising, because stars at similar evolutionary states are not uniformly bright X-ray sources. Therefore, we suggest that both stars are in binary systems that contain either OB stars whose winds collide with those of the luminous supergiants, or compact objects that are accreting from the winds of the supergiants. We also identify X-ray emission from a nitrogen-type Wolf-Rayet star and place upper limits on the X-ray luminosities of three more evolved, massive stars that previously have been identified between 1 and 25 pc from Sgr A*. Finally, we briefly discuss the implications that future searches for young stars will have for our understanding of the recent history of star formation near the Galactic center. (abridged)Comment: 9 pages, including 8 figures. Submitted to ApJ, and modified in response to referee's repor

An Overabundance of Transient X-ray Binaries within 1 pc of the Galactic Center

During five years of Chandra observations, we have identified seven X-ray transients located within 23 pc of Sgr A*. These sources each vary in luminosity by more than a factor of 10, and have peak X-ray luminosities greater than 5e33 erg/s, which strongly suggests that they are accreting black holes or neutron stars. The peak luminosities of the transients are intermediate between those typically considered outburst and quiescence for X-ray binaries. Remarkably four of these transients lie within only 1 pc of Sgr A*. This implies that, compared to the numbers of similar systems located between 1 and 23 pc, transients are over-abundant by a factor of 20 per unit stellar mass within 1 pc of Sgr A*. It is likely that the excess transient X-ray sources are low-mass X-ray binaries that were produced, as in the cores of globular clusters, by three-body interactions between binary star systems and either black holes or neutron stars that have been concentrated in the central parsec through dynamical friction. Alternatively, they could be high-mass X-ray binaries that formed among the young stars that are present in the central parsec.Comment: 4 pages, including 2 figures (one color). Submitted to ApJ Letter

Planetary systems around close binary stars: the case of the very dusty, Sun-like, spectroscopic binary BD+20 307

Field star BD+20 307 is the dustiest known main sequence star, based on the fraction of its bolometric luminosity, 4%, that is emitted at infrared wavelengths. The particles that carry this large IR luminosity are unusually warm, comparable to the temperature of the zodiacal dust in the solar system, and their existence is likely to be a consequence of a fairly recent collision of large objects such as planets or planetary embryos. Thus, the age of BD+20 307 is potentially of interest in constraining the era of terrestrial planet formation. The present project was initiated with an attempt to derive this age using the Chandra X-ray Observatory to measure the X-ray flux of BD+20 307 in conjunction with extensive photometric and spectroscopic monitoring observations from Fairborn Observatory. However, the recent realization that BD+20 307 is a short period, double-line, spectroscopic binary whose components have very different lithium abundances, vitiates standard methods of age determination. We find the system to be metal-poor; this, combined with its measured lithium abundances, indicates that BD+20 307 may be several to many Gyr old. BD+20 307 affords astronomy a rare peek into a mature planetary system in orbit around a close binary star (because such systems are not amenable to study by the precision radial velocity technique).Comment: accepted for ApJ, December 10, 200

The Spectra and Variability of X-ray Sources in a Deep Chandra Observation of the Galactic Center

We examine the X-ray spectra and variability of the sample of X-ray sources with L_X = 10^{31}-10^{33} erg s^{-1} identified within the inner 9' of the Galaxy. Very few of the sources exhibit intra-day or inter-month variations. We find that the spectra of the point sources near the Galactic center are very hard between 2--8 keV, even after accounting for absorption. When modeled as power laws the median photon index is Gamma=0.7, while when modeled as thermal plasma we can only obtain lower limits to the temperature of kT>8 keV. The combined spectra of the point sources is similarly hard, with a photon index of Gamma=0.8. Strong line emission is observed from low-ionization, He-like, and H-like Fe, both in the average spectra and in the brightest individual sources. The line ratios of the highly-ionized Fe in the average spectra are consistent with emission from a plasma in thermal equilibrium. This line emission is observed whether average spectra are examined as a function of the count rate from the source, or as a function of the hardness ratios of individual sources. This suggests that the hardness of the spectra may in fact to due local absorption that partially-covers the X-ray emitting regions in the Galactic center systems. We suggest that most of these sources are intermediate polars, which (1) often exhibit hard spectra with prominent Fe lines, (2) rarely exhibit either flares on short time scales or changes in their mean X-ray flux on long time scales, and (3) are the most numerous hard X-ray sources with comparable luminosities in the Galaxy.Comment: 27 pages, including 13 figures. To appear in ApJ, 1 October 2004, v613 issue. An electronic version of table 2 is on http://astro.ucla.edu/~mmuno/sgra/table2_electronic.txt and reduced data files for each source are available on http://www.astro.psu.edu/users/niel/galcen-xray-data/galcen-xray-data.htm

Spectroscopy of Infrared Flares from the Microquasar GRS 1915+105

We present near-infrared medium-resolution ($R \sim 875$) spectra of the microquasar GRS 1915+105 on 1997 August 13-15 UTC from the Hale 200-inch telescope. The spectra showed broad emission lines of He I (2.058 $\mu$m) and H I (2.166 $\mu$m - Br$\gamma$), consistent with previous work. On August 14 UTC, we took spectra with $\sim 6$-minute time resolution during infrared flaring events similar to those reported in Eikenberry et al. (1998a), which appear to reveal plasma ejection from the system. During the flares, the emission line fluxes varied in approximately linear proportionality to the IR continuum flux, implying that the lines are radiatively pumped by the flares. We also detected a weak He II (2.189 $\mu$m) emission line on August 14 UTC. The nature of the line variability and the presence of the He II feature indicate that the emission lines in GRS 1915+105 arise in an accretion disk around the compact object, rather than in the circumstellar disk of a proposed Oe/Be companion. The radiative line pumping also implies that the flare emission originates from ejecta which have moved out of the accretion disk plane.Comment: 13 pages plus 4 figures, to appear in ApJ Letter

A catalog of diffuse X-ray-emitting features within 20 pc of Sagittarius A*: Twenty pulsar wind nebulae?

We present a catalog of 34 diffuse features identified in X-ray images of the Galactic center taken with the Chandra X-Ray Observatory. Several of the features have been discussed in the literature previously, including seven that are associated with a complex of molecular clouds that exhibits fluorescent line emission, four that are superimposed on the supernova remnant Sgr A East, two that are coincident with radio features that are thought to be the shell of another supernova remnant, and one that is thought to be a pulsar wind nebula only a few arcseconds in projection from Sgr A*. However, this leaves 20 features that have not been reported previously. On the basis of the weakness of the iron emission in their spectra, we propose that most of them are nonthermal. One long, narrow feature points toward Sgr A*, and so we propose that this feature is a jet of synchrotron-emitting particles ejected from the supermassive black hole. For the others, we show that their sizes (0.1-2 pc in length for D = 8 kpc), X-ray luminosities (between 10^32 and 10^34 erg s^−1; 2-8 keV), and spectra (power laws with Γ = 1-3) are consistent with those of pulsar wind nebulae. On the basis of the star formation rate at the Galactic center, we expect that ~20 pulsars have formed in the last 300 kyr and could be producing pulsar wind nebulae. Only 1 of the 19 candidate pulsar wind nebulae is securely detected in an archival radio image of the Galactic center; the remainder have upper limits corresponding to L_R ≾ 10^31 erg s^−1. These radio limits do not strongly constrain their natures, which underscores the need for further multiwavelength studies of this unprecedented sample of Galactic X-ray-emitting structures

Discovery of Variable Iron Fluorescence from Reflection Nebulae in the Galactic Center

Based on three years of deep observations of the Galactic center with the Chandra X-ray Observatory, we report the discovery of changes in the intensities and morphologies of two hard X-ray nebulosities. The nebulosities are dominated by fluorescent iron emission, and are coincident with molecular clouds. The morphological changes are manifest on parsec scales, which requires that these iron features are scattered X-rays from a 2 or 3-year-long outburst of a point source (either Sgr A* or an X-ray binary) with a luminosity of at least 1e37 erg/s. The variability precludes the hypotheses that these nebulae either are produced by keV electrons bombarding molecular clouds, or are iron-rich ejecta from supernovae. Moreover, the morphologies of the reflection nebulae implies that the dense regions of the clouds are filamentary, with widths of ~0.3 pc and lengths of ~2 pc

Accreting Neutron Stars in Low-Mass X-Ray Binary Systems

Using the Rossi X-ray Timing Explorer (RossiXTE), astronomers have discovered that disk-accreting neutron stars with weak magnetic fields produce three distinct types of high-frequency X-ray oscillations. These oscillations are powered by release of the binding energy of matter falling into the strong gravitational field of the star or by the sudden nuclear burning of matter that has accumulated in the outermost layers of the star. The frequencies of the oscillations reflect the orbital frequencies of gas deep in the gravitational field of the star and/or the spin frequency of the star. These oscillations can therefore be used to explore fundamental physics, such as strong-field gravity and the properties of matter under extreme conditions, and important astrophysical questions, such as the formation and evolution of millisecond pulsars. Observations using RossiXTE have shown that some two dozen neutron stars in low-mass X-ray binary systems have the spin rates and magnetic fields required to become millisecond radio-emitting pulsars when accretion ceases, but that few have spin rates above about 600 Hz. The properties of these stars show that the paucity of spin rates greater than 600 Hz is due in part to the magnetic braking component of the accretion torque and to the limited amount of angular momentum that can be accreted in such systems. Further study will show whether braking by gravitational radiation is also a factor. Analysis of the kilohertz oscillations has provided the first evidence for the existence of the innermost stable circular orbit around dense relativistic stars that is predicted by strong-field general relativity. It has also greatly narrowed the possible descriptions of ultradense matter.Comment: 22 pages, 7 figures, updated list of sources and references, to appear in "Short-period Binary Stars: Observation, Analyses, and Results", eds. E.F. Milone, D.A. Leahy, and D. Hobill (Dordrecht: Springer, http://www.springerlink.com

Eddington-limited X-ray Bursts as Distance Indicators. I. Systematic Trends and Spherical Symmetry in Bursts from 4U 1728-34

We investigate the limitations of thermonuclear X-ray bursts as a distance indicator for the weakly-magnetized accreting neutron star 4U 1728-34. We measured the unabsorbed peak flux of 81 bursts in public data from the Rossi X-Ray Timing Explorer (RXTE). The distribution of peak fluxes was bimodal: 66 bursts exhibited photospheric radius expansion and were distributed about a mean bolometric flux of 9.2e-8 erg/cm^2/s, while the remaining (non-radius expansion) bursts reached 4.5e-8 erg/cm^2/s, on average. The peak fluxes of the radius-expansion bursts were not constant, exhibiting a standard deviation of 9.4% and a total variation of 46%. These bursts showed significant correlations between their peak flux and the X-ray colors of the persistent emission immediately prior to the burst. We also found evidence for quasi-periodic variation of the peak fluxes of radius-expansion bursts, with a time scale of approximately 40 d. The persistent flux observed with RXTE/ASM over 5.8 yr exhibited quasi-periodic variability on a similar time scale. We suggest that these variations may have a common origin in reflection from a warped accretion disk. Once the systematic variation of the peak burst fluxes is subtracted, the residual scatter is only approximately 3%, roughly consistent with the measurement uncertainties. The narrowness of this distribution strongly suggests that i) the radiation from the neutron star atmosphere during radius-expansion episodes is nearly spherically symmetric, and ii) the radius-expansion bursts reach a common peak flux which may be interpreted as a standard candle intensity.Adopting the minimum peak flux for the radius-expansion bursts as the Eddington flux limit, we derive a distance for the source of 4.4-4.8 kpc.Comment: 9 pages, 7 figures, accepted by ApJ. Minor referee's revisions, also includes 9 newly public X-ray burst