117 research outputs found
Discovery of a 3.6-hr Eclipsing Luminous X-Ray Binary in the Galaxy NGC 4214
We report the discovery of an eclipsing X-ray binary with a 3.62-hr period
within 24" of the center of the dwarf starburst galaxy NGC 4214. The orbital
period places interesting constraints on the nature of the binary, and allows
for a few very different interpretations. The most likely possibility is that
the source lies within NGC 4214 and has an X-ray luminosity of up to 7 e38
ergs/s. In this case the binary may well be comprised of a naked He-burning
donor star with a neutron-star accretor, though a stellar-mass black-hole
accretor cannot be completely excluded. There is no obvious evidence for a
strong stellar wind in the X-ray orbital light curve that would be expected
from a massive He star; thus, the mass of the He star should be <3-4 solar
masses. If correct, this would represent a new class of very luminous X-ray
binary -- perhaps related to Cyg X-3. Other less likely possibilities include a
conventional low-mass X-ray binary that somehow manages to produce such a high
X-ray luminosity and is apparently persistent over an interval of years; or a
foreground AM Her binary of much lower luminosity that fortuitously lies in the
direction of NGC 4214. Any model for this system must accommodate the lack of
an optical counterpart down to a limiting magnitude of 22.6 in the visible.Comment: 7 pages, ApJ accepted versio
Automatic Identification of Solar X-Ray Bright Points in Hinode X-Ray Data
We have automated a method that is used to find point sources in Chandra X-ray telescope data, to identify solar bright points in Hinode X-ray data. This tool, called lextrct, first identifies candidate sources that are brighter than the surrounding background. The algorithm also allows selected pixels to be excluded from the source-finding, thus allowing saturated pixels (from flares and/or active regions) to be ignored. We then use lextrct to fit the sources to two-dimensional, elliptical Gaussians. The size and orientation give an approximation of the shape of the bright points. We are in the process of analyzing observations through the Al_poly filter with a four-second exposure time, to obtain a catalogue of bright points, which will include their sizes, lifetimes, intensities, and position on the solar dis
Do Ultraluminous X-Ray Sources Exist in Dwarf Galaxies?
A thorough search for Ultraluminous X-ray source candidates within the Local
Volume is made. The search spatially matches potential ULXs detected in X-ray
images or cataloged in the literature with galaxies tabulated in the Catalog of
Neighboring Galaxies compiled by Karachentsev et al. (2004). The specific ULX
frequency (occurrence rate per unit galaxy mass) is found to be a decreasing
function of host galaxy mass for host masses above solar mass.
There is too little mass in galaxies below this point to determine if this
trend continues to lower galaxy mass. No ULXs have yet been detected in
lower-mass galaxies. Systematic differences between dwarf and giant galaxies
that may explain an abundance of ULXs in dwarf galaxies and what they may imply
about the nature of ULXs are discussed.Comment: Accepted to Ap
On the Nature of the Bright Short-Period X-ray Source in the Circinus Galaxy Field
The spectrum and light curve of the bright X-ray source CG X-1 in the field
of the Circinus galaxy are re-examined. Previous analyses have concluded that
the source is an accreting black hole of about 50 solar masses although it was
noted that the light curve resembles that of an AM Her system. Here we show
that the short period and an assumed main sequence companion constrain the mass
of the companion to less than one solar mass. Further a possible eclipse seen
during one of the Chandra observations and a subsequent XMM-Newton observation
constrains the mass of the compact object to less than about 60 solar masses.
If such a system lies in the Circinus galaxy, then the accreting object must
either radiate anisotropically or strongly violate the Eddington limit. Even if
the emission is beamed, then the companion star which intercepts this flux
during eclipse will be driven out of thermal equilibrium and evaporate within
about 1000 years. We find that the observations cannot rule out an AM Her
system in the Milky Way and that such a system can account for the variations
seen in the light curve.Comment: 25 pages, 8 figures, accepted for publication in the Astrophysical
Journa
Optical Spectroscopy of the environment of a ULX in NGC 7331
Optical photometric and spectroscopic data are presented that show an
association of an ultraluminous X-ray source in NGC 7331 with a young star
cluster of mass 1.1e5 solar masses and age 4.25 Myr. If the ULX is part of the
bright stellar cluster, then the mass of the progenitor of the compact accretor
must have been greater than about 40-50 solar masses in order to already have
evolved through the supernova stage to a compact object. The companion star is
also likely an evolved massive star. The emission line spectrum of the nebula
surrounding the cluster can be interpreted as a result of photoionization by
the cluster OB stars with an additional source of shock excitation producing
strong [SII], [OI] and NII lines. This additional source appears to be as much
as five times more powerful than the supernovae and stellar winds in the
cluster can provide. Additional mechanical energy input associated with the ULX
itself can help explain the residual shock excited line luminosities of the
emission region.Comment: 17 pages, accepted to Ap
Chandra Phase-Resolved X-ray Spectroscopy of the Crab Pulsar II
We present a new study of the X-ray spectral properties of the Crab Pulsar. The superb angular resolution of the Chandra X-ray Observatory enables distinguishing the pulsar from the surrounding nebulosity. Analysis of the spectrum as a function of pulse phase allows the least-biased measure of interstellar X-ray extinction due primarily to photoelectric absorption and secondarily to scattering by dust grains in the direction of the Crab Nebula. We modify previous findings that the line-of-sight to the Crab is under-abundant in oxygen and provide measurements with improved accuracy and less bias. Using the abundances and cross sections from Wilms, Allen & McCray (2000) we find [O/H] = (5.28+\-0.28) x 10(exp -4) (4.9 x 10(exp -4) is solar abundance). \rVe also measure for the first time the impact of scattering of flux out of the image by interstellar grains. \rYe find T(sub scat) = 0.147+/-0.043. Analysis of the spectrum as a function of pulse phase also measures the X-ray spectral index even at pulse minimum - albeit with increasing statistical uncertainty. The spectral variations are, by and large, consistent with a sinusoidal variation. The only significant variation from the sinusoid occurs over the same phase range as some rather abrupt behavior in the optical polarization magnitude and position angle. We compare these spectral variations to those observed in Gamma-rays and conclude that our measurements are both a challenge and a guide to future modeling and will thus eventually help us understand pair cascade processes in pulsar magnetospheres. The data were also used to set new. and less biased, upper limits to the surface temperature of the neutron star for different models of the neutron star atmosphere
On the nature of the ultraluminous X-ray transient in Cen~A (NGC 5128)
We combine 9 ROSAT, 9 Chandra, and 2 XMM-Newton observations of the Cen~A
galaxy to obtain the X-ray light curve of 1RXH J132519.8-430312 (=CXOU
J132519.9430317) spanning 1990 to 2003. The source reached a peak 0.1-2.4
keV flux F_X>10^{-12} ergs cm^{-2} s^{-1} during a 10~day span in 1995 July.
The inferred peak isotropic luminosity of the source therefore exceeded 3
10^{39} ergs s^{-1}, which places the source in the class of ultra-luminous
X-ray sources. Coherent pulsations at 13.264 Hz are detected during a second
bright episode (F_X >3 times 10^{-13} ergs cm^{-2} s^{-1}) in 1999 December.
The source is detected and varies significantly within three additional
observations but is below the detection threshold in 7 observations. The X-ray
spectrum in 1999 December is best described as a cut-off power law or a
disk-blackbody (multi-colored disk). We also detect an optical source, m_F555W
~ 24.1 mag, within the Chandra error circle of 1RXH J132519.8-430312 in HST
images taken 195~days before the nearest X-ray observation. The optical
brightness of this source is consistent with a late O or early B star at the
distance of Cen A. If the optical source is the counterpart, then the X-ray and
optical behavior of 1RXH J132519.8-430312 are similar to the transient Be/X-ray
pulsar A 0538-66.Comment: 7 pages, 8 figures. ApJ (accepted
New Observations of the Crab Nebula and Pulsar
We present a phase-resolved study of the X-ray spectrum of the Crab Pulsar, using data obtained in a special mode with the Chandra X-ray Observatory. The superb angular resolution easily enables discerning the Pulsar from the surrounding nebulosity, even at pulse minimum. We find that the Pulsar's X-ray spectral index varies sinusoidally with phase---except over the same phase range for which rather abrupt changes in optical polarization magnitude and position angle have been reported. In addition, we use the X-ray data to constrain the surface temperature for various neutron-star equations of state and atmospheres. Finally, we present new data on dynamical variations of structure within the Nebula
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