A Limit on the Number of Isolated Neutron Stars Detected in the ROSAT Bright Source Catalogue

The challenge in searching for non-radio-pulsing isolated neutron stars (INSs) is in excluding association with objects in the very large error boxes (~13'', 1 σ radius) typical of sources from the largest X-ray all-sky survey, the ROSAT All-Sky Survey Bright Source Catalogue (RASS BSC). We search for candidate INSs using statistical analysis of optical (USNO-A2), infrared (IRAS), and radio (NVSS) sources near the ROSAT X-ray localization and show that this selection would find 20% of the INSs in the RASS BSC. This selection finds 32 candidates at declinations δ > -39°, among which are two previously known INSs, 17 sources that we show are not INSs, and 13 the classification of which are as yet undetermined. These results require a limit of less than 67 INSs (90% confidence, full sky, assuming isotropy) in the RASS BSC. This limit modestly constrains a naive and optimistic model for cooling neutron stars in the Galaxy.

[1]  V. M. Lipunov,et al.  The Neutron Star Census , 2000 .

[2]  S. Kulkarni,et al.  Accepted to ApJL Preprint typeset using L ATEX style emulateapj v. 14/09/00 A PROBABLE OPTICAL COUNTERPART FOR THE ISOLATED NEUTRON STAR RX J1308.6+2127 , 2002 .

[3]  R. Danner.  Searching for old neutron stars with ROSAT II. Soft X-ray sources in galactic dark clouds , 1998 .

[4]  E. Greisen,et al.  The NRAO VLA Sky Survey , 1996 .

[5]  P. Dokkum.  Cosmic-Ray Rejection by Laplacian Edge Detection , 2001, astro-ph/0108003.

[6]  X-Ray Spectra from Neutron Stars Accreting at Low Rates , 1994, astro-ph/9407067.

[7]  California Institute of Technology,et al.  The Pulsar kick velocity distribution , 1997, astro-ph/9708071.

[8]  O. Blaes,et al.  Can we observe accreting, isolated neutron stars ? , 1993 .

[9]  RBS 1774: A New Isolated Neutron Star Candidate , 2001, astro-ph/0108456.

[10]  J. Greiner,et al.  Supersoft X-Ray Source in the Globular Cluster M3 (NGC 5272) , 1999 .

[11]  L. Matthews,et al.  The optical counterpart of the isolated neutron star RX J185635−3754 , 1997, Nature.

[13]  D. Lorimer,et al.  Pulsar statistics - IV. Pulsar velocities , 1997 .

[14]  Aldo Treves,et al.  Isolated Neutron Stars: Accretors and Coolers , 1999, astro-ph/9911430.

[15]  A. Schwope,et al.  The ROSAT Bright Survey: I. Identification of an AGN sample with hard ROSAT X-ray spectra † , 1998, astro-ph/9811139.

[16]  Wolfgang Voges,et al.  Identification of a complete sample of northern ROSAT All-Sky Survey X-ray sources - II. The optical observations , 1997 .

[17]  Propagation of Magnetized Neutron Stars through the Interstellar Medium , 2001, astro-ph/0105422.

[18]  Wolfgang Voges,et al.  The ROSAT Bright Survey: II. Catalogue of all high-galactic latitude RASS sources with PSPC countrate CR > 0.2 s -1 , 2000 .

[19]  O. Blaes,et al.  Constraints on Accreting, Isolated Neutron Stars from the ROSAT and EUVE Surveys , 1994 .

[20]  R. Danner.  Searching for old neutron stars with ROSAT. I. Soft X-ray sources in molecular clouds at high galactic latitude , 1998 .

[21]  Johns Hopkins University,et al.  Characterization of M, L, and T Dwarfs in the Sloan Digital Sky Survey , 2002, astro-ph/0204065.

[22]  Robert J. Brunner,et al.  XID: Cross-Association of ROSAT/Bright Source Catalog X-Ray Sources with USNO A-2 Optical Point Sources , 2000 .

[23]  H. Bondi,et al.  On spherically symmetrical accretion , 1952 .

[24]  Massachusetts Institute of Technology,et al.  Discovery of 5.16 s pulsations from the isolated neutron star RBS 1223 , 2001, astro-ph/0110365.

[25]  Magnetically Accreting Isolated Old Neutron Stars , 2001, astro-ph/0101550.

[26]  S. R. Kulkarni,et al.  Optical Observations of the Isolated Neutron Star RX J0720.4–3125 , 1998, astro-ph/9803024.

[27]  T. Boller,et al.  The Hamburg/RASS Catalogue of optical identifications , 1998 .

[28]  J. Cordes,et al.  Neutron Star Population Dynamics. II. Three-dimensional Space Velocities of Young Pulsars , 1997, astro-ph/9707308.

[29]  R. Nichol,et al.  Cataclysmic Variables from The Sloan Digital Sky Survey. I. The First Results , 2002 .

[30]  F. Hoyle,et al.  On the Mechanism of Accretion by Stars , 1944 .

[31]  University of Oxford,et al.  The Elusiveness of Old Neutron Stars , 1998, astro-ph/9802032.

[32]  R. Nichol,et al.  Cataclysmic Variables from The Sloan Digital Sky Survey. I. The First Results , 2001, astro-ph/0110291.

[33]  A New Magnetic Cataclysmic Variable: WGA J1047.1+6335 , 1995 .

[34]  J. Dickey,et al.  H I in the Galaxy , 1990 .

[35]  M. Zombeck.  Handbook of Space Astronomy and Astrophysics , 1982 .

[36]  D. Kaplan,et al.  The Optical Counterpart of the Isolated Neutron Star RX J1605.3+3249 , 2003, astro-ph/0302370.