96 research outputs found
Evaluating the Expertise of Experts
Professor Shrader-Frechette maintains that a rigid distinction between risk assessment and risk management is unwise. Concerned about procedural fairness, she argues that the public should have a voice in both
Black Hole Mass of the Ultraluminous X-ray source M82 X-1
We report the first clear evidence for the simultaneous presence of a low
frequency break and a QPO in the fluctuation power spectrum of a well known
ultraluminous X-ray source (ULX) in M82 using long XMM-Newton observations. The
break occurs at a frequency of 34.2_{-3}^{+6}mHz. The QPO has a centroid at
114.3\pm1.5mHz, a coherence Q~3.5 and an amplitude (rms) of 19% in the 2-10keV
band. The power spectrum is approximately flat below the break frequency and
then falls off above the break frequency as a power law with the QPO
superimposed. This form of the power spectrum is characteristic of the Galactic
X-ray binaries (XRBs) in their high or intermediate states. M82 X-1 was likely
in an intermediate state during the observation. The EPIC PN spectrum is well
described by a model comprising an absorbed power-law (Gamma~2) and an iron
line at ~6.6keV with a width sigma~0.2keV and an equivalent width of ~180eV.
Using the well established correlations between the power and energy spectral
parameters for XRBs, we estimate a black hole mass for M82 X-1 in the range of
25-520Msun including systematic errors that arise due to the uncertainty in the
calibration of the photon spectral index versus QPO frequency relation.Comment: Final version, accepted for publication in ApJ Letter
Compact X-ray Sources in Nearby Galaxy Nuclei
We have found compact, near-nuclear X-ray sources in 21 (54\%) of a complete
sample of 39 nearby face-on spiral and elliptical galaxies with available ROSAT
HRI data. ROSAT X-ray luminosities (0.2 2.4 keV) of these compact X-ray
sources are 1010 erg~s. The mean displacement
between the location of the compact X-ray source and the optical photometric
center of the galaxy is 390 pc. ASCA spectra of six of the 21 galaxies
show the presence of a hard component with relatively steep (
2.5) spectral slope. A multicolor disk blackbody plus power-law model fits the
data from the spiral galaxies well, suggesting that the X-ray objects in these
galaxies may be similar to a black hole candidate (BHC) in its soft (high)
state. ASCA data from the elliptical galaxies indicate that hot (kT
0.7 keV) gas dominates the emission. The fact that the spectral slope of the
spiral galaxy sources is steeper than in normal type 1 active galactic nuclei
(AGNs) and that relatively low absorbing columns (N 10
cm) were found to the power-law component indicates that these objects
are somehow geometrically and/or physically different from AGNs in normal
active galaxies. The X-ray sources in the spiral galaxies may be BHCs,
low-luminosity AGNs, or possibly X-ray luminous supernovae. We estimate the
black hole masses of the X-ray sources in the spiral galaxies (if they are BHCs
or AGNs) to be 1010 M. The X-ray sources in the
elliptical galaxies may be BHCs, AGNs or young X-ray supernova also.Comment: 4 pages, TeX, two postscript figures, to be published in proceedings
of 32nd COSPAR Session E1.2 (1998 July 15-17 Nagoya) "The AGN-Normal Galaxy
Connection
Ethics and Uncertainty: In Vitro Fertilization and Risks to Women\u27s Health
Dr. de Melo-Martin examines the risks, uncertainties and public policies surrounding in vitro fertilization and women\u27s health issues
The hard X-ray tails in neutron star low mass X-ray binaries: BeppoSAX observations and possible theoretical explanation of the GX 17+2 case
We report results of a new spectral analysis of two BeppoSAX observations of
the Z source GX 17+2. In one of the two observations the source exhibits a
powerlaw-like hard (> 30 keV) X-ray tail which was described in a previous work
by a hybrid Comptonization model. Recent high-energy observations with INTEGRAL
of a sample of Low Mass X-Ray Binaries including both Z and atoll classes have
shown that bulk (dynamical) Comptonization of soft photons can be a possible
alternative mechanism for producing hard X-ray tails in such systems. We start
from the INTEGRAL results and we exploit the broad-band capability of BeppoSAX
to better investigate the physical processes at work. We use GX 17+2 as a
representative case. Moreover, we suggest that weakening (or disappearance) of
the hard X-ray tail can be explained by increasing radiation pressure
originated at the surface of the neutron star (NS). As a result the high
radiation pressure stops the bulk inflow and consequently this radiation
feedback of the NS surface leads to quenching the bulk Comptonization.Comment: 6 pages, 3 figures, Accepted for publication in Ap
A Chandra view of NGC 3621: a bulgeless galaxy hosting an AGN in its early phase?
We report the detection of a weak X-ray point source coincident with the
nucleus of the bulgeless disk galaxy NGC 3621, recently discovered by Spitzer
to display high ionization mid-infrared lines typically associated with AGN.
These Chandra observations provide confirmation for the presence of an AGN in
this galaxy, adding to the growing evidence that black holes do form and grow
in isolated bulgeless disk galaxies. Although the low signal-to-noise ratio of
the X-ray spectrum prevents us from carrying out a detailed spectral analysis
of the nuclear source, the X-ray results, combined with the IR and optical
spectroscopic results, suggests that NGC 3621 harbors a heavily absorbed AGN,
with a supermassive black hole of relatively small mass accreting at a high
rate. Chandra also reveals the presence of two bright sources straddling the
nucleus located almost symmetrically at 20" from the center. Both sources have
X-ray spectra that are well-fitted by an absorbed power-law model. Assuming
they are at the distance of NGC 3621, these two sources have luminosities of
the order of 1.e39 erg/s, which make them ULXs and suggest that they are black
hole systems. Estimates of the black hole mass based on the X-ray spectral
analysis and scaling laws of black hole systems suggest that the 2 bright
sources might be intermediate mass black holes with M_BH of the order of a few
thousand solar masses. However, higher quality X-ray data combined with
multi-wavelength observations are necessary to confirm these conclusions.Comment: 12 pages, 9 figures, 1 table, accepted for publication in Ap
Nonthermal processes and neutrino emission from the black hole GRO J0422+32 in a bursting state
GRO J0422+32 is a member of the class of low-mass X-ray binaries (LMXBs). It
was discovered during an outburst in 1992. During the entire episode a
persistent power-law spectral component extending up to MeV was
observed, which suggests that nonthermal processes should have occurred in the
system. We study relativistic particle interactions and the neutrino production
in the corona of GRO J0422+32, and explain the behavior of GRO J0422+32 during
its recorded flaring phase. We have developed a magnetized corona model to fit
the spectrum of GRO J0422+32 during the low-hard state. We also estimate
neutrino emission and study the detectability of neutrinos with 1 km
detectors, such as IceCube. The short duration of the flares ( hours) and
an energy cutoff around a few TeV in the neutrino spectrum make neutrino
detection difficult. There are, however, many factors that can enhance neutrino
emission. The northern-sky coverage and full duty cycle of IceCube make it
possible to detect neutrino bursts from objects of this kind through
time-dependent analysis.Comment: 12 pages, 11 figures, accepted for publication in A&
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