54 research outputs found
An Extended Galactic Population of Low-Luminosity X-Ray Sources (CVs?) and the Diffuse X-Ray Background
Summary of abstract: The existence of a new population of yet unrecognized
x-ray sources has been often suggested to resolve some pecularities in the
properties of the x-ray background (XRB), and has recently been indicated by an
analysis of resolved sources in deep ROSAT observations (Hasinger \etal 1993).
We investigate the possibility that the indicated new population of x-ray
sources is Galactic in origin, and derive its properties which would resolve
the discrepancy found in the number counts of faint sources, and be consistent
with observational constraints on the total background intensity, the XRB
anisotropy, the number of unidentified bright sources, the Galaxy's total x-ray
luminosity, and with the results of fluctuation analyses of the unresolved XRB.
We find that a flattened Galactic halo (or a thick disk) distribution with a
scale height of a few Kpc satisfies all the above requirements. The typical
x-ray luminosity of the sources is erg/s in the 0.5-2 KeV
band, the number density of sources in the Solar vicinity is , their total number in the Galaxy is , and they
contribute erg/s to the Galaxy's total x-ray luminosity.
We discuss the possible nature of these sources (subdwarfs, LMXBs, old
neutron stars), and argue that the inferred x-ray and optical luminosities of
the sources, their KeV spectrum, and the derived local number
density and spatial distribution are all consistent with them being intrinsicly
faint cataclysmic variables with low accretion rates. We discuss the possible
origin of such population, including an origin from disrupted globular clusters
or dark clusters, make predictions, and suggest observational tests.Comment: 20 pages, PostScript file (330K including figures), submitted to Ap
A STRINGENT CONSTRAINT ON ALTERNATIVES TO A MASSIVE BLACK HOLE AT THE CENTER OF NGC 4258
There is now dynamical evidence for massive dark objects at the center of
several galaxies, but suggestions that these are supermassive black holes are
based only on indirect astrophysical arguments. The recent unprecedented
measurement of the rotation curve of maser emission sources at the center of
NGC 4258, and the remarkable discovery that it is Keplerian to high precision,
provides us a unique opportunity for testing alternatives to a BH (e.g., a
massive cluster of stellar remnants, brown dwarfs, low-mass stars, or halo dark
matter).
We use a conservative upper limit on the systematic deviation from a
Keplerian rotation curve to constrain the mass distribution at the galaxy
center. Based on evaporation and physical collision time-scale arguments, we
show that a central cluster is ruled out, *unless* the cluster consists of
*extremely* dense objects with mass less than about 0.05 solar masses (e.g.,
low mass BHs or elementary particles). Since both of these dynamically-allowed
systems are very improbable for other astrophysical reasons, we conclude that a
central dense cluster at the center of NGC 4258 is *very* improbable, thus
leaving the alternative possibility of a massive BH. We also show that the mass
of the BH must be at least 98% of the mass enclosed within the inner edge of
the masering disk (3.6*10^7 solar masses). A substantial contribution to that
mass from a density cusp in the background mass distribution is excluded.Comment: Submitted to ApJ (Letters) on March 15, 1995. 11 pages including 1
figure; uuencoded, compressed postscript
Dynamical Constraints On Alternatives To Massive Black Holes In Galactic Nuclei
The compelling dynamical evidence for massive dark objects in galactic nuclei
does not uniquely imply massive black holes (BHs). To argue convincingly that
these objects are BHs we must rule out alternatives to a BH, and the
alternative to a point mass is a cluster of some sort of nonluminous objects,
such as a cluster of brown dwarfs or stellar remnants.
We use simple physical considerations to derive the maximum possible lifetime
of a dark cluster which may consist of any plausible form of non-luminous
gravitating objects -- from brown dwarfs and very low-mass objects of cosmic
composition, to white dwarfs, neutron stars, and black holes. The lower this
limit relative to the galaxy age, the more implausible is the cluster
hypothesis, thus arguing for a point mass. A cluster with a lifetime much
shorter than ~10 Gyr is unacceptable, since observing it at the present epoch
would be highly improbable.
Since the goal is to rule out a dark cluster by showing that its lifetime
must be very short, we make the most generous assumptions possible under the
observational constraints to allow for its survival. We find that the lifetime
of such an hypothetical cluster must be much shorter than the galaxy age only
in the cases of NGC 4258 and our Galaxy, thus strongly arguing for a point
mass. In all other galaxies, the case of a massive BH, although compelling, is
not yet watertight. We also note that there are two exotic alternatives to a
massive BH that cannot be ruled out even in the cases of NGC 4258 and the
Galaxy: clusters of elementary particles (e.g. bosons), and clusters of very
low-mass BHs. We point out, however, serious difficulties with these
alternatives, and argue that they are highly implausible.Comment: ApJ (Letters), in press. v2: corrected figure, 11 page
A revised Cepheid distance to NGC 4258 and a test of the distance scale
In a previous paper (Maoz et al. 1999), we reported a Hubble Space Telescope
(HST) Cepheid distance to the galaxy NGC 4258 obtained using the calibrations
and methods then standard for the Key Project on the Extragalactic Distance
Scale. Here, we reevaluate the Cepheid distance using the revised Key Project
procedures described in Freedman et al. (2001). These revisions alter the zero
points and slopes of the Cepheid Period-Luminosity (P-L) relations derived at
the Large Magellanic Cloud (LMC), the calibration of the HST WFPC2 camera, and
the treatment of metallicity differences. We also provide herein full
information on the Cepheids described in Maoz et al. 1999. Using the refined
Key Project techniques and calibrations, we determine the distance modulus of
NGC 4258 to be 29.47 +/- 0.09 mag (unique to this determination) +/- 0.15 mag
(systematic uncertainties in Key Project distances), corresponding to a metric
distance of 7.8 +/- 0.3 +/- 0.5 Mpc and 1.2 sigma from the maser distance of
7.2 +/- 0.5 Mpc. We also test the alternative Cepheid P-L relations of Feast
(1999), which yield more discrepant results. Additionally, we place weak limits
upon the distance to the LMC and upon the effect of metallicity in Cepheid
distance determinations.Comment: 26 pages in emulateapj5 format, including 6 figures and 5 tables.
Accepted for publication in the Astrophysical Journa
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