167 research outputs found
Characterization of the optical and X-ray properties of the northwestern wisps in the Crab Nebula
We have studied the wisps to the north-west of the Crab pulsar as part of a
multi-wavelength campaign in the visible and in X-rays. Optical observations
were obtained using the Nordic Optical Telescope in La Palma and X-ray
observations were made with the Chandra X-ray Observatory. The observing
campaign took place from 2010 October until 2012 September. About once per year
we observe wisps forming and peeling off from (or near) the region commonly
associated with the termination shock of the pulsar wind. We find that the
exact locations of the northwestern wisps in the optical and in X-rays are
similar but not coincident, with X-ray wisps preferentially located closer to
the pulsar. This suggests that the optical and X-ray wisps are not produced by
the same particle distribution. Our measurements and their implications are
interpreted in terms of a Doppler-boosted ring model that has its origin in
magne- tohydrodynamic (MHD) modelling. While the Doppler boosting factors
inferred from the X-ray wisps are consistent with current MHD simulations of
pulsar wind nebulae (PWN), the optical boosting factors are not, and typically
exceed values from MHD simulations by about a factor of 3.Comment: 11 pages, 12 figure
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
Implications of the Observed Ultraluminous X-Ray Source Luminosity Function
We present the X-ray luminosity function (XLF) of ultraluminous X-ray (ULX) sources with 0.3-10.0 keV luminosities in excess of 10(sup 39) erg/s in a complete sample of nearby galaxies. The XLF shows a break or cut-off at high luminosities that deviates from its pure power law distribution at lower luminosities. The cut-off is at roughly the Eddington luminosity for a 90-140 solar mass accretor. We examine the effects on the observed XLF of sample biases, of small-number statistics (at the high luminosity end) and of measurement uncertainties. We consider the physical implications of the shape and normalization of the XLF. The XLF is also compared and contrasted to results of other recent surveys
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
Analysis of an Anemone-Type Eruption in an On-Disk Coronal Hole
We report on an eruption seen in a very small coronal hole (about 120 arcseconds across), beginning at approximately 19:00 Universal Time on March 3, 2016. The event was initially observed by an amateur astronomer (RW) in an H-alpha movie from the Global Oscillation Network Group (GONG); the eruption attracted the attention of the observer because there was no nearby active region. To examine the region in detail, we use data from the Solar Dynamics Observatory (SDO), provided by the Atmospheric Imaging Assembly (AIA) in wavelengths 193 angstroms, 304 angstroms, and 94 angstroms, and the Helioseismic and Magnetic Imager (HMI). Data analysis and calibration activities such as scaling, rotation so that north is up, and removal of solar rotation are accomplished with SunPy. The eruption in low-cadence HMI data begins with the appearance of a bipole in the location of the coronal hole, followed by (apparent) expansion outwards when the intensity of the AIA wavelengths brighten; as the event proceeds, the coronal hole disappears. From high-cadence data, we will present results on the magnetic evolution of this structure, how it is related to intensity brightenings seen in the various SDO/AIA wavelengths, and how this event compares with the standard-anemone picture
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
Composition of the Chandra ACIS contaminant
The Advanced CCD Imaging Spectrometer (ACIS) on the Chandra X-ray Observatory
is suffering a gradual loss of low energy sensitivity due to a buildup of a
contaminant. High resolution spectra of bright astrophysical sources using the
Chandra Low Energy Transmission Grating Spectrometer (LETGS) have been analyzed
in order to determine the nature of the contaminant by measuring the absorption
edges. The dominant element in the contaminant is carbon. Edges due to oxygen
and fluorine are also detectable. Excluding H, we find that C, O, and F
comprise >80%, 7%, and 7% of the contaminant by number, respectively. Nitrogen
is less than 3% of the contaminant. We will assess various candidates for the
contaminating material and investigate the growth of the layer with time. For
example, the detailed structure of the C-K absorption edge provides information
about the bonding structure of the compound, eliminating aromatic hydrocarbons
as the contaminating material.Comment: To appear in Proceedings SPIE volume 5165; paper is 12 pages long
with 13 figure
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
An X-ray View of Star Formation in the Central 3 kpc of NGC 2403
Archival Chandra observations are used to study the X-ray emission associated
with star formation in the central region of the nearby SAB(s)cd galaxy NGC
2403. The distribution of X-ray emission is compared to the morphology visible
at other wavelengths using complementary Spitzer, GALEX, and ground-based
Halpha imagery. In general, the brightest extended X-ray emission is associated
with HII regions and to other star-forming structures but is more pervasive;
existing also in regions devoid of strong Halpha and UV emission. This X-ray
emission has the spectral properties of diffuse hot gas (kT ~ 0.2keV) whose
likely origin is in gas shock-heated by stellar winds and supernovae with < 20%
coming from faint unresolved X-ray point sources. This hot gas may be
slowly-cooling extra-planar remnants of past outflow events, or a disk
component that either lingers after local star formation activity has ended or
that has vented from active star-forming regions into a porous interstellar
medium.Comment: 25 pages, accepted to A
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