Re-examining the X-ray versus spin-down luminosity correlation of rotation powered pulsars

¹ Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy e-mail: phd@tucanae.bo.astro.it
² Dipartimento di Fisica, Università di Milano , via Celoria 16, 20133 Milano, Italy e-mail: rossella@ifctr.mi.cnr.it
³ Dipartimento di Fisica, Università di Milano Bicocca, Piazza della Scienza 3, 20126 Milano, Italy e-mail: colpi@castore.mib.infn.it
⁴ Istituto di Astrofisica Spaziale e Fisica Cosmica, Sezione di Milano “G.Occhialini”, CNR, via Bassini 15, 20133 Milano, Italy e-mail: sandro@ifctr.mi.cnr.it

Corresponding author: M. Colpi, monica.colpi@mib.infn.it

Received: 25 September 2001
Accepted: 22 March 2002

Abstract

The empirical relation between the X-ray luminosity (in the 2–10 keV band) and the rate of spin-down energy loss of a sample of 39 pulsars is re-examined considering recent data from ASCA, RXTE, BeppoSAX, Chandra, and XMM-Newton and including statistical and systematic errors. The data show a significant scatter around an average correlation between L_{x, (2-10)} and . By fitting a dependence of L_{x, (2-10)} on the period P and period derivative of the type , we obtain and (i.e. ). This translates into the relation with a normalization . However, the reduced is large (=7.2) making the fit unacceptable on statistical ground. All the X-ray luminosities lie below a critical line L_x,crit: the corresponding efficiency of conversion of rotational energy into 2–10 keV X-rays is and varies, within the sample, between 0.1 and 80%. The large dispersion of L_x below L_x,crit indicates that other physical parameters uncorrelated with P and  need to be included to account for the observed emission at X-ray energies. We indicate a few possibilities that all conspire to reduce L_{x, (2-10)}.