Fastest pulsar so far seen

[Martin]

press release from NRAO

http://www.nrao.edu/pr/2005/fastpulsar/

The fastest-moving neutron star ever seen, clocked at 1100 kilometers per second, was given its initial kick by the supernova that formed it, observers think, and that speed will take it out of our galaxy.

 

journal article

http://arxiv.org/abs/astro-ph/0509031

Getting Its Kicks: A VLBA Parallax for the Hyperfast Pulsar B1508+55

S. Chatterjee et al.

5 pages, including 2 figures

Astrophys.J. 630 (2005) L61-L64

"The highest velocity neutron stars establish stringent constraints on natal kicks, asymmetries in supernova core collapse, and the evolution of close binary systems. Here we present the first results of a long-term pulsar astrometry program using the VLBA. We measure a proper motion and parallax for the pulsar B1508+55, leading to model-independent estimates of its distance (2.37+0.23-0.20 kpc) and transverse velocity (1083+103-90 km/s), the highest velocity directly measured for a neutron star. We trace the pulsar back from its present Galactic latitude of 52.3 degrees to a birth site in the Galactic plane near the Cyg OB associations, and find that it will inevitably escape the Galaxy. Binary disruption alone is insufficient to impart the required birth velocity, and a natal kick is indicated. A composite scenario including a large kick along with binary disruption can plausibly account for the high velocity.

 

popular magazine account:

http://www.astronomy.com/asy/default.aspx?c=a&id=3471

 

a discussion of various "kick" mechanisms that might give neutron stars high speeds is in this paper, see bottom of page 7

http://arxiv.org/abs/astro-ph/0106159

[Martin]

the earth speed in its orbit around the sun is 30 kilometers per second

 

which is one tenthousandth of the speed of light-----a hundredth of one percent.

 

OK this neutron star is going 1100 km per second. Almost 40 times faster than the earth in its orbit.

 

A science question, if anybody wants to explain, is HOW DID IT GET GOING THAT FAST?

 

FOUR TENTHS OF A PERCENT of the speed of light.

 

What could boost a STAR to go that fast?

[Martin]

I'm gradually finding out a little more about "asymmetric supernova explosions"

 

this is the LOPSIDED BLAST which can give the neutronstar remnant a KICK

 

I used to imagine that supernova explosions were symmetric, equally forceful outwards on all sides---that's how I think of explosions being, mostly. but apparently they dont have to be.

 

great rocket engine if one could only harness it

 

 

here's some more stuff

http://newton.ex.ac.uk/aip/glimpse.txt/physnews.181.1.html

 

ASYMMETRIC SUPERNOVA EXPLOSIONS can impart a "kick" to the neutron star remnants born during the explosions. After a new reassessment, the proper motions (the motions across the sky) of 86 pulsars were found to have a mean velocity of 450 km/sec, which exceeds the escape velocities for a number of celestial systems, such as binary stars, globular clusters, and even for our galaxy. The University of Manchester (UK) astronomers who performed the study conclude that more than half of all pulsars will escape from the Milky Way and that those that stay will assume a larger and more spherical distribution than was previously thought. Therefore, the astronomers assert, this population of old, high-velocity pulsars might be responsible for more of the gamma bursts seen by the Gamma Ray Observatory than previously expected. (A.G. Lyne and D.R. Lorimer, Nature, 12 May 1994.)

[Martin]

here is a recent paper (2005), maybe it has references to previous stuff that would help. this is online free for download as PDF

 

 

http://arxiv.org/astro-ph/0505199

An Asymmetric, Energetic Type Ic Supernova Viewed Off-Axis, and a Link to Gamma-Ray Bursts

Mazzali et al

Science on 27 May 2005. 14 pages, 4 figures

 

"Type Ic supernovae, the explosions following the core collapse of massive stars that have previously lost their hydrogen and helium envelopes, are particularly interesting because of the link with long-duration gamma-ray bursts. Although indications exist that these explosions are aspherical, direct evidence has still been missing. Late-time observations of SN 2003jd, a luminous Type Ic supernova, provide such evidence. Recent Subaru and Keck spectra reveal double-peaked profiles in the nebular lines of neutral oxygen and magnesium. These profiles are different from those of known Type Ic supernovae, with or without a gamma-ray burst, and they can be understood if SN 2003jd was an aspherical, axisymmetric explosion viewed from near the equatorial plane. If SN 2003jd was associated with a gamma-ray burst, we missed the burst as it was pointing away from us."

 

SAMPLE TEXT FROM INTRODUCTION

 

When a massive star reaches the end of its life and exhausts its nuclear fuel, the core itself collapses to form a compact remnant (a neutron star or a black hole). Although the exact mechanism is not well-understood, the resulting release of energy leads to the ejection of the envelope of the star at high velocities, producing a supernova (SN)...

[Martin]

they are talking about an axi-symmetric SN explosion where iron and nickel from the core comes out in a JET

 

so if we are in line with the jet we see iron and nickel atoms coming at us FASTER than the other crud, like carbon and oxygen, which was in the outer layers

 

usually, in a symmetric, spherical, explosion the lighter atoms C and O come tearing out faster because they are lighter, so you watch the expanding cloud which is there for weeks and months after the SN

and you can tell by the colors what sort of hot atom is in different parts of the cloud, and by dopplershift how fast stuff is moving towards us.

 

but where there was this JET that bursts up out of the depth, you look at the light and recognize that the Ni is coming at us faster than the C and O.

 

So they observed this. It is evidence confirming that this Jet type of SN explosion can happen. does happen.

 

MORE QUOTE

"Asphericity can explain this peculiar situation (11). In a typical, spherical SN explosion, heavier elements are produced in deeper layers of the progenitor star, and as a consequence of the hydrodynamical properties of the explosion they are given less kinetic energy per unit mass than external layers, which typically contain lighter elements. However, in a jet-like explosion the heavier elements (in particular 56Ni) are probably synthesized near the jet at the time of core collapse, and are ejected at high velocities. Lighter elements such as oxygen, which are not produced in the explosion but rather by the progenitor star during its evolution, are ejected near the equatorial plane with a smaller kinetic energy, and are distributed in a disc-like structure.

 

Given this scenario, the observed line profiles depend on the orientation of the explosion with respect to our line of sight. Iron can be observed to be approaching us at a higher velocity than oxygen if we view the explosion near the jet direction, which is also the requirement for the GRB to be observed (10,11). The [O i] line, on the other hand, will appear as a narrow, sharp line in the case of a polar view, since in that case oxygen moves almost perpendicular with respect to our line of sight (the case of SN 1998bw), but it will show a broader, double-peaked profile for an equatorial view, since a large fraction of the oxygen would then be moving either toward or away from the observer (11)."

 

here are some other Keiichi Maeda links

http://arxiv.org/abs/astro-ph/0304172

Bipolar Supernova Explosions...

http://arxiv.org/abs/astro-ph/0305183

Jets and Black Holes in Hypernova Explosions

 

none of this is entirely satisfactory, I'm getting a taste of it but I still dont understand how a SN explosion can deliver a KICK in one particular direction that results in the neutronstar remnant from the explosion flying off at several hundred clicks per second. or a thousand km/s like in that news item

[Martin]

this might help

http://arxiv.org/abs/astro-ph/0504584

A statistical study of 233 pulsar proper motions

G. Hobbs, D. R. Lorimer, A. G. Lyne, M. Kramer

 

Lyne and Lorimer seem to be experts in this area

here's another thing of theirs that could be relevant

http://arxiv.org/abs/astro-ph/0411796

 

 

here is from the final paragraph of the Statistical Study conclusions section

 

The implications of these results for ‘kick’ mechanisms may be summarised by stating that the true space velocities of young pulsars range from a few tens to well over 1000 kms^-1 with a mean velocity of 400(40) kms^-1. According to Lai, Chernoff & Cordes (2001): (1) local convective instabilities in the collapsed stellar core can account for velocities up to approx. 100 kms^-1; (2) global asymmetric perturbations can create velocities over 1000 kms^-1; (3) asymmetric neutrino emission can provide kick velocities up to approx. 1000 kms^-1; (4) the electromagnetic rocket effect can accelerate pulsars up to similarly high velocities. Our results suggest that (1) is unlikely. (4) is testable by studying the alignment between the direction of motion of a pulsar and its spin-axis (see Deshpande, Ramachandran & Radhakrishnan 1999) although the duration of the kick will also affect the observed alignment (Spruit & Phinney 1998). More proper motions will become available within the next few years both from interferometry and from timing. The Jodrell Bank data archive will be able to provide values or limits on the proper motions of many hundreds of pulsars in the near future. These will further improve the constraints on the distribution and origin of pulsar velocities.

[Martin]

this might be the best so far, about the "kick" mechanism:

 

http://arxiv.org/abs/astro-ph/0007272

Pulsar Jets: Implications for Neutron Star Kicks and Initial Spins

Dong Lai, David F. Chernoff, James M. Cordes (Cornell)

ApJ, Vol.549 (March 10, 2001)

We study implications for the apparent alignment of the spin axes, proper-motions, and polarization vectors of the Crab and Vela pulsars. The spin axes are deduced from recent Chandra X-ray Observatory images that reveal jets and nebular structure having definite symmetry axes. The alignments indicate these pulsars were born either in isolation or with negligible velocity contributions from binary motions. We examine the effects of rotation and the conditions under which spin-kick alignment is produced for various models of neutron star kicks. If the kick is generated when the neutron star first forms by asymmetric mass ejection or/and neutrino emission, then the alignment requires that the protoneutron star possesses an original spin with period P_s much less than the kick timescale, thus spin-averaging the kick forces. The kick timescale ranges from 100 ms to 10 s depending on whether the kick is hydrodynamically driven or neutrino-magnetic field driven. For hydrodynamical models, spin-kick alignment further requires the rotation period of an asymmetry pattern at the radius near shock breakout (>100 km) to be much less than approx 100 ms; this is difficult to satisfy unless rotation plays a dynamically important role in the core collapse and explosion (P_s less than ms). Aligned kick and spin vectors are inherent to the slow process of asymmetric electromagnetic radiation from an off-centered magnetic dipole. We reassess the viability of this effect, correcting a factor of 4 error in Harrison and Tademaru's calculation that increases the size of the effect. To produce a kick velocity of order a few hundred km/s requires that the neutron star be born with an initial spin close to 1 ms and that spindown due to r-mode driven gravitational radiation be inefficient compared to standard magnetic braking."

 

WELL FOLKS, IT LOOKS LIKE A HANDFUL OF KICK MECHANISMS HAVE BEEN PROPOSED BUT IT IS STILL NOT CLEAR WHAT IS RESPONSIBLE for all these neutron stars flying around some of them at incredible speeds. At least I couldnt find anything solidly conclusive.

[Martin]

More stuff on this:

 

a popular article in space.com

http://www.space.com/scienceastronomy/050906_fast_star.html

 

a new article with some discussion of how a supernova can give the collapsed core a kick

 

http://arxiv.org/abs/astro-ph/0509484

Neutron Star Kicks in Isolated and Binary Pulsars: Observational Constraints and Implications for Kick Mechanisms

Chen Wang, Dong Lai, JinLin Han

30 pages, 2 figures, submitted to ApJ on Aug. 24, 2005

 

"We study observational constraints on neutron star (NS) kicks for isolated pulsars and for neutron stars in binary systems. We are particularly interested in the evidence of kick-spin alignment/misalignment and its dependence on the neutron star initial spin period. ... We discuss the implication of our result for various NS kick mechanisms.

 

The authors outline 4 general types of kick mechanism that have been proposed. Two of these they say they can exclude based on their observation. the two remaining proposed mechanisms are

 

1. hydrodynamic asymmetry. because of unstable convections currents the collapsing star has some lopsided stuff going on inside and the supernova explosion is actually stronger in one direction than another.

 

this seems to work best with their observations---easiest way to account for most of them

 

2. some partly new physics (not yet established) is required about how the intense magnetic field in a collapsing star with iron core can affect neutrinos. the supernova explosion turns out asymmentric (it is conjectured) because of asymmetry in the magnetic field.

 

this has been proposed by other people and the authors do not seem very enthusiastic about it, but their observations cannot exclude this mechanism.

 

I have to conclude that although many neutron stars are observed to have high velocities, and something in the supernova explosion must have given them a kick, the astrophysicists don't seem to be sure about how.

 

the mechanism still seems to be an open problem