Blackbody excess in persistent Be pulsars

0.14 MB·English

by  N. La Palombara

Mem.S.A.It.Vol.84,1
(cid:13)c SAIt 2008 Memoriedella
Blackbody excess in persistent Be pulsars
N. LaPalombara1,S. Mereghetti1,L. Sidoli1,A. Tiengo1,2 andP. Esposito1
3
1 INAF-IASFMilano,viaBassini15,I–20133Milano,Italy
1
2 IUSS,PiazzadellaVittoria15,I–27100Pavia,Italy
0
2

n
Abstract.Wereportonthemainresultsobtainedthankstoanobservationcampaign,per-
a
formed with XMM–Newton, of four persistent, low–luminosity (L ∼ 1034 erg s−1) and
J X
long–period (P > 200s)Beaccretingpulsars.Wefound thatallsourcesconsidered here
2
arecharacterizedbyaspectralexcessthatcanbedescribed withablackbody component
2
ofhightemperature(kT >1keV)andsmallarea(R <0.5km).Weshowthat:1)this
BB BB
featureisacommonpropertyofseverallow–luminosityX–raybinaries;2)formostsources
]
E theblackbodyparameters(radiusandtemperature)arewithinanarrowrangeofvalues;3)
itcanbeinterpretedasemissionfromtheNSpolarcaps.
H
h. Keywords.X–rays:binaries–accretion,accretiondisks–stars:emissionline,Be–X–
p rays:individual:4U0352+309,RXJ0146.9+6121,RXJ0440.9+4431,RXJ1037.5–5647
-
o
r
t We have analyzed the XMM–Newton jected by the data. On the other hand, a good
s
observations of the four persistent Be fit is obtained with a PL+BB model. In all
a
[ pulsars originally identified by Reig & casestheBB componentischaracterizedbya
1 Roche (1999), i.e. RXJ0146.9+6121 hightemperature(kTBB > 1keV)andasmall
(La Palombara & Mereghetti 2006), emission radius (R < 0.5 km), and con-
v BB
4U0352+309 (La Palombara & Mereghetti tributesto30–40%ofthesourcefluxbelow10
0
2007),RXJ1037.5-5647(La Palombaraet al. keV (Table 1): therefore this hot–BB spectral
2
1 2009), and RXJ0440.9+4431 (La Palombara componentcanbeconsideredasanadditional
5 etal.2012);theirmainparametersarereported commonpropertyofthepersistentBepulsars,
. in Table 1. These sources have persistently whichstandsbesidethosepreviouslyknown.
1
low luminosity (L ∼ 1034−35 erg s−1) and
0 X
3 long pulse period(P > 200s), two properties Based on the emission models proposed
1 whichsuggestthattheneutronstar(NS)orbits by Hickox et al. (2004), the low luminosity
v: the Be star in a wide and nearly circular ofthesepulsarssuggeststhattheobservedBB
orbit,continuouslyaccretingmaterialfromthe
i componentisduetothermalemissionfromthe
X low–densityouterregionsof thecircumstellar
NS polar cap. Assuming the standard NS pa-
r envelope. rameters M = 1.4 M , R = 106 cm, and
a NS ⊙ NS
The XMM–Newton spectra of these Be B = 1012 G, from the source luminosities
NS
pulsars cannot be described with a single- we can estimate the accretion rate and, then,
component model: the fits with a power–law theradiusoftheaccretioncolumnR .Forall
col
(PL) or a blackbody (BB) model are affected sources we found that R ∼ R , thus con-
col BB
by large residuals, while other models are re- firmingtheprevioushypothesis.
2 LaPalombara:BlackbodyexcessinpersistentBe/NSbinarypulsars
1036 3388
Blackbody temperature (keV)Blackbody temperature (keV)Blackbody temperature (keV)Blackbody temperature (keV)Blackbody temperature (keV)Blackbody temperature (keV)Blackbody temperature (keV)Blackbody temperature (keV)Blackbody temperature (keV)Blackbody temperature (keV)Blackbody temperature (keV) 121212121212121212121212121212 11003345 Log(L) (2−10 keV, erg s)Log(L) (2−10 keV, erg s)−1−1XX 3366
3344
000000000000000...............555555555555555 1033
555555555555555000000000000000 111111111111111000000000000000000000000000000 222222222222222000000000000000000000000000000 555555555555555000000000000000000000000000000 111111111111111000000000000000000000000000000000000000000000 2222000000000000 00..0011 00..11 11 1100 110000 11000000 110044
BBBBBBBBBBBlllllllllllaaaaaaaaaaaccccccccccckkkkkkkkkkkbbbbbbbbbbbooooooooooodddddddddddyyyyyyyyyyy rrrrrrrrrrraaaaaaaaaaadddddddddddiiiiiiiiiiiuuuuuuuuuuusssssssssss (((((((((((mmmmmmmmmmm))))))))))) PPuullssee PPeerriioodd ((ss))
Fig.1. Left: Best–fit values for R and kT of the low–luminosity high–mass X–ray binaries with a
BB BB
hot–BBspectralcomponent(differentsymbolsrefertodifferentsources);thecontinuouslinesconnectthe
BB parameters corresponding tofour different levels of L (in ergs−1). Right: X–ray luminosity (inthe
X
2–10 keV energy range) of the pulsars withadetected thermal excess asa function of the pulse period;
filledcirclesrefertothehot–BBpulsars,emptysquarestothesoft–excesssources.
Table1.MainparametersofthepersistentBeX–raybinaries
Source 4U0352+309 RXJ0146.9+6121 RXJ1037.5-5647 RXJ0440.9+4431
OpticalSpectralType 09.5IIIe B0IIIe B0III-Ve B0.2Ve
Distance(kpc) ≃1 ≃2.5 ≃5 ≃3.3
PulsePeriod(s) 839.3 1396.1 853.4 204.98
L (0.3–10keV,ergs−1) 1.4×1035 1.5×1034 1.2×1034 8.3×1034
X
L (0.3–10keV,ergs−1) 5.5×1034 3.6×1033 5.0×1033 2.9×1034
BB
L /L (%) 39 24 42 35
BB X
T (keV) 1.42±0.03 1.11±0.06 1.26+0.16 1.34±0.04
BB −0.09
R (m) 361±3 140±15 128+13 273±16
BB −21
R (m) ∼330 ∼230 ∼200 ∼320
col
A spectral feature similar to the hot BB we report the luminosity and pulse period of
ofthepersistentBepulsarshasbeenobserved bothtypesofpulsars.Ontheaverage,thehot–
also in other low–luminosity (L ≤ 1036 erg BB pulsarsarecharacterizedbythelowestlu-
X
s−1)high–massX–raybinaries(LaPalombara minosities and the longest periods; their hot–
et al. 2012). In Fig. 1 (left panel) we report BB spectral component is a common feature
the best–fitradiusandtemperaturefortheBB which separates them from all the other pul-
componentofthesesources:itshowsthat,for sars, stronglysuggestingthatthey forma dis-
all the sources, kT > 0.5 keV and R < 2 tinctandwell–definedclassof binarypulsars.
BB BB
km; moreover,forthe less luminosity sources
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X
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