213 research outputs found
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 () 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 m) and H
I (2.166 m - Br), consistent with previous work. On August 14 UTC,
we took spectra with -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 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
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
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^â1; 2-8 keV), and spectra (power laws with Î = 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^â1. 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
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
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