271 research outputs found
Blandford's Argument: The Strongest Continuous Gravitational Wave Signal
For a uniform population of neutron stars whose spin-down is dominated by the
emission of gravitational radiation, an old argument of Blandford states that
the expected gravitational-wave amplitude of the nearest source is independent
of the deformation and rotation frequency of the objects. Recent work has
improved and extended this argument to set upper limits on the expected
amplitude from neutron stars that also emit electromagnetic radiation. We
restate these arguments in a more general framework, and simulate the evolution
of such a population of stars in the gravitational potential of our galaxy. The
simulations allow us to test the assumptions of Blandford's argument on a
realistic model of our galaxy. We show that the two key assumptions of the
argument (two dimensionality of the spatial distribution and a steady-state
frequency distribution) are in general not fulfilled. The effective scaling
dimension D of the spatial distribution of neutron stars is significantly
larger than two, and for frequencies detectable by terrestrial instruments the
frequency distribution is not in a steady state unless the ellipticity is
unrealistically large. Thus, in the cases of most interest, the maximum
expected gravitational-wave amplitude does have a strong dependence on the
deformation and rotation frequency of the population. The results strengthen
the previous upper limits on the expected gravitational-wave amplitude from
neutron stars by a factor of 6 for realistic values of ellipticity.Comment: 12 pages, 6 Figures, published in Phys. Rev. D, v3: final published
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Einstein@Home Discovery of 24 Pulsars in the Parkes Multi-Beam Pulsar Survey
We have conducted a new search for radio pulsars in compact binary systems in the Parkes multi-beam pulsar survey (PMPS) data, employing novel methods to remove the Doppler modulation from binary motion. This has yielded unparalleled sensitivity to pulsars in compact binaries. The required computation time of 17, 000 CPU core years was provided by the distributed volunteer computing project Einstein@Home, which has a sustained computing power of about 1 PFlop sâ1. We discovered 24 new pulsars in our search, 18 of which were isolated pulsars, and 6 were members of binary systems. Despite the wide filterbank channels and relatively slow sampling time of the PMPS data, we found pulsars with very large ratios of dispersion measure (DM) to spin period. Among those is PSR J1748â3009, the millisecond pulsar with the highest known DM (420 pc cmâ3). We also discovered PSR J1840â0643, which is in a binary system with an orbital period of 937 days, the fourth largest known. The new pulsar J1750â2536 likely belongs to the rare class of intermediate-mass binary pulsars. Three of the isolated pulsars show long-term nulling or intermittency in their emission, further increasing this growing family. Our discoveries demonstrate the value of distributed volunteer computing for data-driven astronomy and the importance of applying new analysis methods to extensively searched data
Timing of a Young Mildly Recycled Pulsar with a Massive White Dwarf Companion
We report on timing observations of the recently discovered binary pulsar PSR
J1952+2630 using the Arecibo Observatory. The mildly recycled 20.7-ms pulsar is
in a 9.4-hr orbit with a massive, M_WD > 0.93 M_sun, white dwarf (WD)
companion. We present, for the first time, a phase-coherent timing solution,
with precise spin, astrometric, and Keplerian orbital parameters. This shows
that the characteristic age of PSR J1952+2630 is 77 Myr, younger by one order
of magnitude than any other recycled pulsar-massive WD system. We derive an
upper limit on the true age of the system of 50 Myr. We investigate the
formation of PSR J1952+2630 using detailed modelling of the mass-transfer
process from a naked helium star on to the neutron star following a
common-envelope phase (Case BB Roche-lobe overflow). From our modelling of the
progenitor system, we constrain the accretion efficiency of the neutron star,
which suggests a value between 100 and 300% of the Eddington accretion limit.
We present numerical models of the chemical structure of a possible
oxygen-neon-magnesium WD companion. Furthermore, we calculate the past and the
future spin evolution of PSR J1952+2630, until the system merges in about 3.4
Gyr due to gravitational wave emission. Although we detect no relativistic
effects in our timing analysis we show that several such effects will become
measurable with continued observations over the next 10 years; thus PSR
J1952+2630 has potential as a testbed for gravitational theories.Comment: 12 pages, 10 figures, to be published in MNRA
Doxycycline delays aneurysm rupture in a mouse model of Marfan syndrome
ObjectivesThoracic aneurysms are the main cardiovascular complication of Marfan syndrome (MFS) resulting in premature death. MFS has been associated with mutations of the gene encoding fibrillin-1 (FBN1), a major constituent of the elastic fibers. Matrix metalloproteinases (MMPs) are important in the pathogenesis of abdominal aortic aneurysms but their precise role in MFS is not clear. Doxycycline is a nonspecific MMP inhibitor. The objective of the study was to determine whether docycycline can attenuate matrix degradation and prolong the survival of mice with MFS.MethodsThe study employed a well-characterized animal model of MFS, namely fibrillin-1 under-expressing mice (mgR/mgR mice) that die spontaneously from rupture of the thoracic aorta between 2 to 4 months of age. Mutant and wild type mice were given doxycycline in their drinking water at a concentration designed to provide 100 mg/kg/day beginning at postnatal day (PD) 1, whereas control mice were given water. Treated mice were divided into two groups. One group of animals was followed until death or for 7 months to determine lifespan. In the second group of mice, the ascending thoracic aortas were collected for histological analysis (H&E staining, trichrome staining) and zymography for examining MMP-2 and MMP-9 levels at 6 weeks.ResultsMMP-2 and MMP-9 levels were higher in the thoracic aorta of mgR/mgR mice compared with wild type littermates. Doxycycline-treated mgR/mgR mice lived 132 ± 14.6 days (n = 16) or significantly longer than untreated mutant mice (79 ± 6.7 days, n = 30) (P < 0.01). Connective tissue staining showed that doxycycline treatment decreased elastic fiber degradation in mgR/mgR mice. Furthermore, mgR/mgR mice treated with doxycycline had lower MMP-2 and MMP-9 levels compared with untreated mgR/mgR mice.ConclusionsThis study demonstrates that doxycycline significantly delays aneurysm rupture in MFS-like mice by inhibiting expression of tissue MMP-2 and MMP-9 and thus, degradation of the elastic matrix. The results suggest that MMPs contribute to the progression of thoracic aneurysm in MFS and that doxycycline has the potential to significantly alter the course of the disease.Clinical RelevanceAortic aneurysms are the main cardiovascular complication of Marfan syndrome (MFS) resulting in premature death. ÎČ-blockers offer some benefit but do not address the underlying cause of the progressive aortic degradation. Medical treatment that actually targets recently identified pathogenic factors leading to progressive matrix destruction could significantly impact the clinical course of the disease. A recent study using a mouse model of MFS has demonstrated that TGF- ÎČ antibodies or the angiotensin II type I receptor (AT1) antagonist losartan can both effectively rescue aneurysm progression. We have found that doxycycline, a nonspecific inhibitor of matrix metalloproteinases (MMPs), can decrease elastin degradation and prolong the lifespan of genetically engineered mice that mimic the human disease process. Based on these results, further testing may be warranted to determine if doxycycline could favorable impact the natural history of Marfan syndrome
Timing of a Young Mildly Recycled Pulsar with a Massive White Dwarf Companion
We report on timing observations of the recently discovered binary pulsar PSR J1952+2630 using the Arecibo Observatory. The mildly recycled 20.7-ms pulsar is in a 9.4-hr orbit with a massive, M_WD > 0.93 M_sun, white dwarf (WD) companion. We present, for the first time, a phase-coherent timing solution, with precise spin, astrometric, and Keplerian orbital parameters. This shows that the characteristic age of PSR J1952+2630 is 77 Myr, younger by one order of magnitude than any other recycled pulsar-massive WD system. We derive an upper limit on the true age of the system of 50 Myr. We investigate the formation of PSR J1952+2630 using detailed modelling of the mass-transfer process from a naked helium star on to the neutron star following a common-envelope phase (Case BB Roche-lobe overflow). From our modelling of the progenitor system, we constrain the accretion efficiency of the neutron star, which suggests a value between 100 and 300% of the Eddington accretion limit. We present numerical models of the chemical structure of a possible oxygen-neon-magnesium WD companion. Furthermore, we calculate the past and the future spin evolution of PSR J1952+2630, until the system merges in about 3.4 Gyr due to gravitational wave emission. Although we detect no relativistic effects in our timing analysis we show that several such effects will become measurable with continued observations over the next 10 years; thus PSR J1952+2630 has potential as a testbed for gravitational theories
The PALFA Survey: Going to great depths to find radio pulsars
The on-going PALFA survey is searching the Galactic plane (|b| < 5 deg., 32 <
l < 77 deg. and 168 < l < 214 deg.) for radio pulsars at 1.4 GHz using ALFA,
the 7-beam receiver installed at the Arecibo Observatory. By the end of August
2012, the PALFA survey has discovered 100 pulsars, including 17 millisecond
pulsars (P < 30 ms). Many of these discoveries are among the pulsars with the
largest DM/P ratios, proving that the PALFA survey is capable of probing the
Galactic plane for millisecond pulsars to a much greater depth than any
previous survey. This is due to the survey's high sensitivity, relatively high
observing frequency, and its high time and frequency resolution. Recently the
rate of discoveries has increased, due to a new more sensitive spectrometer,
two updated complementary search pipelines, the development of online
collaborative tools, and access to new computing resources. Looking forward,
focus has shifted to the application of artificial intelligence systems to
identify pulsar-like candidates, and the development of an improved
full-resolution pipeline incorporating more sophisticated radio interference
rejection. The new pipeline will be used in a complete second analysis of data
already taken, and will be applied to future survey observations. An overview
of recent developments, and highlights of exciting discoveries will be
presented.Comment: Proceedings of IAUS 291 "Neutron Stars and Pulsars: Challenges and
Opportunities after 80 years", J. van Leeuwen (ed.); 6 pages, 4 figure
Fast Radio Burst Discovered in the Arecibo Pulsar ALFA Survey
Recent work has exploited pulsar survey data to identify temporally isolated,
millisecond-duration radio bursts with large dispersion measures (DMs). These
bursts have been interpreted as arising from a population of extragalactic
sources, in which case they would provide unprecedented opportunities for
probing the intergalactic medium; they may also be linked to new source
classes. Until now, however, all so-called fast radio bursts (FRBs) have been
detected with the Parkes radio telescope and its 13-beam receiver, casting some
concern about the astrophysical nature of these signals. Here we present FRB
121102, the first FRB discovery from a geographic location other than Parkes.
FRB 121102 was found in the Galactic anti-center region in the 1.4-GHz Pulsar
ALFA survey with the Arecibo Observatory with a DM = 557.4 3 pc
cm, pulse width of ms, and no evidence of interstellar
scattering. The observed delay of the signal arrival time with frequency agrees
precisely with the expectation of dispersion through an ionized medium. Despite
its low Galactic latitude (), the burst has three times the
maximum Galactic DM expected along this particular line-of-sight, suggesting an
extragalactic origin. A peculiar aspect of the signal is an inverted spectrum;
we interpret this as a consequence of being detected in a sidelobe of the ALFA
receiver. FRB 121102's brightness, duration, and the inferred event rate are
all consistent with the properties of the previously detected Parkes bursts.Comment: 9 pages, 3 figures, submitted to Ap
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Mast cells modulate the pathogenesis of elastase-induced abdominal aortic aneurysms in mice
Abdominal aortic aneurysm (AAA), an inflammatory disease, involves leukocyte recruitment, immune responses, inflammatory cytokine production, vascular remodeling, neovascularization, and vascular cell apoptosis, all of which contribute to aortic dilatation. This study demonstrates that mast cells, key participants in human allergic immunity, participate in AAA pathogenesis in mice. Mast cells were found to accumulate in murine AAA lesions. Mast cellâdeficient KitW-sh/KitW-sh mice failed to develop AAA elicited by elastase perfusion or periaortic chemical injury. KitW-sh/KitW-sh mice had reduced aortic expansion and internal elastic lamina degradation; decreased numbers of macrophages, CD3+ T lymphocytes, SMCs, apoptotic cells, and CD31+ microvessels; and decreased levels of aortic tissue IL-6 and IFN-Îł. Activation of mast cells in WT mice via C48/80 injection resulted in enhanced AAA growth while mast cell stabilization with disodium cromoglycate diminished AAA formation. Mechanistic studies demonstrated that mast cells participated in angiogenesis, aortic SMC apoptosis, and matrix-degrading protease expression. Reconstitution of KitW-sh/KitW-sh mice with bone marrowâderived mast cells from WT or TNF-αâ/â mice, but not from IL-6â/â or IFN-Îłâ/â mice, caused susceptibility to AAA formation to be regained. These results demonstrate that mast cells participate in AAA pathogenesis in mice by releasing proinflammatory cytokines IL-6 and IFN-Îł, which may induce aortic SMC apoptosis, matrix-degrading protease expression, and vascular wall remodeling, important hallmarks of arterial aneurysms
On detecting millisecond pulsars at the galactic center
The lack of detected pulsars at the Galactic Center (GC) region is a long-standing mystery. We argue that the high stellar density in the central parsec around the GC is likely to result in a pulsar population dominated by millisecond pulsars (MSPs), similar to the situation in globular cluster environments. Earlier GC pulsar searches have been largely insensitive to such an MSP population, accounting for the lack of pulsar detections. We estimate the best search frequency for such an MSP population with present and upcoming broad-band radio telescopes for two possible scattering scenarios, the âweak-scatteringâ case suggested by the recent detection of a magnetar close to the GC, and the âstrong-scatteringâ case, with the scattering screen located close to the GC. The optimal search frequencies are â 8 GHz ( weak-scattering ) and â 25 GHz ( strong-scattering ) , for pulsars with periods 1 â 20 ms, assuming that GC pulsars have a luminosity distribution similar to that those in the rest of the Milky Way. We find that 10 â 30 hr integrations with the Very Large Array and the Green Bank Telescope would be sufficient to detect MSPs at the GC distance in the weak-scattering case. However, if the strong-scattering case is indeed applicable to the GC, observations with the full Square Kilometre Array would be needed to detect the putative MSP population
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