48 research outputs found
X-RAY SCATTERING ECHOES AND GHOST HALOS FROM THE INTERGALACTIC MEDIUM: RELATION TO THE NATURE OF AGN VARIABILITY
X-ray bright quasars might be used to trace dust in the circumgalactic and intergalactic medium through the phenomenon of X-ray scattering, which is observed around Galactic objects whose light passes through a sufficient column of interstellar gas and dust. Of particular interest is the abundance of gray dust larger than 0.1 µm, which is difficult to detect at other wavelengths. To calculate X-ray scattering from large grains, one must abandon the traditional Rayleigh-Gans approximation. The Mie solution for the X-ray scattering optical depth of the universe is ~1%. This presents a great difficulty for distinguishing dust scattered photons from the point source image of Chandra, which is currently unsurpassed in imaging resolution. The variable nature of AGNs offers a solution to this problem, as scattered light takes a longer path and thus experiences a time delay with respect to non-scattered light. If an AGN dims significantly ([> over ~]3 dex) due to a major feedback event, the Chandra point source image will be suppressed relative to the scattering halo, and an X-ray echo or ghost halo may become visible. I estimate the total number of scattering echoes visible by Chandra over the entire sky: N[subscript ech] ~ 10[superscript 3](ν[subscript fb]/yr[superscript -1]), where ν[subscript fb] is the characteristic frequency of feedback events capable of dimming an AGN quickly.United States. National Aeronautics and Space Administration (Earth and Space Science Fellowship Program Grant NNX11AO09H
The Impact of Accurate Extinction Measurements for X-ray Spectral Models
Interstellar extinction includes both absorption and scattering of photons
from interstellar gas and dust grains, and it has the effect of altering a
source's spectrum and its total observed intensity. However, while multiple
absorption models exist, there are no useful scattering models in standard
X-ray spectrum fitting tools, such as XSPEC. Nonetheless, X-ray halos, created
by scattering from dust grains, are detected around even moderately absorbed
sources and the impact on an observed source spectrum can be significant, if
modest, compared to direct absorption. By convolving the scattering cross
section with dust models, we have created a spectral model as a function of
energy, type of dust, and extraction region that can be used with models of
direct absorption. This will ensure the extinction model is consistent and
enable direct connections to be made between a source's X-ray spectral fits and
its UV/optical extinction.Comment: 8 pages, 4 figure
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High Energy Studies of Astrophysical Dust
Astrophysical dust -- any condensed matter ranging from tens of atoms to micron sized grains -- accounts for about one third of the heavy elements produced in stars and disseminated into space. These tiny pollutants are responsible for producing the mottled appearance in the spray of light we call the "Milky Way." However these seemingly inert particles play a strong role in the physics of the interstellar medium, aiding star and planet formation, and perhaps helping to guide galaxy evolution. Most dust grains are transparent to X-ray light, leaving a signature of atomic absorption, but also scattering the light over small angles. Bright X-ray objects serendipitously situated behind large columns of dust and gas provide a unique opportunity to study the dust along the line of sight. I focus primarily on X-ray scattering through dust, which produces a diffuse halo image around a central point source. Such objects have been observed around X-ray bright Galactic binaries and extragalactic objects that happen to shine through the plane of the Milky Way. I use the Chandra X-ray Observatory, a space-based laboratory operated by NASA, which has imaging resolution ideal for studying X-ray scattering halos. I examine several bright X-ray objects with dust-free sight lines to test their viability as templates and develop a parametric model for the Chandra HETG point spread function (PSF)
CYGNUS X-3: ITS LITTLE FRIEND’S COUNTERPART, THE DISTANCE TO CYGNUS X-3, AND OUTFLOWS/JETS
Chandra observations have revealed a feature within 16'' of Cygnus X-3 that varied in phase with Cygnus X-3. This feature was shown to be a Bok globule that is along the line of sight to Cygnus X-3. We report on observations made with the Submillimeter Array to search for molecular emission from this globule, also known as Cygnus X-3's "Little Friend." We have found a counterpart in both [superscript 12]CO (2-1) and [superscript 13]CO (2-1) emission. From the velocity shift of the molecular lines we are able to find two probable distances based on the Bayesian model of Milky Way kinematics of Reid et al. For the LF velocity of −47.5 km s[superscript −1], we find distances of 6.1 ± 0.6 kpc (62% probability) and 7.8 ± 0.6 kpc (38% probability). This yields distances to Cyg X-3 of 7.4 ± 1.1 kpc and 10.2 ± 1.2 kpc, respectively. Based on the probabilities entailed, we take 7.4 ± 1.1 kpc as the preferred distance to Cyg X-3. We also report the discovery of bipolar molecular outflow, suggesting that there is active star formation occurring within the Little Friend
Dust in the Circumgalactic Medium of Low-Redshift Galaxies
Using spectroscopically selected galaxies from the Sloan Digital Sky Survey
we present a detection of reddening due to dust in the circumgalactic medium of
galaxies. We detect the mean change in the colors of "standard crayons"
correlated with the presence of foreground galaxies at z ~0.05 as a function of
angular separation. Following Peek & Graves (2010), we create standard crayons
using passively evolving galaxies corrected for Milky Way reddening and
color-redshift trends, leading to a sample with as little as 2% scatter in
color. We devise methods to ameliorate possible systematic effects related to
the estimation of colors, and we find an excess reddening induced by foreground
galaxies at a level ranging from 10 to 0.5 millimagnitudes on scales ranging
from 30 kpc to 1 Mpc. We attribute this effect to a large-scale distribution of
dust around galaxies similar to the findings of Menard et al. 2010. We find
that circumgalactic reddening is a weak function of stellar mass over the range
-- and note that this
behavior appears to be consistent with recent results on the distribution of
metals in the gas phase.Comment: Submitted to Ap
The X-Ray Variable Sky as Seen by MAXI: The Future of Dust-echo Tomography with Bright Galactic X-Ray Bursts
Bright, short duration X-ray flares from accreting compact objects produce thin, dust scattering rings that enable dust-echo tomography: high-precision distance measurements and mapping of the line-of-sight distribution of dust. This work looks to the past activity of X-ray transient outbursts in order to predict the number of sight lines available for dust-echo tomography. We search for and measure the properties of 3σ significant flares in the 2-4 keV light curves of all objects available in the public MAXI archive. We derive a fluence sensitivity limit of 10-3 erg cm-2 for the techniques used to analyze the light curves. This limits the study mainly to flares from Galactic X-ray sources. We obtain the number density of flares and estimate the total fluence of the corresponding dust echoes. However, the sharpness of a dust-echo ring depends on the duration of a flare relative to quiescence. We select flares that are shorter than their corresponding quiescent period to calculate a number density distribution for dust-echo rings as a function of fluence. The results are fit with a power law of slope -2.3 ± 0.1. Extrapolating this to dimmer flares, we estimate that the next generation of X-ray telescopes will be 30 times more sensitive than current observatories, resulting in 10-30 dust ring echoes per year. The new telescopes will also be 10-100 times more sensitive than Chandra to dust ring echoes from the intergalactic medium
The X-Ray Variable Sky as Seen by MAXI: The Future of Dust-echo Tomography with Bright Galactic X-Ray Bursts
Bright, short duration X-ray flares from accreting compact objects produce thin, dust scattering rings that enable dust-echo tomography: high-precision distance measurements and mapping of the line-of-sight distribution of dust. This work looks to the past activity of X-ray transient outbursts in order to predict the number of sight lines available for dust-echo tomography. We search for and measure the properties of 3σ significant flares in the 2–4 keV light curves of all objects available in the public MAXI archive. We derive a fluence sensitivity limit of 10−3 erg cm−2 for the techniques used to analyze the light curves. This limits the study mainly to flares from Galactic X-ray sources. We obtain the number density of flares and estimate the total fluence of the corresponding dust echoes. However, the sharpness of a dust-echo ring depends on the duration of a flare relative to quiescence. We select flares that are shorter than their corresponding quiescent period to calculate a number density distribution for dust-echo rings as a function of fluence. The results are fit with a power law of slope −2.3 ± 0.1. Extrapolating this to dimmer flares, we estimate that the next generation of X-ray telescopes will be 30 times more sensitive than current observatories, resulting in 10–30 dust ring echoes per year. The new telescopes will also be 10–100 times more sensitive than Chandra to dust ring echoes from the intergalactic medium
Cosmological X-ray Scattering from Intergalactic Dust
High resolution X-ray imaging offers a unique opportunity to probe the nature
of dust in the z ~< 2 universe. Dust grains 0.1- 1 um in size will scatter soft
X-rays, producing a diffuse "halo" image around an X-ray point source, with a
brightness ~ few % confined to an arcminute-sized region. We derive the
formulae for scattering in a cosmological context and calculate the surface
brightness of the scattering halo due to (i) an IGM uniformly enriched (Omega_
d ~ 10^-5) by a power-law distribution of grain sizes, and (ii) a DLA-type (N_H
~ 10^21 cm^-2) dust screen at cosmological distances. The morphology of the
surface brightness profile can distinguish between the two scenarios above,
place size constraints on dusty clumps, and constrain the homogeneity of the
IGM. Thus X-ray scattering can gauge the relative contribution of the first
stars, dwarf galaxies, and galactic outflows to the cosmic metallicity budget
and cosmic history of dust. We show that, because the amount of intergalactic
scattering is overestimated for photon energies < 1 keV, the non-detection of
an X-ray scattering halo by Petric et al. (2006) is consistent with `grey'
intergalactic dust grains (Omega_d ~ 10^-5$) when the data is restricted to the
1-8 keV band. We also calculate the systematic offset in magnitude, delta m ~
0.01, for such a population of graphite grains, which would affect the type of
supernova survey ideal for measuring dark energy parameters within ~ 1%
precision.Comment: Published in ApJ, figures updated to published versio