Blackbody excess in persistent Be pulsars
N. La Palombara, S. Mereghetti, L. Sidoli, A. Tiengo, P. Esposito
aa r X i v : . [ a s t r o - ph . H E ] J a n Mem. S.A.It. Vol. 84, 1 c (cid:13) SAIt 2008
Memorie della
Blackbody excess in persistent Be pulsars
N. La Palombara , S. Mereghetti , L. Sidoli , A. Tiengo , and P. Esposito INAF - IASF Milano, via Bassini 15, I–20133 Milano, Italy IUSS, Piazza della Vittoria 15, I–27100 Pavia, Italy
Abstract.
We report on the main results obtained thanks to an observation campaign, per-formed with
XMM–Newton , of four persistent, low–luminosity ( L X ∼ erg s − ) andlong–period ( P >
200 s) Be accreting pulsars. We found that all sources considered hereare characterized by a spectral excess that can be described with a blackbody componentof high temperature ( kT BB > R BB < . Key words.
X–rays: binaries – accretion, accretion disks – stars: emission line, Be – X–rays: individual: 4U 0352 + + + We have analyzed the
XMM–Newton observations of the four persistent
Bepulsars originally identified by Reig &Roche (1999), i.e. RX J0146.9 + +
309 (La Palombara & Mereghetti2007), RX J1037.5-5647 (La Palombara et al.2009), and RX J0440.9 + L X ∼ − erg s − ) andlong pulse period ( P >
200 s), two propertieswhich suggest that the neutron star (NS) orbitsthe Be star in a wide and nearly circularorbit, continuously accreting material from thelow–density outer regions of the circumstellarenvelope.The
XMM–Newton spectra of these Bepulsars cannot be described with a single-component model: the fits with a power–law(PL) or a blackbody (BB) model are a ff ectedby large residuals, while other models are re- jected by the data. On the other hand, a goodfit is obtained with a PL + BB model. In allcases the BB component is characterized by ahigh temperature ( kT BB > R BB < . hot–BB spectralcomponent can be considered as an additionalcommon property of the persistent Be pulsars,which stands beside those previously known.Based on the emission models proposedby Hickox et al. (2004), the low luminosityof these pulsars suggests that the observed BB component is due to thermal emission from theNS polar cap. Assuming the standard NS pa-rameters M NS = M ⊙ , R NS = cm, and B NS = G, from the source luminositieswe can estimate the accretion rate and, then,the radius of the accretion column R col . For allsources we found that R col ∼ R BB , thus con-firming the previous hypothesis. La Palombara: Blackbody excess in persistent Be / NS binary pulsars
100 100050 200 50010.52 B l a ck bod y t e m pe r a t u r e ( k e V ) Blackbody radius (m)100 100050 200 50010.52 B l a ck bod y t e m pe r a t u r e ( k e V ) Blackbody radius (m)100 100050 200 50010.52 B l a ck bod y t e m pe r a t u r e ( k e V ) Blackbody radius (m)100 100050 200 50010.52 B l a ck bod y t e m pe r a t u r e ( k e V ) Blackbody radius (m)100 100050 200 50010.52 B l a ck bod y t e m pe r a t u r e ( k e V ) Blackbody radius (m)100 100050 200 50010.52 B l a ck bod y t e m pe r a t u r e ( k e V ) Blackbody radius (m)100 100050 200 50010.52 B l a ck bod y t e m pe r a t u r e ( k e V ) Blackbody radius (m)100 100050 200 50010.52 B l a ck bod y t e m pe r a t u r e ( k e V ) Blackbody radius (m)100 100050 200 50010.52 B l a ck bod y t e m pe r a t u r e ( k e V ) Blackbody radius (m)100 100050 200 50010.52 B l a ck bod y t e m pe r a t u r e ( k e V ) Blackbody radius (m)100 100050 200 50010.52 B l a ck bod y t e m pe r a t u r e ( k e V ) Blackbody radius (m)100 100050 200 500 200010.52 10
100 100050 200 500 200010.52 10
100 100050 200 500 200010.52 10
100 100050 200 500 200010.52 10 Log ( L X ) ( − k e V , e r g s − ) Pulse Period (s)0.01 0.1 1 10 100 1000 10 Log ( L X ) ( − k e V , e r g s − ) Pulse Period (s)
Fig. 1.
Left : Best–fit values for R BB and kT BB of the low–luminosity high–mass X–ray binaries with a hot–BB spectral component (di ff erent symbols refer to di ff erent sources); the continuous lines connect theBB parameters corresponding to four di ff erent levels of L X (in erg s − ). Right : X–ray luminosity (in the2–10 keV energy range) of the pulsars with a detected thermal excess as a function of the pulse period; filled circles refer to the hot–BB pulsars, empty squares to the soft–excess sources.
Table 1.
Main parameters of the persistent Be X–ray binaries
Source 4U 0352 +
309 RX J0146.9 + + ≃ ≃ ≃ ≃ L X (0.3–10 keV, erg s − ) 1 . × . × . × . × L BB (0.3–10 keV, erg s − ) 5 . × . × . × . × L BB / L X (%) 39 24 42 35 T BB (keV) 1.42 ± ± + . − . ± R BB (m) 361 ± ±
15 128 + − ± R col (m) ∼ ∼ ∼ ∼ A spectral feature similar to the hot BB of the persistent Be pulsars has been observedalso in other low–luminosity ( L X ≤ ergs − ) high–mass X–ray binaries (La Palombaraet al. 2012). In Fig. 1 ( left panel ) we reportthe best–fit radius and temperature for the BBcomponent of these sources: it shows that, forall the sources, kT BB > R BB < L X ∼ erg s − ), the spectral parametersare within a narrow range of values ( kT BB ∼ R BB <
200 m), with a 20–40 %contribution of the blackbody component.In contrast to this sample of sources, sev-eral pulsars are characterized by a soft ex-cess, since the fit of this component with athermal emission model provides low temper-atures ( kT SE < . R SE >
100 km). In Fig. 1 ( right panel ) we report the luminosity and pulse period ofboth types of pulsars. On the average, the hot–BB pulsars are characterized by the lowest lu-minosities and the longest periods; their hot–BB spectral component is a common featurewhich separates them from all the other pul-sars, strongly suggesting that they form a dis-tinct and well–defined class of binary pulsars.