93 research outputs found
Radio detection prospects for a bulge population of millisecond pulsars as suggested by Fermi LAT observations of the inner Galaxy
Analogously to globular clusters, the dense stellar environment of the
Galactic center has been proposed to host a large population of as-yet
undetected millisecond pulsars (MSPs). Recently, this hypothesis found support
in the analysis of gamma rays from the inner Galaxy seen by the Large Area
Telescope (LAT) aboard the Fermi satellite, which revealed a possible excess of
diffuse GeV photons in the inner 15 deg about the Galactic center (Fermi GeV
excess). The excess can be interpreted as the collective emission of thousands
of MSPs in the Galactic bulge, with a spherical distribution that strongly
peaks towards the Galactic center. In order to fully establish the MSP
interpretation, it is essential to find corroborating evidence in
multi-wavelength searches, most notably through the detection of radio
pulsation from individual bulge MSPs. Based on globular cluster observations
and the gamma-ray emission from the inner Galaxy, we investigate the prospects
for detecting MSPs in the Galactic bulge. While previous pulsar surveys failed
to identify this population, we demonstrate that, in the upcoming years, new
large-area surveys with focus on regions a few degrees north or south of the
Galactic center should lead to the detection of dozens of bulge MSPs.
Additionally, we show that, in the near future, deep targeted searches of
unassociated Fermi sources should be able to detect the first few MSPs in the
bulge. The prospects for these deep searches are enhanced by a tentative
gamma-ray/radio correlation that we infer from high-latitude gamma-ray MSPs.
Such detections would constitute the first clear discoveries of field MSPs in
the Galactic bulge, with far-reaching implications for gamma-ray observations,
the formation history of the central Milky Way and strategy optimization for
future radio observations.Comment: 24 pages, 17 figures, 5 tables. Minor clarifications. Matches version
published in Ap
A Massive Neutron Star in the Globular Cluster M5
We report the results of 19 years of Arecibo timing for two pulsars in the
globular cluster NGC 5904 (M5), PSR B1516+02A (M5A) and PSR B1516+02B (M5B).
This has resulted in the measurement of the proper motions of these pulsars
and, by extension, that of the cluster itself. M5B is a 7.95-ms pulsar in a
binary system with a > 0.13 solar mass companion and an orbital period of 6.86
days. In deep HST images, no optical counterpart is detected within ~2.5 sigma
of the position of the pulsar, implying that the companion is either a white
dwarf or a low-mass main-sequence star. The eccentricity of the orbit (e =
0.14) has allowed a measurement of the rate of advance of periastron: (0.0142
+/-0.0007) degrees per year. We argue that it is very likely that this
periastron advance is due to the effects of general relativity, the total mass
of the binary system then being 2.29 +/-0.17 solar masses. The small measured
mass function implies, in a statistical sense, that a very large fraction of
this total mass is contained in the pulsar: 2.08 +/- 0.19 solar masses (1
sigma); there is a 5% probability that the mass of this object is < 1.72 solar
masses and a 0.77% probability that is is between 1.2 and 1.44 solar masses.
Confirmation of the median mass for this neutron star would exclude most
``soft'' equations of state for dense neutron matter. Millisecond pulsars
(MSPs) appear to have a much wider mass distribution than is found in double
neutron star systems; about half of these objects are significantly more
massive than 1.44 solar masses. A possible cause is the much longer episode of
mass accretion necessary to recycle a MSP, which in some cases corresponds to a
much larger mass transfer.Comment: 10 pages in ApJ emulate format, 2 tables, 6 figures. Added February
2008 data, slightly revised mass limits. Accepted for publication in Ap
Investigating Galactic supernova remnant candidates with LOFAR
We investigate six supernova remnant (SNR) candidates --- G51.21+0.11,
G52.37-0.70, G53.07+0.49, G53.41+0.03, G53.84-0.75, and the possible shell
around G54.1-0.3 --- in the Galactic Plane using newly acquired LOw-Frequency
ARray (LOFAR) High-Band Antenna (HBA) observations, as well as archival
Westerbork Synthesis Radio Telescope (WSRT) and Very Large Array Galactic Plane
Survey (VGPS) mosaics. We find that G52.37-0.70, G53.84-0.75, and the possible
shell around pulsar wind nebula G54.1+0.3 are unlikely to be SNRs, while
G53.07+0.49 remains a candidate SNR. G51.21+0.11 has a spectral index of
, but lacks X-ray observations and as such requires further
investigation to confirm its nature. We confirm one candidate, G53.41+0.03, as
a new SNR because it has a shell-like morphology, a radio spectral index of
and it has the X-ray spectral characteristics of a
1000-8000 year old SNR. The X-ray analysis was performed using archival
XMM-Newton observations, which show that G53.41+0.03 has strong emission lines
and is best characterized by a non-equilibrium ionization model, consistent
with an SNR interpretation. Deep Arecibo radio telescope searches for a pulsar
associated with G53.41+0.03 resulted in no detection, but place stringent upper
limits on the flux density of such a source if it is beamed towards Earth.Comment: 9 pages, 4 figures, 1 tabl
Pulsar Wind Nebulae in EGRET Error Boxes
A remarkable number of pulsar wind nebulae (PWN) are coincident with EGRET
gamma-ray sources. X-ray and radio imaging studies of unidentified EGRET
sources have resulted in the discovery of at least 6 new pulsar wind nebulae
(PWN). Stationary PWN (SPWN) appear to be associated with steady EGRET sources
with hard spectra, typical for gamma-ray pulsars. Their toroidal morphologies
can help determine the geometry of the pulsar which is useful for constraining
models of pulsed gamma-ray emission. Rapidly moving PWN (RPWN) with more
cometary morphologies seem to be associated with variable EGRET sources in
regions where the ambient medium is dense compared to what is typical for the
ISM.Comment: 8 pages, 5 figures, to appear in the proceedings of "The
Multiwavelength Approach to Unidentified Sources", ed. G. Romero & K.S. Chen
The Millisecond Pulsars in NGC 6760
We present the results of recent Arecibo and Green Bank observations of the
globular cluster NGC 6760. Using Arecibo, a phase-coherent timing solution has
been obtained for the previously known binary pulsar in this cluster, PSR
J1911+0102A. We have also discovered a new millisecond pulsar in NGC 6760, PSR
J1911+0101B, an isolated object with a rotational period of 5.38 ms and a
dispersion measure DM = 196.7 cm-3 pc. Both pulsars are located within 1.3 core
radii of the cluster center and have negative period derivatives. The resulting
lower limits for the accelerations of the pulsars are within the range expected
given a simple model of the cluster. A search for eclipses in the PSR
J1911+0102A binary system using both telescopes yielded negative results. The
corresponding limits on the extra gas column density at superior conjunction
are consistent with the hypothesis that the observational properties of
ultra-low-mass binary pulsars like PSR J1911+0102A are strongly affected by the
inclination of the orbital plane of the system. Among globular cluster pulsar
populations, that of NGC 6760 exhibits one of the largest known spreads in DM.
This quantity seems to be roughly proportional to a cluster's central DM; this
suggests that the observed spread is caused by a turbulent interstellar medium
at spatial scales of 1 pc.Comment: 10 pages in referee format, 4 figures, one table, re-submitted to the
Astrophysical Journa
Twenty-One Millisecond Pulsars in Terzan 5 Using the Green Bank Telescope
We have discovered 21 millisecond pulsars (MSPs) in the globular cluster
Terzan 5 using the Green Bank Telescope, bringing the total of known MSPs in
Terzan 5 to 24. These discoveries confirm fundamental predictions of globular
cluster and binary system evolution. Thirteen of the new MSPs are in binaries,
of which two show eclipses and two have highly eccentric orbits. The
relativistic periastron advance for the two eccentric systems indicates that at
least one of these pulsars has a mass >1.68 Msun at 95% confidence. Such large
neutron star masses constrain the equation of state of matter at or beyond the
nuclear equilibrium density.Comment: 12 pages, 2 figures. Accepted by Science. Published electronically
via Science Express 13 Jan 200
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