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Issue A&A
Volume 387, Number 3, June I 2002
Page(s) 993 - 1002
Section Formation, structure and evolution of stars
DOI http://dx.doi.org/10.1051/0004-6361:20020472



A&A 387, 993-1002 (2002)
DOI: 10.1051/0004-6361:20020472

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

A. Possenti1, R. Cerutti2, M. Colpi3 and S. Mereghetti4

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

(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 $ {{L}}_{\rm sd}$ 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 $L_{\rm x, (2-10)}$ and $ {{L}}_{\rm sd}$. By fitting a dependence of  $L_{\rm x, (2-10)}$ on the period P and period derivative $\dot P$ of the type $L_{\rm x, (2-10)}\propto P^a\, {\dot {P}} ^b$, we obtain a=-4.00 and b=+1.34 (i.e. $a\simeq -3b$). This translates into the relation $L_{\rm x, (2-10)}=L_{\rm x,nor}\left ({ {{L}}_{\rm sd}/ {\rm {erg \,\,s}^{-1}} }\right)^{1.34}$ with a normalization $L_{\rm x,nor}=10^{-15.3}~ {\rm {erg \,\,s}^{-1}} $. However, the reduced $\chi^2$ is large ( =7.2) making the fit unacceptable on statistical ground. All the X-ray luminosities lie below a critical line $L_{\rm x,crit}$: the corresponding efficiency of conversion of rotational energy into 2-10 keV X-rays is $\eta_{\rm x}=(L_{\rm x,crit}/ {{L}}_{\rm sd})\propto {{L}}_{\rm sd}^{0.48}$ and varies, within the sample, between 0.1 and 80%. The large dispersion of $L_{\rm x}$ below $L_{\rm x,crit}$ indicates that other physical parameters uncorrelated with  P and  $\dot P$ need to be included to account for the observed emission at X-ray energies. We indicate a few possibilities that all conspire to reduce  $L_{\rm x, (2-10)}$.


Key words: radiation mechanisms: non-thermal -- star: neutron -- stars: pulsar: general -- X-rays: general -- $\gamma$-rays: observations

Offprint request: M. Colpi, monica.colpi@mib.infn.it

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© ESO 2002


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