Some New Papers

arXiv:1102.5094 [pdf, ps, other]
Reassessing The Fundamentals: New Constraints on the Evolution, Ages and Masses of Neutron Stars
Bulent Kiziltan
Comments: 4 pages, 4 figures; To appear in the AIP proceedings of "Astrophysics of Neutron Stars-2010", eds. E. Gogus, T. Belloni, U. Ertan
Subjects: Galaxy Astrophysics (astro-ph.GA); Applications (stat.AP)

The ages and masses of neutron stars (NSs) are two fundamental threads that make pulsars accessible to other sub-disciplines of astronomy and physics. A realistic and accurate determination of these two derived parameters play an important role in understanding of advanced stages of stellar evolution and the physics that govern relevant processes. Here I summarize new constraints on the ages and masses of NSs with an evolutionary perspective. I show that the observed P-Pdot demographics is more diverse than what is theoretically predicted for the standard evolutionary channel. In particular, standard recycling followed by dipole spin-down fails to reproduce the population of millisecond pulsars with higher magnetic fields (B > 4 x 10^{8} G) at rates deduced from observations. A proper inclusion of constraints arising from binary evolution and mass accretion offers a more realistic insight into the age distribution. By analytically implementing these constraints, I propose a "modified" spin-down age for millisecond pulsars that gives estimates closer to the true age. Finally, I independently analyze the peak, skewness and cutoff values of the underlying mass distribution from a comprehensive list of radio pulsars for which secure mass measurements are available. The inferred mass distribution shows clear peaks at 1.35 Msun and 1.50 Msun for NSs in double neutron star (DNS) and neutron star-white dwarf (NS-WD) systems respectively. I find a mass cutoff at 2 Msun for NSs with WD companions, which establishes a firm lower bound for the maximum mass of NSs.

arXiv:1102.5192 [pdf, other]
On the unreasonable effectiveness of the post-Newtonian approximation in gravitational physics
Clifford M. Will
Comments: 9 pages, 2 figures, submitted to Proceedings of the National Academy of Sciences (US)
Subjects: General Relativity and Quantum Cosmology (gr-qc); High Energy Astrophysical Phenomena (astro-ph.HE); High Energy Physics - Theory (hep-th)

The post-Newtonian approximation is a method for solving Einstein's field equations for physical systems in which motions are slow compared to the speed of light and where gravitational fields are weak. Yet it has proven to be remarkably effective in describing certain strong-field, fast-motion systems, including binary pulsars containing dense neutron stars and binary black hole systems inspiraling toward a final merger. The reasons for this effectiveness are largely unknown. When carried to high orders in the post-Newtonian sequence, predictions for the gravitational-wave signal from inspiraling compact binaries will play a key role in gravitational-wave detection by laser-interferometric observatories.

arXiv:1102.5340 [pdf, other]
Arecibo PALFA Survey and Einstein@Home: Binary Pulsar Discovery by Volunteer Computing
B. Knispel, P. Lazarus, B. Allen, D. Anderson, C. Aulbert, N. D. R. Bhat, O. Bock, S. Bogdanov, A. Brazier, F. Camilo, S. Chatterjee, J. M. Cordes, F. Crawford, J. S. Deneva, G. Desvignes, H. Fehrmann, P. C. C. Freire, D. Hammer, J. W. T. Hessels, F. A. Jenet, V. M. Kaspi, M. Kramer, J. van Leeuwen,D. R. Lorimer, A. G. Lyne, B. Machenschalk, M. A. McLaughlin, C. Messenger, D. J. Nice, M. A. Papa, H. J. Pletsch, R. Prix, S. M. Ransom, X. Siemens, I. H. Stairs, B. W. Stappers, K. Stovall, A. Venkataraman
Comments: 5 pages, 3 figures, 1 table
Subjects: Galaxy Astrophysics (astro-ph.GA)

We report the discovery of the 20.7-ms binary pulsar J1952+2630, made using the distributed computing project Einstein@Home in Pulsar ALFA survey observations with the Arecibo telescope. Follow-up observations with the Arecibo telescope confirm the binary nature of the system. We obtain a circular orbital solution with an orbital period of 9.4 hr, a projected orbital radius of 2.8 lt-s, and a mass function of f = 0.15 solar masses by analysis of spin period measurements. No evidence of orbital eccentricity is apparent; we set a 2-sigma upper limit e < 1.7e-3. The orbital parameters suggest a massive white dwarf companion with a minimum mass of 0.95 solar masses, assuming a pulsar mass of 1.4 solar masses. Most likely, this pulsar belongs to the rare class of intermediate mass binary pulsars. Future timing observations will aim to determine the parameters of this system further, measure relativistic effects, and elucidate the nature of the companion star.

arXiv:1103.0006 [pdf, ps, other]
SGR 1806-20 distance and dust properties in molecular clouds by analysis of a flare x-ray echoes
Gilad Svirski, Ehud Nakar, Eran O. Ofek
Comments: 12 pages, 7 figures, 3 tables, submitted to MNRAS
Subjects: High Energy Astrophysical Phenomena (astro-ph.HE)

The soft gamma repeater SGR 1806-20 is most famous for its giant flare from 2004, which yielded the highest gamma-ray flux ever observed on Earth. The flare emphasized the importance of determining the distance to the SGR, thus revealing the flare's energy output, with implications on SGRs energy budget and giant flare rates. We analyze x-ray scattering echoes observed by Swift/XRT following the 2006 August 6 intermediate burst of SGR 1806-20. Assuming positions and opacities of the molecular clouds along the line-of-sight from previous works, we derive direct constrains on the distance to SGR 1806-20, setting a lower limit of 9.4 kpc and an upper limit of 18.6 kpc (90% confidence), compared with a 6-15 kpc distance range by previous works. This distance range matches an energy output of ~10^46 erg/s for the 2004 giant flare. We further use, for the first time, the x-ray echoes in order to study the dust properties in molecular clouds. Analyzing the temporal evolution of the observed flux using a dust scattering model, which assumes a power-law size distribution of the dust grains, we find a power-law index of -3.3_{-0.7}^{+0.6} (1 sigma) and a lower limit of 0.1 micron (2 sigma) on the dust maximal grain size, both conforming to measured dust properties in the diffused interstellar medium (ISM). We advocate future burst follow-up observations with Swift, Chandra and the planned NuSTAR telescopes, as means of obtaining much superior results from such an analysis.

arXiv:1103.0789 [pdf]
A 5 GHz LNA for a Radio-Astronomy Experiment
Miguel Bergano, Luis Cupido, Armando Rocha, Domingos Barbosa
Comments: IEEE Xplore, Accepted for Publication at IEEE EuronCon 2011 - International Conference on Computer as Tool / ConfTel 2011, Lisbon, April 2011
Subjects: Instrumentation and Methods for Astrophysics (astro-ph.IM)

The paper describes the project, implementation and test of a C-band (5GHz) Low Noise Amplifier (LNA) using new low noise Pseudomorphic High Electron Mobility Transistors (pHEMTS) from Avago. The amplifier was developed to be used as a cost effective solution in a receiver chain for Galactic Emission Mapping (GEM-P) project in Portugal with the objective of finding affordable solutions not requiring strong cryogenic operation, as is the case of massive projects like the Square Kilometer Array (SKA), in Earth Sensing projects and other niches like microwave reflectometry. The particular application and amplifier requirements are first introduced. Several commercially available low noise devices were selected and the noise performance simulated. An ultra-low noise pHEMT was used for an implementation that achieved a Noise Figure of 0.6 dB with 13 dB gain at 5 GHz. The design, simulation and measured results of the prototype are presented and discussed.

arXiv:1103.0819 [pdf, ps, other]
New Black Widows and Redbacks in the Galactic Field
Mallory S.E. Roberts, Fermi Pulsar Search Consortium, GBT Drift Scan Survey Collaboration
Comments: 4 pages, 1 figure, to appear in AIP Conference Proceedings of Pulsar Conference 2010 "Radio Pulsars: a key to unlock the secrets of the Universe", Sardinia, October 2010
Subjects: High Energy Astrophysical Phenomena (astro-ph.HE)

There has recently been a large increase in the number of known eclipsing radio millisecond pulsars in the Galactic field, many of which are associated with Fermi gamma-ray sources. All are in tight binaries (P_b < 24hr) many of which are classical "black widows" with very low mass companions (M_c << 0.1 M_sol) but some are "redbacks" with probably non-degenerate low mass companions (M_c ~ 0.2 M_sol). I review the new discoveries, briefly discuss the distance uncertainties and the implications for high-energy emission.

arXiv:1103.0880 [pdf, ps, other]
On the excitation of f-modes and torsional modes by magnetar giant flares
Yuri Levin (Monash, Leiden), Maarten van Hoven (Leiden)
Comments: submitted to MNRAS
Subjects: High Energy Astrophysical Phenomena (astro-ph.HE); General Relativity and Quantum Cosmology (gr-qc)

Magnetar giant flares may excite vibrational modes of neutron stars. Here we compute an estimate of initial post-flare amplitudes of both the torsional modes in the magnetar's crust and of the global f-modes. We show that while the torsional crustal modes can be strongly excited, only a small fraction of the flare's energy is converted directly into the lowest-order f-modes. For a conventional model of a magnetar, with the external magnetic field of about 10^{15} Gauss, the gravitational-wave detection of these f-modes with advanced LIGO is unlikely.