Could electron-positron annihilation lines in the Galactic center result from pulsar winds?

Free access article

Issue		A&A Volume 446, Number 3, February II 2006


Page(s)		943 - 948
Section		Galactic structure, stellar clusters and populations
DOI		http://dx.doi.org/10.1051/0004-6361:20053559

A&A 446, 943-948 (2006)
DOI: 10.1051/0004-6361:20053559

Could electron-positron annihilation lines in the Galactic center result from pulsar winds?

W. Wang^{1, 2}, C. S. J. Pun³ and K. S. Cheng³

¹ Max-Planck-Institut für extraterrestrische Physik, Postfach 1312, 85741 Garching, Germany
e-mail: wwang@mpe.mpg.de
² National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012, PR China
³ Department of Physics, University of Hong Kong, Pokfulam Road, Hong Kong, PR China

(Received 2 June 2005 / Accepted 20 September 2005)

Abstract
Observations of a strong and extended positron-electron annihilation line emission in the Galactic center (GC) region by the Spectrometer on the International Gamma-Ray Astrophysical Laboratory (SPI/INTEGRAL) are challenging to the existing models of positron sources in the Galaxy. In this paper, we study the possibility that pulsar winds in the GC produce the 511 keV line. We propose that three possible scenarios of pulsar winds may exist as the positron sources: normal pulsars, rapidly spinning strongly magnetized neutron stars (magnetars) in gamma-ray burst (GRB) progenitors, a population of millisecond pulsars in the Galactic center. These e $^\pm$ pairs could be trapped in the region by the magnetic field in the Galactic center, and cool through synchrotron radiation and Coulomb interactions with the medium, thereby becoming non-relativistic particles. The cooling timescales are shorter than the diffuse timescale of positrons, so low-energy positrons could annihilate directly with electrons into 511 keV photons or could form positronium before annihilation. We find that normal pulsars cannot be a significant contributor to the positron sources. Although magnetars in the GC could be potential sources of positrons, their birth rate and birth locations may pose some problems for this scenario. We believe that the most likely candidates for positron sources in the GC may be a population of millisecond pulsars in the GC. Our preliminary estimations predict that the e $^\pm$ annihilation rate in the GC is ${\geq} 5\times 10^{42}$ s^-1, which is consistent with the present observational constraints. Therefore, the e $^\pm$ pairs from pulsars winds can contribute significantly to the positron sources in the Galactic center region. Furthermore, since the diffusion length of positrons is short, we predict that the intensity distribution of the annihilation line should follow the distribution of millisecond pulsars, which should then correlate to the mass distribution in the GC.

Key words: Galaxy: center -- gamma-rays: theory -- pulsars: general

SIMBAD Objects

What is OpenURL?

The OpenURL standard is a protocol for transmission of metadata describing the resource that you wish to access. An OpenURL link contains article metadata and directs it to the OpenURL server of your choice. The OpenURL server can provide access to the resource and also offer complementary services (specific search engine, export of references...). The OpenURL link can be generated by different means.

If your librarian has set up your subscription with an OpenURL resolver, OpenURL links appear automatically on the abstract pages.
You can define your own OpenURL resolver with your EDPS Account. In this case your choice will be given priority over that of your library.
You can use an add-on for your browser (Firefox or I.E.) to display OpenURL links on a page (see http://www.openly.com/openurlref/). You should disable this module if you wish to use the OpenURL server that you or your library have defined.