A Potential Cyclotron Line Signature in Low-Luminosity X-Ray Sources
Abstract
Estimates indicate there may be $\simgreat 10^3$ low luminosity X-ray pulsars ($L \simless 10^{34} \rom{erg\;s^{-1}}$) in the Galaxy undergoing ``low-state'' wind accretion in Be/X-ray binary systems, and $\sim 10^8-10^9$ isolated neutron stars which may be accreting directly from the interstellar medium. Despite their low effective temperatures ($kT_e \simless 300 \rom{eV}$), low luminosity accreting neutron stars with magnetic fields $B \sim (0.7-7) \times 10^{12}$ G could emit a substantial fraction ($0.5 - 5\%$) of their total luminosity in a moderately broadened ($E/\Delta E \sim 2-4$) cyclotron {\it emission} line which peaks in the energy range $ \sim 5-20\rom{keV}$. The bulk of the thermal emission from these stars will be in the extreme ultraviolet/soft X-ray regime, which is subject to strong interstellar absorption and would be difficult to distinguish from spectra of other types of objects. In sharp contrast, the {\it nonthermal} cyclotron component predicted here will not be strongly absorbed, and consequently it may be the only distinguishing signature for the bulk of these low luminosity sources. We propose a search for this cyclotron emission feature in long pointed observations of the newly discovered candidate isolated neutron star MS0317.7-6477, and the Be/X-ray transient
- Publication:
-
The Astrophysical Journal
- Pub Date:
- January 1995
- DOI:
- 10.1086/187725
- arXiv:
- arXiv:astro-ph/9410017
- Bibcode:
- 1995ApJ...438L..99N
- Keywords:
-
- Accretion Disks;
- Astrophysics;
- Binary Stars;
- Cyclotron Frequency;
- Neutron Stars;
- Pulsars;
- X Ray Astronomy;
- X Ray Sources;
- Interstellar Matter;
- Radiative Transfer;
- Spectral Signatures;
- Stellar Winds;
- Ultraviolet Spectra;
- Astrophysics;
- ACCRETION;
- ACCRETION DISKS;
- LINE: FORMATION;
- RADIATIVE TRANSFER;
- STARS: NEUTRON;
- X-RAYS: STARS;
- Astrophysics
- E-Print:
- Accepted by ApJLett, 10 pages, uuencoded tar-compressed postscript, CITA-94-44