572 research outputs found
The physical implications of an isothermal model for the hot intracluster medium
X-ray fluxes from HEAO-1 A2 and Einstein Imaging Proportional Counter (IPC) observations of clusters of galaxies were used to constrain the parameter beta in the isothermal surface brightness profile. Beta is found primarily to have values between .50 and .75 for 15 clusters. Eight of these objects have values of beta previously measured using imaging observations. For these clusters good agreement is found with the values reported here implying that this profile is a good description of the surface brightness out to 8 to 10 core radii. The total gas mass and radial distribution (assuming spherical symmetry) within the cluster resulting from the isothermal model imply an extended halo of hot gas which has 30 to 60% of the virial mass for some clusters
Spectral constraints on models of gas in clusters of galaxies
The HEAO 1A2 spectra of clusters of galaxies are used to determine the temperature profile which characterizes the X-ray emitting gas. Strong evidence of nonisothermality is found for the Coma, A85, and A1795 clusters. Properties of the cluster potential which binds the gas are calculated for a range of model parameters. The typical binding mass, if the gas is adiabatic, is 2-4E14 solar masses and is quite centrally concentrated. In addition, the Fe abundance in Coma is .26 + or - .06 solar, less than the typical value (.5) found for rich clusters. The results for the gas in Coma may imply a physical description of the cluster which is quite different from what was previously believed
PKS 2155-304 relativistically beamed synchrotron radiation from BL LAC object
The newly discovered BL Lacertae object, PKS 2155-304, was observed with the medium and high intensity energy detectors of the HEAO-1 A2 experiment. The variability by a factor of two in less than a day reported by Snyder, et al (1979) is confirmed. Two spectra, obtained a year apart, while the satellite was in scanning mode, are well fit by simple power laws with energy spectral index alpha sub 1 equals approximately 1.4. A third spectrum, of higher statistical quality, obtained while the satellite was pointed at its source, has has two components. An acceptable fit was obtained using a two power law model, with indices alpha sub 1 equals 2.0 (+1.2, -0.6) and alpha sub 2 equals -1.5 (+1.5, -2.3). An interpretation of the overall spectrum from radio through X-rays in terms of a synchrotron self-Compton model gives a good description of the data if allowance is made for relativistic beaming. Thus, from a consideration of the spectrum, combined with an estimate of the size of the source, the presence of jets is inferred without their observation
LHEA contributions to the Future of Ultraviolet Astronomy Based on Six Years of IUE Research
Astronomical models of galactic nuclei emission spectra are reassessed in light of ultraviolet and X-ray spectroscopic observations. Spectral analysis of BL Lacertae objects using data collected by the International Ultraviolet Explorer (IUE) and other astronomical observatories is presented
Are the virial masses of clusters smaller than we think?
The constraints that the available X-ray spectral and imaging data place on the mass distribution and mass to light ratio of rich clusters are considered. It was found for the best determined cases that the mass to light ratio is less than 125 h sub 50 at radii exceeding 1 h sub 50 Mpc. The mass to light ratio is approximately constant at radii exceeding 1 h sub 50 Mpc but may rise to values of roughly 200 h sub 50 in the central regions. The fraction of the total mass that is in baryons, primarily the hot X-ray emitting gas, is roughly 30% thus setting the mass to light ratio of the dark material to roughly 70. The model that fits the X-ray data for Coma is in good agreement with the observed optical velocity dispersion vs. radius data
The star-forming environment of a ULX in NGC 4559: an optical study
We have studied the candidate optical counterparts and the stellar population
in the star-forming complex around a bright ULX in NGC4559, using HST/WFPC2,
XMM-Newton/OM, and ground-based data. We find that the ULX is located near a
small group of OB stars. The brightest point source in the Chandra error circle
is consistent with a single blue supergiant of mass ~ 20 M_sun and age ~ 10
Myr. A few other stars are resolved inside the error circle: mostly blue and
red supergiants with masses ~ 10-15 M_sun and ages ~ 20 Myr. This is consistent
with the interpretation of this ULX as a black hole (BH) accreting from a
high-mass donor star in its supergiant phase, via Roche-lobe overflow. The
observed optical colors and the blue-to-red supergiant ratio suggest a low
metal abundance: 0.2 <~ Z/Z_sun <~ 0.4 (Padua tracks), or 0.05 <~ Z/Z_sun <~
0.2 (Geneva tracks). The age of the star-forming complex is <~ 30 Myr. H-alpha
images show that this region has a ring-like appearance. We propose that it is
an expanding wave of star formation, triggered by an initial density
perturbation, in a region where the gas was only marginally stable to
gravitational collapse. A possible trigger was the collision with a satellite
dwarf galaxy, visible a few arcsec north-west of the complex, going through the
gas-rich outer disk of NGC4559. The X-ray data favour a BH more massive (M > 50
M_sun) than typical Milky Way BH candidates. The optical data favour a
``young'' BH originating in the recent episode of massive star formation;
however, they also rule out an association with young massive star clusters. We
speculate that other mechanisms may lead to the formation of relatively massive
BHs (~ 50-100 M_sun) from stellar evolution processes in low-metallicity
environments, or when star formation is triggered by galactic collisions.Comment: MNRAS accepted, 19 pages. Contact the first author for
full-resolution picture
Groups and the Entropy Floor- XMM-Newton Observations of Two Groups
Using XMM-Newton spatially resolved X-ray imaging spectroscopy we obtain the
temperature, density, entropy, gas mass, and total mass profiles for two groups
of galaxies out to ~0.3 Rvir (Rvir, the virial radius). Our density profiles
agree well with those derived previously, and the temperature data are broadly
consistent with previous results but are considerably more precise. Both of
these groups are at the mass scale of 2x10^13 Msolar but have rather different
properties. They have considerably lower gas mass fractions at r<0.3 Rvir than
the rich clusters. NGC2563, one of the least luminous groups for its X-ray
temperature, has a very low gas mass fraction of ~0.004 inside 0.1 Rvir, which
rises with radius. NGC4325, one of the most luminous groups at the same average
temperature, has a higher gas mass fraction of 0.02. The entropy profiles and
the absolute values of the entropy as a function of virial radius also differ,
with NGC4325 having a value of ~100 keV cm-2 and NGC2563 a value of ~300 keV
cm-2 at r~0.1 Rvir. For both groups the profiles rise monotonically with radius
and there is no sign of an entropy "floor". These results are inconsistent with
pre-heating scenarios which have been developed to explain the entropy floor in
groups but are broadly consistent with models of structure formation which
include the effects of heating and/or the cooling of the gas. The total entropy
in these systems provides a strong constraint on all models of galaxy and group
formation, and on the poorly defined feedback process which controls the
transformation of gas into stars and thus the formation of structure in the
universe.Comment: 22 pages, 2 figure
- …