42 research outputs found
The Dark Matter Distribution in Galaxy Cluster Cores
Determining the structure of galaxy clusters is essential for an
understanding of large scale structure in the universe, and may hold important
clues to the identity and nature of dark matter particles. Moreover, the core
dark matter distribution may offer insight into the structure formation
process. Unfortunately, cluster cores also tend to be the site of complicated
astrophysics. X-ray imaging spectroscopy of relaxed clusters, a standard
technique for mapping their dark matter distributions, is often complicated by
the presence of their putative ``cooling flow'' gas, and the dark matter
profile one derives for a cluster is sensitive to assumptions made about the
distribution of this gas. Here we present a statistical analysis of these
assumptions and their effect on our understanding of dark matter in galaxy
clusters.Comment: Poster contribution to the 13th Annual Astrophysics Conference in
Maryland, The Emergence of Cosmic Structure; 4 page
The X-ray Binary GRS 1741.9-2853 in Outburst and Quiescence
We report Chandra and XMM-Newton observations of the transient neutron star
low-mass X-ray binary GRS 1741.9-2853. Chandra detected the source in outburst
on 2000 October 26 at an X-ray luminosity of ~10^{36} erg/s (2--8 keV; 8 kpc),
and in quiescence on 2001 July 18 at ~10^{32} erg/s. The latter observation is
the first detection of GRS 1741.9-2853 in quiescence. We obtain an accurate
position for the source of 17h 45m 2.33s, -28o 54' 49.7" (J2000), with an
uncertainty of 0.7". GRS 1741.9-2853 was not detected significantly in three
other Chandra observations, nor in three XMM-Newton observations, indicating
that the luminosity of the source in quiescence varies by at least a factor of
5 between (< 0.9 - 5.0) \times 10^{32} erg/s (2--8 keV). A weak X-ray burst
with a peak luminosity of 5 \times 10^{36} erg/s above the persistent level was
observed with Chandra during the outburst on 2000 October 26. The energy of
this burst, 10^{38} erg, is unexpectedly low, and may suggest that the accreted
material is confined to the polar caps of the neutron star. A search of the
literature reveals that GRS 1741.9-2853 was observed in outburst with ASCA in
Fall 1996 as well, when the BeppoSAX WFC detected the three previous X-ray
bursts from this source. The lack of X-ray bursts from GRS 1741.9-2853 at other
epochs suggests that it produces bursts only during transient outbursts when
the accretion rate onto the surface of the neutron star is about 10^{-10}
M_sun/yr. A similar situation may hold for other low-luminosity bursters
recently identified from WFC data.Comment: Submitted to ApJ. 9 pages, including 5 figure
Extracting the Dark Matter Profile of a Relaxed Galaxy Cluster
Knowledge of the structure of galaxy clusters is essential for an
understanding of large scale structure in the universe, and may provide
important clues to the nature of dark matter. Moreover, the shape of the dark
matter distribution in the cluster core may offer insight into the structure
formation process. Unfortunately, cluster cores also tend to be the site of
complicated astrophysics. X-ray imaging spectroscopy of relaxed clusters, a
standard technique for mapping their dark matter distributions, is often
complicated by the presence of cool components in cluster cores, and the dark
matter profile one derives for a cluster is sensitive to assumptions made about
the distribution of this component. In addition, fluctuations in the
temperature measurements resulting from normal statistical variance can produce
results which are unphysical. We present here a procedure for extracting the
dark matter profile of a spherically symmetric, relaxed galaxy cluster which
deals with both of these complications. We apply this technique to a sample of
galaxy clusters observed with the Chandra X-ray Observatory, and comment on the
resulting mass profiles. For some of the clusters we compare their masses with
those derived from weak and strong gravitational measurements.Comment: final version to match accepted ApJ version; 29 page
Measuring Molecular, Neutral Atomic, and Warm Ionized Galactic Gas Through X-Ray Absorption
We study the column densities of neutral atomic, molecular, and warm ionized
Galactic gas through their continuous absorption of extragalactic X-ray spectra
at |b| > 25 degrees. For N(H,21cm) < 5x10^20 cm^-2 there is an extremely tight
relationship between N(H,21cm) and the X-ray absorption column, N(xray), with a
mean ratio along 26 lines of sight of N(xray)/N(H,21cm) = 0.972 +- 0.022. This
is significantly less than the anticpated ratio of 1.23, which would occur if
He were half He I and half He II in the warm ionized component. We suggest that
the ionized component out of the plane is highly ionized, with He being mainly
He II and He III. In the limiting case that H is entirely HI, we place an upper
limit on the He abundance in the ISM of He/H <= 0.103.
At column densities N(xray) > 5x10^20 cm^-2, which occurs at our lower
latitudes, the X-ray absorption column N(xray) is nearly double N(H,21cm). This
excess column cannot be due to the warm ionized component, even if He were
entirely He I, so it must be due to a molecular component. This result implies
that for lines of sight out of the plane with |b| ~ 30 degrees, molecular gas
is common and with a column density comprable to N(H,21cm).
This work bears upon the far infrared background, since a warm ionized
component, anticorrelated with N(H,21cm), might produce such a background. Not
only is such an anticorrelation absent, but if the dust is destroyed in the
warm ionized gas, the far infrared background may be slightly larger than that
deduced by Puget et al. (1996).Comment: 1 AASTeX file, 14 PostScript figure files which are linked within the
TeX fil
Model-independent X-ray mass determinations
A new method is introduced for making X-ray mass determinations of spherical
clusters of galaxies. Treating the distribution of gravitating matter as
piecewise constant and the cluster atmosphere as piecewise isothermal, X-ray
spectra of a hydrostatic atmosphere are determined up to a single overall
normalizing factor. In contrast to more conventional approaches, this method
relies on the minimum of assumptions, apart from the conditions of hydrostatic
equilibrium and spherical symmetry. The method has been implemented as an XSPEC
mixing model called CLMASS, which was used to determine masses for a sample of
nine relaxed X-ray clusters. Compared to conventional mass determinations,
CLMASS provides weak constraints on values of M_500, reflecting the quality of
current X-ray data for cluster regions beyond r_500. At smaller radii, where
there are high quality X-ray spectra inside and outside the radius of interest
to constrain the mass, CLMASS gives confidence ranges for M_2500 that are only
moderately less restrictive than those from more familiar mass determination
methods. The CLMASS model provides some advantages over other methods and
should prove useful for mass determinations in regions where there are high
quality X-ray data.Comment: 12 pages, 8 figures, accepted for publication in Ap