Ira Wasserman

A relativistic model of pulsar polarization

The rotating vector model of pulsar polarization of Radhakrishnan and Cooke (1969) is extended here to include first-order special relativistic effects. The model predicts that the centroid of the position angle curve arrives later than the centroid of the intensity profile by 4r/c, where r is the emission radius. Application of the model to pulsars with well ordered position-angle swings and periods between 0.06 and 3.7 s give emission radii of not more than 2000 km for 0.43 and 1.4 GHz. Symmetry-breaking effects of the corotation velocity may help explain a general asymmetry found in pulsar intensity profiles and may strongly affect the intensity profiles of short-period pulsars. 66 refs.

Cosmological constraints on tachyon matter

Abstract We examine whether tachyon matter is a viable candidate for the cosmological dark matter. First, we demonstrate that in order for the density of tachyon matter to have an acceptable value today, the magnitude of the tachyon potential energy at the onset of rolling must be finely tuned. For a tachyon potential V(T)∼MPl4exp(−T/τ), the tachyon must start rolling at T≃60τ in order for the density of tachyon matter today to satisfy Ω T,0 ∼1 , provided that standard big bang cosmology begins at the same time as the tachyon begins to roll. In this case, the value of Ω T,0 is exponentially sensitive to T/τ at the onset of rolling, so smaller T/τ is unacceptable, and larger T/τ implies a tachyon density that is too small to have interesting cosmological effects. If instead the universe undergoes a second inflationary epoch after the tachyon has already rolled considerably, then the tachyon can begin with T near zero, but the increase of the scale factor during inflation must still be finely tuned in order for Ω T,0 ∼1 . Second, we show that tachyon matter, unlike quintessence, can cluster gravitationally on very small scales. If the starting value of T/τ is tuned finely enough that Ω T,0 ∼1 , then tachyon matter clusters more or less identically to pressureless dust. Thus, if the fine-tuning problem can be explained, tachyon matter is a viable candidate for cosmological dark matter.

Supergiant pulses from extragalactic neutron stars

We consider radio bursts that originate from extragalactic neutron stars (NSs) by addressing three questions about source distances. What are the physical limitations on coherent radiation at GHz frequencies? Do they permit detection at cosmological distances? How many bursts per NS are needed to produce the inferred burst rate 10 3 -10 4 sky 1 day 1 ? The burst rate is comparable to the NS formation rate in a Hubble volume, requiring only one per NS if they are bright enough. However, radiation physics causes us to favor a closer population. More bursts per NS are then required but repeats in 10 to 100 yr could still be negligible. Bursts are modeled as sub-ns, coherent shot pulses superposed incoherently to produce msduration 1 Jy amplitudes; each shot-pulse can be much weaker than the burst amplitude, placing less restrictive requirements on the emission process. Nonetheless, single shot pulses are similar to the extreme, unresolved (< 0:4 ns) MJy shot pulse seen from the Crab pulsar, which is consistent with coherent curvature radiation emitted near the light cylinder by an almost neutral clump with net charge 10 21 e and total energy & 10 23 ergs. Bursts from Gpc distances require incoherent superposition of 10 12 d 2 shot pulses or a total energy & 10 35 d 2 erg. The energy reservoir near the light cylinder limits the detection distance to . few 100 Mpc for a fluence 1 Jy ms unless conditions are more extreme than for the Crab pulsar. Similarly, extreme single pulses from ordinary pulsars and magnetars could be detectable from throughout the Local Group and perhaps farther. Contributions to dispersion measures from galaxy clusters will be significant for some of the bursts. We discuss tests for the signatures of bursts associated with extragalactic NSs.

Cosmological expansion in the Randall-Sundrum brane world scenario

The cosmology of the Randall-Sundrum scenario for a positive tension brane in a 5D universe with localized gravity has been studied previously. In the radiation-dominated universe, it was suggested that there are two solutions for the cosmic scale factor

Type Ia Supernovae, Evolution, and the Cosmological Constant

a(t):

Neutrino-driven winds from young, hot neutron stars

the standard solution

The fate of accreted CNO elements in neutron star atmospheres : X-ray bursts and gamma-ray lines

a\ensuremath{\sim}{t}^{1/2},

Mimicking dark energy with Lemaitre-Tolman-Bondi models: Weak central singularities and critical points

and a solution

Pulsar Spin Evolution, Kinematics, and the Birthrate of Neutron Star Binaries

a\ensuremath{\sim}{t}^{1/4},

Rolling tachyon in brane world cosmology from superstring field theory

which is incompatible with standard big bang nucleosynthesis. In this paper, we reconsider expansion of the Universe in this scenario. We derive and solve a first order, linear differential equation for