649 research outputs found
Time-domain modelling of Extreme-Mass-Ratio Inspirals for the Laser Interferometer Space Antenna
When a stellar-mass compact object is captured by a supermassive black hole
located in a galactic centre, the system losses energy and angular momentum by
the emission of gravitational waves. Subsequently, the stellar compact object
evolves inspiraling until plunging onto the massive black hole. These EMRI
systems are expected to be one of the main sources of gravitational waves for
the future space-based Laser Interferometer Space Antenna (LISA). However, the
detection of EMRI signals will require of very accurate theoretical templates
taking into account the gravitational self-force, which is the responsible of
the stellar-compact object inspiral. Due to its potential applicability on
EMRIs, the obtention of an efficient method to compute the scalar self-force
acting on a point-like particle orbiting around a massive black hole is being
object of increasing interest. We present here a review of our time-domain
numerical technique to compute the self-force acting on a point-like particle
and we show its suitability to deal with both circular and eccentric orbits.Comment: 4 pages, 2 figures, JPCS latex style. Submitted to JPCS (special
issue for the proceedings of the Spanish Relativity Meeting (ERE2010)
The Kinematic and Plasma Properties of X-ray Knots in Cassiopeia A from the Chandra HETGS
We present high-resolution X-ray spectra from the young supernova remnant Cas
A using a 70-ks observation taken by the Chandra High Energy Transmission
Grating Spectrometer (HETGS). Line emission, dominated by Si and S ions, is
used for high-resolution spectral analysis of many bright, narrow regions of
Cas A to examine their kinematics and plasma state. These data allow a 3D
reconstruction using the unprecedented X-ray kinematic results: we derive
unambiguous Doppler shifts for these selected regions, with values ranging
between -2500 and +4000 km/s. Plasma diagnostics of these regions, derived from
line ratios of resolved He-like triplet lines and H-like lines of Si, indicate
temperatures largely around 1 keV, which we model as O-rich reverse-shocked
ejecta. The ionization age also does not vary considerably over these regions
of the remnant. The gratings analysis was complemented by the non-dispersed
spectra from the same dataset, which provided information on emission measure
and elemental abundances for the selected Cas A regions. The derived electron
density of X-ray emitting ejecta varies from 20 to 200 cm^{-3}. The measured
abundances of Mg, Si, S and Ca are consistent with O being the dominant element
in the Cas A plasma. With a diameter of 5 arcmin, Cas A is the largest source
observed with the HETGS to date. We, therefore, describe the technique we use
and some of the challenges we face in the HETGS data reduction from such an
extended, complex object.Comment: 26 pages, 16 figures, evised version (minor changes), accepted for
publication in ApJ (Oct 20 2006
Chandra Observations of the X-ray Narrow-Line Region in NGC 4151
We present the first high-resolution X-ray spectrum of the Seyfert 1.5 galaxy
NGC 4151. Observations with the Chandra High Energy Transmission Grating
Spectrometer reveal a spectrum dominated by narrow emission lines from a
spatially resolved (1.6 kpc), highly ionized nebula. The X-ray narrow-line
region is composite, consisting of both photoionized and collisionally ionized
components. The X-ray emission lines have similar velocities, widths, and
spatial extent to the optical emission lines, showing that they arise in the
same region. The clouds in the narrow-line region must contain a large range of
ionization states in order to explain both the optical and X-ray photoionized
emission. Chandra data give the first direct evidence of X-ray line emission
from a hot plasma (T~1e7 K) which may provide pressure confinement for the
cooler (T=3e4 K) photoionized clouds.Comment: 13 pages, 3 figures, to be published in Astrophysical Journal Letter
Ionization Structure and the Reverse Shock in E0102-72
The young oxygen-rich supernova remnant E0102-72 in the Small Magellanic
Cloud has been observed with the High Energy Transmission Grating Spectrometer
of Chandra. The high resolution X-ray spectrum reveals images of the remnant in
the light of individual emission lines of oxygen, neon, magnesium and silicon.
The peak emission region for hydrogen-like ions lies at larger radial distance
from the SNR center than the corresponding helium-like ions, suggesting passage
of the ejecta through the "reverse shock". We examine models which test this
interpretation, and we discuss the implications.Comment: 4 pages, 6 figures; To appear in "Young Supernova Remnants" (11th
Annual Astrophysics Conference in Maryland), S. S. Holt & U. Hwang (eds),
AIP, New York (2001
High-Resolution X-ray Spectroscopy of SNR 1987A: Chandra LETG and HETG Observations in 2007
We present an extended analysis of the deep Chandra LETG and HETG
observations of the supernova remnant 1987A (SNR 1987A) carried out in 2007.
The global fits to the grating spectra show that the temperature of the X-ray
emitting plasma in the slower shocks in this system has remained stable for the
last three years, while that in the faster shocks has decreased. This
temperature evolution is confirmed by the first light curves of strong X-ray
emission lines and their ratios. On the other hand, bulk gas velocities
inferred from the X-ray line profiles are too low to account for the post-shock
plasma temperatures inferred from spectral fits. This suggests that the X-ray
emission comes from gas that has been shocked twice, first by the blast wave
and again by shocks reflected from the inner ring of SNR 1987A. A new model
that takes these considerations into account gives support to this physical
picture.Comment: 36 pages, 10 figures, Accepted for publication in Ap
A Chandra View of the Normal SO Galaxy NGC 1332: II: Solar Abundances in the Hot Gas and Implications for SN Enrichment
We present spectral analysis of the diffuse emission in the normal, isolated,
moderate-Lx S0 NGC 1332, constraining both the temperature profile and the
metal abundances in the ISM. The characteristics of the point source population
and the gravitating matter are discussed in two companion papers. The diffuse
emission comprises hot gas, with an ~isothermal temperature profile (~0.5 keV),
and emission from unresolved point-sources. In contrast with the cool cores of
many groups and clusters, we find a small central temperature peak. We obtain
emission-weighted abundance contraints within 20 kpc for several key elements:
Fe, O, Ne, Mg and Si. The measured iron abundance (Z_Fe=1.1 in solar units;
>0.53 at 99% confidence) strongly excludes the very sub-solar values often
historically reported for early-type galaxies but agrees with recent
observations of brighter galaxies and groups. The abundance ratios, with
respect to Fe, of the other elements were also found to be ~solar, although
Z_o/Z_Fe was significantly lower (<0.4). Such a low O abundance is not
predicted by simple models of ISM enrichment by Type Ia and Type II supernovae,
and may indicate a significant contribution from primordial hypernovae.
Revisiting Chandra observations of the moderate-Lx, isolated elliptical NGC
720, we obtain similar abundance constraints. Adopting standard SNIa and SNII
metal yields, our abundance ratio constraints imply 73+/-5% and 85+/-6% of the
Fe enrichment in NGC 1332 and NGC 720, respectively, arises from SNIa. Although
these results are sensitive to the considerable systematic uncertainty in the
SNe yields, they are in good agreement with observations of more massive
systems. These two moderate-Lx early-type galaxies reveal a consistent pattern
of metal enrichment from cluster scales to moderate Lx/Lb galaxies. (abridged)Comment: 12 pages, 4 figures, accepted for publication in ApJ. Minor changes
to match published versio
An HST/COS Observation of Broad Ly Emission and Associated Absorption Lines of the BL Lacertae Object H 2356-309
Weak spectral features in BL Lacertae objects (BL Lac) often provide a unique
opportunity to probe the inner region of this rare type of active galactic
nucleus. We present a Hubble Space Telescope/Cosmic Origins Spectrograph
observation of the BL Lac H 2356-309. A weak Ly emission line was
detected. This is the fourth detection of a weak Ly emission feature in
the ultraviolet (UV) band in the so-called "high energy peaked BL Lacs", after
Stocke et al. Assuming the line-emitting gas is located in the broad line
region (BLR) and the ionizing source is the off-axis jet emission, we constrain
the Lorentz factor () of the relativistic jet to be with a
maximum viewing angle of 3.6. The derived is somewhat larger
than previous measurements of , implying a covering
factor of 3% of the line-emitting gas. Alternatively, the BLR clouds
could be optically thin, in which case we constrain the BLR warm gas to be
. We also detected two HI and one OVI absorption
lines that are within of the BL Lac object.
The OVI and one of the HI absorbers likely coexist due to their nearly
identical velocities. We discuss several ionization models and find a
photoionization model where the ionizing photon source is the BL Lac object can
fit the observed ion column densities with reasonable physical parameters. This
absorber can either be located in the interstellar medium of the host galaxy,
or in the BLR.Comment: 7 pages, 2 figures, accepted for publication in Ap
- …