475 research outputs found
A Strong Jet Signature in the Late-Time Lightcurve of GW170817
We present new 0.6-10 GHz observations of the binary neutron star merger
GW170817 covering the period up to 300 days post-merger, taken with the Karl G.
Jansky Very Large Array, the Australia Telescope Compact Array, the Giant
Metrewave Radio Telescope and the MeerKAT telescope. We use these data to
precisely characterize the decay phase of the late-time radio light curve. We
find that the temporal decay is consistent with a power-law slope of t^-2.2,
and that the transition between the power-law rise and decay is relatively
sharp. Such a slope cannot be produced by a quasi-isotropic (cocoon-dominated)
outflow, but is instead the classic signature of a relativistic jet. This
provides strong observational evidence that GW170817 produced a successful jet,
and directly demonstrates the link between binary neutron star mergers and
short-hard GRBs. Using simple analytical arguments, we derive constraints on
the geometry and the jet opening angle of GW170817. These results are
consistent with those from our companion Very Long Baseline Interferometry
(VLBI) paper, reporting superluminal motion in GW170817.Comment: 11 pages, 3 figures, 3 tables. Accepted for publication in ApJ
Letter
PSR J1833-1034: Discovery of the Central Young Pulsar in the Supernova Remnant G21.5-0.9
We have discovered the pulsar associated with the supernova remnant
G21.5-0.9. PSR J1833-1034, with spin period P = 61.8 ms and dispersion measure
169 pc/cc, is very faint, with pulse-averaged flux density of approximately 70
microJy at a frequency of 1.4 GHz, and was first detected in a deep search with
the Parkes telescope. Subsequent observations with Parkes and the Green Bank
Telescope have confirmed this detection, and yield a period derivative dP/dt =
2.02e-13. These spin parameters imply a characteristic age tau_c = 4.8 kyr and
a spin-down luminosity dE/dt = 3.3e37 ergs/s, the latter value exceeded only by
the Crab pulsar among the rotation-powered pulsars known in our Galaxy. The
pulsar has an unusually steep radio spectrum in the 0.8-2.0 GHz range, with
power law index ~3.0, and a narrow single-peaked pulse profile with full-width
at half maximum of 0.04P. We have analyzed 350 ks of archival Chandra X-ray
Observatory High Resolution Camera (HRC) data, and find a point-like source of
luminosity approximately 3e-5dE/dt, offset from the center of an elliptical
region of size ~7"x5" and luminosity approximately 1e-3dE/dt within which
likely lies the pulsar wind termination shock. We have searched for X-ray
pulsations in a 30 ks HRC observation without success, deriving a pulsed
fraction upper limit for a sinusoidal pulse shape of about 70% of the pulsar
flux. We revisit the distance to G21.5-0.9 based on HI and CO observations,
arguing that it is 4.7+-0.4 kpc. We use existing X-ray and radio observations
of the pulsar wind nebula, along with the measured properties of its engine and
a recent detection of the supernova remnant shell, to argue that G21.5-0.9 and
PSR J1833-1034 are much younger than tau_c, and likely their true age is ~<1000
yr. In that case, the initial spin period of the pulsar was >~55 ms.Comment: To appear in ApJ (submitted 2005 May 22; accepted 2005 September 26).
3 figures, 11 pages using emulateapj.cls. Independent discovery of the pulsar
reported by Gupta et al. (astro-ph/0508257
The X-ray Structure of the Pulsar Bow Shock G189.22+2.90 in the Supernova Remnant IC 443
We present a deep observation with the Chandra X-ray Observatory of the
neutron star bow shock G189.22+2.90 in the supernova remnant (SNR) IC 443. Our
data confirm the cometary morphology and central point source seen previously,
but also reveal considerable new structure. Specifically, we find that the
X-ray nebula consists of two distinct components: a "tongue" of bright emission
close to the neutron star, enveloped by a larger, fainter "tail". We interpret
the tongue and tail as delineating the termination shock and the post-shock
flow, respectively, as previously identified also in the pulsar bow shock
G359.23-0.82 ("the Mouse"). However, for G189.22+2.90 the tongue is much less
elongated than for the Mouse, while the tail is much broader. These differences
are consistent with the low Mach number, M >~ 2, expected for a neutron star
moving through the hot gas in a SNR's interior, supporting the case for a
physical association between G189.22+2.90 and IC 443. We resolve the stand-off
distance between the star and the head of the bow shock, which allows us to
estimate a space velocity for the neutron star of ~230 km/s, independent of
distance. We detect thermal emission from the neutron star surface at a
temperature of 102 +/- 22 eV, which is consistent with the age of SNR IC 443
for standard neutron star cooling models. We also identify two compact knots of
hard emission located 1-2 arcsec north and south of the neutron star.Comment: 8 pages, including 3 color EPS figure. Expaneded to include new
figures, plus discussion of the association between the pulsar and the
supernova remnant. ApJ, in pres
Eight gamma-ray pulsars discovered in blind frequency searches of Fermi LAT data
We report the discovery of eight gamma-ray pulsars in blind frequency
searches using the LAT, onboard the Fermi Gamma-ray Space Telescope. Five of
the eight pulsars are young (tau_c10^36 erg/s), and
located within the Galactic plane (|b|<3 deg). The remaining three are older,
less energetic, and located off the plane. Five pulsars are associated with
sources included in the LAT bright gamma-ray source list, but only one, PSR
J1413-6205, is clearly associated with an EGRET source. PSR J1023-5746 has the
smallest characteristic age (tau_c=4.6 kyr) and is the most energetic
(Edot=1.1E37 erg/s) of all gamma-ray pulsars discovered so far in blind
searches. PSRs J1957+5033 and J2055+25 have the largest characteristic ages
(tau_c~1 Myr) and are the least energetic (Edot~5E33 erg/s) of the
newly-discovered pulsars. We present the timing models, light curves, and
detailed spectral parameters of the new pulsars. We used recent XMM
observations to identify the counterpart of PSR J2055+25 as XMMU
J205549.4+253959. In addition, publicly available archival Chandra X-ray data
allowed us to identify the likely counterpart of PSR J1023-5746 as a faint,
highly absorbed source, CXOU J102302.8-574606. The large X-ray absorption
indicates that this could be among the most distant gamma-ray pulsars detected
so far. PSR J1023-5746 is positionally coincident with the TeV source HESS
J1023-575, located near the young stellar cluster Westerlund 2, while PSR
J1954+2836 is coincident with a 4.3 sigma excess reported by Milagro at a
median energy of 35 TeV. Deep radio follow-up observations of the eight pulsars
resulted in no detections of pulsations and upper limits comparable to the
faintest known radio pulsars, indicating that these can be included among the
growing population of radio-quiet pulsars in our Galaxy being uncovered by the
LAT, and currently numbering more than 20.Comment: Submitted to Ap
The AFLOW Fleet for Materials Discovery
The traditional paradigm for materials discovery has been recently expanded
to incorporate substantial data driven research. With the intent to accelerate
the development and the deployment of new technologies, the AFLOW Fleet for
computational materials design automates high-throughput first principles
calculations, and provides tools for data verification and dissemination for a
broad community of users. AFLOW incorporates different computational modules to
robustly determine thermodynamic stability, electronic band structures,
vibrational dispersions, thermo-mechanical properties and more. The AFLOW data
repository is publicly accessible online at aflow.org, with more than 1.7
million materials entries and a panoply of queryable computed properties. Tools
to programmatically search and process the data, as well as to perform online
machine learning predictions, are also available.Comment: 14 pages, 8 figure
Three Millisecond Pulsars in FERMI LAT Unassociated Bright Sources
We searched for radio pulsars in 25 of the non-variable, unassociated sources
in the Fermi LAT Bright Source List with the Green Bank Telescope at 820 MHz.
We report the discovery of three radio and gamma-ray millisecond pulsars (MSPs)
from a high Galactic latitude subset of these sources. All of the pulsars are
in binary systems, which would have made them virtually impossible to detect in
blind gamma-ray pulsation searches. They seem to be relatively normal, nearby
(<=2 kpc) millisecond pulsars. These observations, in combination with the
Fermi detection of gamma-rays from other known radio MSPs, imply that most, if
not all, radio MSPs are efficient gamma-ray producers. The gamma-ray spectra of
the pulsars are power-law in nature with exponential cutoffs at a few GeV, as
has been found with most other pulsars. The MSPs have all been detected as
X-ray point sources. Their soft X-ray luminosities of ~10^{30-31} erg/s are
typical of the rare radio MSPs seen in X-rays.Comment: Accepted for publication in ApJ Letter
Chandra Study of a Complete Sample of Millisecond Pulsars in 47Tuc and NGC6397
We report Chandra observations of the complete sample of millisecond pulsars
(MSPs) with precise radio positions in the globular clusters 47Tuc (NGC104) and
NGC6397. The 47Tuc MSPs are predominantly soft sources suggestive of thermal
emission from small (r_x < 0.6km) polar caps on the neutron star rather than
magnetospheric emission and are a relatively homogeneous sample, with most
x-ray luminosities in a surprisingly narrow range (Lx ~1-4 X 10^30 erg/s). We
use previously derived intrinsic Pdot values and find a new relation between Lx
and spindown luminosity, Edot: Lx ~ Edot^beta, with beta ~0.5 vs. ~1.0 for both
pulsars and MSPs in the field. This Lx-Edot relation and also the Lx/Edot vs.
spindown age are each similar to that found by Harding & Muslimov (2002) for
thermal emission from polar cap heating although the cluster MSPs are
relatively longer-lived (in thermal x-rays) than either the models or field
MSPs. We suggest the cluster MSPs may have altered surface magnetic field
topology (e.g.multipole) or their neutron stars are more massive from repeated
accretion episodes due to encounters and repeated exchange interactions. MSP
binary companions on or just off the main sequence are likely to have been
re-exchanged and might show anomalous Pdot and Edot values due to relaxation of
misaligned core-crust spins. The radial distribution of 40 soft Chandra sources
in 47Tuc is consistent with a 1.4Msun component. The implied total MSP
population in 47Tuc with Lx >10^30 erg/s is ~35-90, and can constrain the
relative beaming in radio vs. soft x-rays. NGC6397 is relatively deficient in
MSPs; its single detected example may have been re-exchanged out of the cluster
core. (abridged)Comment: 16 pages, 11 figures, 5 tables. Accepted for Ap
Rewriting Logic Semantics of a Plan Execution Language
The Plan Execution Interchange Language (PLEXIL) is a synchronous language
developed by NASA to support autonomous spacecraft operations. In this paper,
we propose a rewriting logic semantics of PLEXIL in Maude, a high-performance
logical engine. The rewriting logic semantics is by itself a formal interpreter
of the language and can be used as a semantic benchmark for the implementation
of PLEXIL executives. The implementation in Maude has the additional benefit of
making available to PLEXIL designers and developers all the formal analysis and
verification tools provided by Maude. The formalization of the PLEXIL semantics
in rewriting logic poses an interesting challenge due to the synchronous nature
of the language and the prioritized rules defining its semantics. To overcome
this difficulty, we propose a general procedure for simulating synchronous set
relations in rewriting logic that is sound and, for deterministic relations,
complete. We also report on two issues at the design level of the original
PLEXIL semantics that were identified with the help of the executable
specification in Maude
Six Years of Chandra Observations of Supernova Remnants
We present a review of the first six years of Chandra X-ray Observatory
observations of supernova remnants. From the official "first-light" observation
of Cassiopeia A that revealed for the first time the compact remnant of the
explosion, to the recent million-second spectrally-resolved observation that
revealed new details of the stellar composition and dynamics of the original
explosion, Chandra observations have provided new insights into the supernova
phenomenon. We present an admittedly biased overview of six years of these
observations, highlighting new discoveries made possible by Chandra's unique
capabilities.Comment: 82 pages, 28 figures, for the book Astrophysics Update
- …