27,733 research outputs found
High signal-to-noise ratio observations and the ultimate limits of precision pulsar timing
We demonstrate that the sensitivity of high-precision pulsar timing
experiments will be ultimately limited by the broadband intensity modulation
that is intrinsic to the pulsar's stochastic radio signal. That is, as the peak
flux of the pulsar approaches that of the system equivalent flux density,
neither greater antenna gain nor increased instrumental bandwidth will improve
timing precision. These conclusions proceed from an analysis of the covariance
matrix used to characterise residual pulse profile fluctuations following the
template matching procedure for arrival time estimation. We perform such an
analysis on 25 hours of high-precision timing observations of the closest and
brightest millisecond pulsar, PSR J0437-4715. In these data, the standard
deviation of the post-fit arrival time residuals is approximately four times
greater than that predicted by considering the system equivalent flux density,
mean pulsar flux and the effective width of the pulsed emission. We develop a
technique based on principal component analysis to mitigate the effects of
shape variations on arrival time estimation and demonstrate its validity using
a number of illustrative simulations. When applied to our observations, the
method reduces arrival time residual noise by approximately 20%. We conclude
that, owing primarily to the intrinsic variability of the radio emission from
PSR J0437-4715 at 20 cm, timing precision in this observing band better than 30
- 40 ns in one hour is highly unlikely, regardless of future improvements in
antenna gain or instrumental bandwidth. We describe the intrinsic variability
of the pulsar signal as stochastic wideband impulse modulated self-noise
(SWIMS) and argue that SWIMS will likely limit the timing precision of every
millisecond pulsar currently observed by Pulsar Timing Array projects as larger
and more sensitive antennae are built in the coming decades.Comment: 16 pages, 9 figures, accepted for publication in MNRAS. Updated
version: added DOI and changed manuscript to reflect changes in the final
published versio
Multi-wavelength signals of dark matter annihilations at the Galactic center
We perform a systematic study of the multi-wavelength signal induced by
weakly interacting massive particle (WIMP) annihilations at the Galactic Center
(GC). Referring to a generic WIMP dark matter (DM) scenario and depending on
astrophysical inputs, we discuss spectral and angular features and sketch
correlations among signals in the different energy bands. None of the
components which have been associated to the GC source Sgr A*, nor the diffuse
emission components from the GC region, have spectral or angular features
typical of a DM source. Still, data-sets at all energy bands, namely, the
radio, near infrared, X-ray and gamma-ray bands, contribute to place
significant constraints on the WIMP parameter space. In general, the gamma-ray
energy range is not the one with the largest signal to background ratio. In the
case of large magnetic fields close to the GC, X-ray data give the tightest
bounds. The emission in the radio-band, which is less model dependent, is very
constraining as well. The recent detection by HESS of a GC gamma-ray source,
and of a diffuse gamma-ray component, limits the possibility of a DM discovery
with next generation of gamma-ray telescopes, like GLAST and CTA. We find that
the most of the region in the parameter space accessible to these instruments
is actually already excluded at other wave-lenghts. On the other hand, there
may be still an open window to improve constraints with wide-field radio
observations.Comment: 26 pages, 32 figures, treatments of starlight and interstellar medium
improved, other minor changes, references adde
Triplets of supermassive black holes: Astrophysics, Gravitational Waves and Detection
Supermassive black holes (SMBHs) found in the centers of many galaxies have
been recognized to play a fundamental active role in the cosmological structure
formation process. In hierarchical formation scenarios, SMBHs are expected to
form binaries following the merger of their host galaxies. If these binaries do
not coalesce before the merger with a third galaxy, the formation of a black
hole triple system is possible. Numerical simulations of the dynamics of
triples within galaxy cores exhibit phases of very high eccentricity (as high
as ). During these phases, intense bursts of gravitational
radiation can be emitted at orbital periapsis. This produces a gravitational
wave signal at frequencies substantially higher than the orbital frequency. The
likelihood of detection of these bursts with pulsar timing and the Laser
Interferometer Space Antenna ({\it LISA}) is estimated using several population
models of SMBHs with masses . Assuming a fraction
of binaries in triple system, we find that few to few dozens of these
bursts will produce residuals ns, within the sensitivity range of
forthcoming pulsar timing arrays (PTAs). However, most of such bursts will be
washed out in the underlying confusion noise produced by all the other
'standard' SMBH binaries emitting in the same frequency window. A detailed data
analysis study would be required to assess resolvability of such sources.
Implementing a basic resolvability criterion, we find that the chance of
catching a resolvable burst at a one nanosecond precision level is 2-50%,
depending on the adopted SMBH evolution model. On the other hand, the
probability of detecting bursts produced by massive binaries (masses \gtrsim
10^7\msun) with {\it LISA} is negligible.Comment: Accepted for publication by MNRAS, minor change
Understanding Spatial and Spectral Morphologies of Ultracompact H II Regions
The spatial morphology, spectral characteristics, and time variability of
ultracompact H II regions provide strong constraints on the process of massive
star formation. We have performed simulations of the gravitational collapse of
rotating molecular cloud cores, including treatments of the propagation of
ionizing and non-ionizing radiation. We here present synthetic radio continuum
observations of H II regions from our collapse simulations, to investigate how
well they agree with observation, and what we can learn about how massive star
formation proceeds. We find that intermittent shielding by dense filaments in
the gravitationally unstable accretion flow around the massive star leads to
highly variable H II regions that do not grow monotonically, but rather
flicker, growing and shrinking repeatedly. This behavior appears able to
resolve the well-known lifetime problem. We find that multiple ionizing sources
generally form, resulting in groups of ultracompact H II regions, consistent
with observations. We confirm that our model reproduces the qualitative H II
region morphologies found in surveys, with generally consistent relative
frequencies. We also find that simulated spectral energy distributions (SEDs)
from our model are consistent with the range of observed H II region SEDs,
including both regions showing a normal transition from optically thick to
optically thin emission, and those with intermediate spectral slopes. In our
models, anomalous slopes are solely produced by inhomogeneities in the H II
region, with no contribution from dust emission at millimeter or submillimeter
wavelengths. We conclude that many observed characteristics of ultracompact H
II regions appear consistent with massive star formation in fast,
gravitationally unstable, accretion flows.Comment: ApJ in pres
- …