The soft quiescent spectrum of the transiently accreting 11 Hz X-ray pulsar in the globular cluster Terzan 5

0.46 MB·English

by  N. Degenaar

Mon.Not.R.Astron.Soc.000,1–6(0000) Printed6January2011 (MNLATEXstylefilev2.2)
The soft quiescent spectrum of the transiently accreting
11 Hz X-ray pulsar in the globular cluster Terzan 5
⋆
1 N. Degenaar & R. Wijnands
1
Astronomical Institute ”Anton Pannekoek”, University of Amsterdam, Postbus 94249, 1090 GEAmsterdam, the Netherlands
0
2
Accepted 2010December 23.Received2010December23;inoriginalform2010November26
n
a
J
ABSTRACT
4
We report on the quiescent X-ray properties of the recently discovered transiently

] accreting 11 Hz X-ray pulsar in the globular cluster Terzan 5. Using two archival
E Chandra observations, we demonstrate that the quiescent spectrum of this neutron
H star low-mass X-ray binary is soft and can be fit to a neutron star atmosphere model
with a temperature of kT∞ ∼73 eV. A powerlaw spectral component is not required
.
h by the data and contributes at most ∼ 20% to the total unabsorbed 0.5–10 keV flux
p of ∼ 9×10−14 erg cm−2 s−1. Such a soft quiescent spectrum is unusual for neutron
-
o stars with relatively high inferred magnetic fields and casts a different light on the
r interpretation of the hard spectral component, which is often attributed to magnetic
st field effects. For a distance of 5.5 kpc, the estimated quiescent thermal bolometric
a luminosity is ∼ 6×1032 erg s−1. If the thermal emission is interpreted as cooling of
[ the neutron star, the observed luminosity requires that the system is quiescent for at

1 least ∼ 100 years. Alternatively, enhanced neutrino emissions can cool the neutron
v star to the observed quiescent luminosity.
2
Key words: globular clusters: individual (Terzan 5) - X-rays: binaries - stars: neu-
4
8 tron-pulsars:individual(CXOGClbJ174804.8–244648,IGRJ174880–2446)-X-rays:
0 individual (XB 1745–25,EXO 1745–248)
.
1
0
1
1 1 INTRODUCTION ically be fitted with a soft thermal model, a hard powerlaw
:
v shape, or a combination of both. Thehard spectral compo-
Low-mass X-ray binaries (LMXBs) are binary star systems
i nenthasbeenattributedtonon-thermalemission processes
X inwhichaneutronstarorablackholeaccretesmatterfrom
relatedtothemagneticfieldoftheneutronstar,e.g.,accre-
a (sub-) solar companion. Two phenomena are thought to
r tion onto the magnetosphere or a pulsar wind mechanism
a uniquelyidentifythecompactprimaryasaneutronstar:co-
(e.g., Campana et al. 1998).
herentX-raypulsations andtype-IX-raybursts.Thelatter
are intense flashes of X-ray emission caused by thermonu- The soft thermal component is generally interpreted
clear runaway of the accreted matter on the surface of the as thermal emission from the neutron star surface. Dur-
neutron star. X-ray pulsations can be observed when the ing accretion outbursts, a chain of nuclear reactions release
neutron star magnetic field is strong enough to funnel the heatdeepintheneutronstarcrust(e.g., Haensel & Zdunik
accretion flow tothe magnetic poles. 2008), which is re-radiated during quiescent episodes. This
Many neutron star LMXBs are transient and spend resultsinanincandescentluminositythatissetbythelong-
most of their lifetime in a quiescent state, during which termaveragedmass-accretion rateofthesystemandtheef-
they are dim with typical 0.5–10 keV X-ray luminosities ficiencyofneutrinoemissionprocessesoccurringintheneu-
of Lq ∼ 1031−33 erg s−1 (e.g., Heinkeet al. 2009). How- tron star core (e.g., Brown et al. 1998). Residual accretion
ever,occasionally theyexhibitoutburstsduringwhichtheir onto the neutron star surface offers an alternative expla-
X-ray luminosity increases orders of magnitude to LX ∼ nation for the quiescent thermal emission (Zampieri et al.
1036−38 erg s−1 (2–10 keV; e.g., Chen et al. 1997). The en- 1995).
hanced activity is ascribed to a sudden strong increase in
Galactic globular clusters are rich targets for studies
themass-accretionrateontotheneutronstar,whereaslittle
ofX-raybinaries, whichcan form indynamicalinteractions
or no matter is accreted duringquiescent episodes.
in thedensecluster environments(e.g., Pooley et al. 2003).
Thespectra of quiescentneutronstar LMXBscan typ-
Numerouslow-luminosity X-raysources havebeen foundin
globularclusters,amongstwhichareseveralcandidatequies-
centLMXBs(e.g.,Verbuntet al.1995;Grindlay et al.2001;
⋆ e-mail:[email protected] Heinkeet al. 2006). Indeed, a few transient LMXBs have
2 N. Degenaar & R. Wijnands
nationofthebinaryorbitalperiod(Porb =21.27h),andthe
identification of the mass donor as a ∼ 0.4−1.5 M⊙ main
sequenceor slightly evolved star (Papitto et al. 2010).
Anaccurate Swift localization revealed that thesource
active in 2010 was likely a different transient than the one
detectedin2000(Kennea et al.2010).Chandraobservations
confirmed this, thereby establishing the existence of a sec-
ond transient neutron star LMXB in Terzan 5, named CX-
OGClbJ174804.8–244648 (Pooley et al.2010).Theangular
separation between the two transients is only ∼ 5.5′′ (see
Figure2),andduetothelimitedangularresolutionofolder
X-raymissionsitisunclearwhichofthetwowasresponsible
forotheractiveepisodesofTerzan5.The2010transientcor-
respondstotheX-raysourceCX25fromtheChandra study
of Heinkeet al. (2006), which was marked as a candidate
quiescent LMXB by theseauthors.
2 OBSERVATIONS, DATA ANALYSIS AND
RESULTS
WesearchedtheChandra dataarchiveandfoundtwoACIS-
Figure 1. RXTE/ASM 5-day averaged lightcurve of Terzan 5,
showingactivityfromtheglobularclusterin2000,2003and2010. S observations of Terzan 5 in which no bright transient
sourcesareactive.Thisdataisthussuitabletoexaminethe
quiescentpropertiesofthenewlydetected11HzX-raypul-
beenidentifiedinglobularclustersduringoutburstepisodes sar.Thefirstobservationwasperformedon2003July13–14
(see e.g., thereview byVerbunt& Lewin 2006). from 13:33–00:46 ut(ID3798) foran exposuretimeof39.5
ks (see also Wijnands et al. 2005; Heinkeet al. 2006). The
second observation had a duration of 36.4 ks and was ob-
1.1 Terzan 5
tainedon 2009 July15–16 from 17:21–04:14 ut(ID10059).
The globular cluster Terzan 5 has long been known to har- Both observations were carried out in the faint data mode,
bour at least one transient neutron star LMXB. In 1980, with thenominal frame timeof 3.2 s and the globular clus-
a number of X-ray bursts were observed with the Haku- terpositioned ontheS3chip.Figure2displaysanimageof
cho satellite (Makishima et al. 1981). Activity from Terzan the2009 Chandra observation.
5 was also detected in 1984 with EXOSAT (Warwick et al. Datareductionwascarriedoutusingtheciaosoftware
1988),andin1990/1991 withROSAT (Verbuntet al.1995; tools(v.4.2).Wereprocessedthelevel-1dataproductswith
Johnston et al. 1995). Furthermore, RXTE/ASM observa- the task acis process events. The 2003 observation con-
tionshaverevealedthreedistinctX-rayoutburstssince1996: tained episodes of background flaring, which were excluded
in 2000 (Markwardt & Swank 2000; Heinkeet al. 2003), from further analysis, resulting in a net exposure time of
2002 (Wijnands et al. 2002) and 2010 (see Figure 1). 34.2 ks. We extracted source count rates and lightcurves
Thelimitedangularresolutionoftheseinstrumentspre- using the tool dmextract, employing a 1′′ circular region
cludes pinpointing the X-ray source that causes the out- centredat thecoordinates reportedbyPooley et al. (2010).
bursts.However,Chandra observationsobtainedduringthe Backgroundeventswerecollectedfromacircularregionwith
2000 outburst of Terzan 5 allowed for an accurate localiza- aradiusof40′′,positionedonasource-freepartoftheCCD
tion of the transient source that was active at that time that was located ∼1.4′ West of the cluster core.
(Heinkeet al. 2003). Multiple X-ray bursts were observed The2010transientisdetectedatcountratesof(0.89±
and the 2000 transient was therefore associated with the 0.17)×10−3 and(1.11±0.18)×10−3 counts s−1 inthe2003
X-ray burster that was detected by earlier X-ray missions and2009data,respectively.Atotalof30–40netsourcepho-
(XB 1745–25/EXO 1745–248). The quiescent counterpart tonswere collected for both observations. Sourceand back-
of thistransient neutronstarLMXBhas anunusually hard ground spectra were obtained using psextract. We sub-
spectrum,with noclear evidencefor thepresenceofather- sequently generated redistribution matrices (rmf) and an-
mal emission component (Wijnands et al. 2005). A Chan- cillary response files (arf) with the tasks mkacisrmf and
dra study revealed 50 distinct X-ray point sources within mkarf, respectively. The spectra were grouped using grp-
the half-mass radius of the cluster, several of which were pha to contain a minimum of 5 photons per bin, and fit
proposed to bequiescent LMXBs (Heinkeet al. 2006). withinXspec(v.12.6)inthe0.5–8keVenergyrange.Given
Renewed activity from Terzan 5 was detected in 2010 the low number of photons, we also fitted the un-binned
OctoberduringIntegral bulgescanmonitoringobservations spectra using the W-statistic in Xspec. This yielded simi-
(Bordas et al. 2010). Subsequent pointed RXTE/PCA ob- lar results as obtained for the binned spectra with the χ2
servations detected coherent 11 Hz pulsations and type-I method applied, which indicates that the spectral fits are
X-raybursts, establishing thenatureof thetransient X-ray not biased by thesmall numberof counts perbin.
sourceasanaccretingneutronstar(Strohmayeret al.2010). Figure 3 displays the quiescent spectra of the new
Timing studies of the X-ray pulsations provided a determi- Terzan5transient.Inbothobservationsthespectrumissoft,
The Terzan 5 X-ray pulsar in quiescence 3
startransientsinquiescence.Forthetwo-component model
weobtainNH =(1.6±1.6)×1022 cm−2 andkT∞ =67.1±
10.5 eV (χν = 0.97 for 8 dof). The powerlaw is not signifi-
cant,butmaycontribute6+13%tothetotalunabsorbed0.5–
−6
10keVluminosity of2.5±2.5×1032 (D/5.5 kpc)2 erg s−1.
The inferred thermal bolometric luminosity is 4.6±3.7×
1032 (D/5.5 kpc)2 erg s−1. Since the additional powerlaw
component is not required to fit the data, we consider the
2000 transient calculated fractional contributionasanupperlimit.Choos-
ing different spectral indices (Γ = 1 − 2) yields spectral
2010 transient
parameters and fluxes that are consistent within the errors
with theresults for Γ=1.5.
The values obtained for the hydrogen column density
are comparable to the results of Heinkeet al. (2006), who
10'' performed spectral fits for the 16 brightest X-ray sources
in Terzan 5 (which does not include the new 2010 tran-
sient),andfoundanaveragevalueofNH ∼1.9×1022 cm−2.
However, Bozzo et al. (2010) report NH = (5.0 ± 0.8) ×
Figure2.Chandra/ACISimageofthecoreofTerzan5,obtained 1021 cm−2, based on spectral analysis of Swift/XRT data
on2009July15–16.Thequiescentcounterpartsofbothtransient obtained duringthe2010 outburst.Therefore, we havealso
neutron star LMXBs that have been identified in this globular performed fits to the quiescent data with thehydrogen col-
clusterareindicated. umndensityfixedatNH =5.0×1021 cm−2.Forthisvalue,
the thermal bolometric luminosity decreases by a factor of
withmostphotonsdetectedbelow∼2keV.Whenfittingthe ∼ 2.5 and the allowed fractional powerlaw contribution in-
2003 and 2009 data separately, we found no spectral differ- creases to ∼25+−2165%.
ences between the two observations. Therefore, we tied all
spectral parameters between the two data sets. We assume
a distance of D = 5.5 kpc (Ortolani et al. 2007) through-
3 DISCUSSION
out this work and all quoted errors refer to 90% confidence
intervals. To account for the interstellar hydrogen absorp- We report on the spectral analysis of the newly discov-
tion, we employ the phabs model with the default Xspec ered11HzX-raypulsarCXOGClbJ174804.8–244648 (IGR
abundancesand cross-sections. J174880–2446) in the globular cluster Terzan 5 during
Fittingthespectraldatawithasimpleabsorbedpower- quiescence. Using two archival Chandra observations, car-
lawyieldsNH =1.3+−31..23×1022 cm−2,Γ=4.8+−42..94 andχν = ried out in 2003 and 2009, we show that the quiescent
0.89for8degreesoffreedom(dof).Suchalargespectralin- spectrum is dominated by thermal emission that fits to
dexisusuallyinterpretedasanindicationthatthespectrum a neutron star atmosphere model with a temperature of
has a thermal shape. We therefore tried an absorbed neu- kT∞ ∼ 73 eV. The inferred thermal bolometric luminosity
tronstaratmospheremodelnsatmos(Heinkeet al.2006b), is Lq ∼6×1032 (D/5.5 kpc)2 erg cm−2 s−1. If a powerlaw
whichisoftenusedtofitthequiescentX-rayspectraofneu- is included in the fits, this spectral component contributes
tron stars. . 20% to the total unabsorbed 0.5–10 keV luminosity of
For the nsatmos model, we fix the neutron star mass ∼3×1032 (D/5.5kpc)2 erg cm−2 s−1.Thereisnoevidence
and radius to canonical values of MNS = 1.4 M⊙ and forspectralvariationsinthequiescentemissionbetweenthe
RNS = 10 km, and the distance is frozen at D = 5.5 kpc. 2003 and 2009 observations.
Furthermore,wekeepthenormalization fixedattherecom-
mendedvalueof 1,which corresponds totheentireneutron
star surface emitting. As such, the only free fit parameters
3.1 The powerlaw spectral component
arethehydrogencolumndensityandtheneutronstareffec-
tivetemperature.ThisyieldsNH =(2.1±0.9)×1022 cm−2 Itisinterestingtocomparethequiescentspectralproperties
and kT∞ = 72.7 ± 7.6 eV (χν = 0.92 for 9 dof). The of the2010 Terzan 5 transient with results obtained for the
corresponding 0.5–10 keV unabsorbed flux is (9.4±4.6)× accretingmillisecondX-raypulsars(AMXPs).Thequiescent
10−14 erg cm−2 s−1. For a distance of 5.5 kpc, the associ- spectraoffiveAMXPscouldbestudiedindetail:NGC6440
ated luminosity is (3.4±1.7) ×1032 erg s−1. Extrapolat- X-1,AqlX-1,SAXJ1808.4–3658, XTEJ0929–314 andIGR
ing the model fit to the 0.01–100 keV energy range yields 00291+5934. The former two are both intermittent X-ray
an estimate of the thermal bolometric flux of (1.7±0.7)× pulsars (i.e., the pulsations are detected only sporadically
10−13 erg cm−2 s−1. This implies a thermal bolometric lu- duringoutburst),thathavepredominantlysoftquiescentX-
minosity of Lq = (6.2±2.5)×1032 (D/5.5 kpc)2 erg s−1. ray spectra and a 0.5–10 keV luminosity of ∼ 1033 erg s−1
The nsatmos model fit is plotted in Figure 3. (Rutledgeet al. 2001; Cackett et al. 2005). SAX J1808.4–
Although the data can be adequately fit by a neutron 3658,XTEJ0929–314 andIGR00291+5934,havequiescent
star atmosphere model alone, we explored fits with an ad- 0.5–10 keV luminosities on the order of ∼ 1032 erg s−1. Of
ditional powerlaw to probe the contribution of a possible thethree,onlyIGR00291+5934showsevidenceforthermal
non-thermalemission component.Wefixthespectralindex emission, contributing ∼ 40% to the total 0.5–10 keV flux
to Γ = 1.5, which is a typical value observed for neutron (Heinkeet al.2009),whilethequiescentspectraoftheother
4 N. Degenaar & R. Wijnands
−30 tion of matter are expected to result in a quiescent bolo-
2×1 metricluminosity thatisgivenbyLq =hM˙iQnuc/mu (e.g.,
Brown et al. 1998; Colpi et al. 2001). Here, Qnuc ∼ 2 MeV
−1V0−3 isnuclearenergydepositedinthecrustperaccretedbaryon
−1unts s ke1−45×10 aa(evt.oegmr.a,igcHedamemanssaseslsu-&naictZcrdaenutdnioinkhMr2˙a0it0e=8o),fhMmt˙houebis=y×s1tt.e6omb6/.×tTre1hc0e−isr2a4tthigoeoitsfimtthheee-
o
d c outburst duration (tob) and the recurrence time (trec) rep-
malize−4×10 resenUtssitnhgedtuhteycaybcloev.e equation, the observed quies-
Nor2 cent thermal bolometric luminosity of Lq ∼ 6 ×
0−4 1032 (D/5.5 kpc)2 erg s−1 suggests a time-averaged mass-
1 accretionrateofhM˙i∼5×10−12 M⊙ yr−1.Combinedwith
−50 theestimatedmass-accretionrateduringthe2010outburst,
1
× this would imply that the system must have a duty cy-
5
2 5 cle on the order of ∼ 0.1%, provided that standard cool-
Energy (keV)
ing mechanisms are operating in the core. The 2010 out-
burstcommenced aroundOctober10andisongoingduring
Figure 3. Chandra/ACIS quiescent spectra of the 11 Hz X-ray RXTE/PCA pointed observations performed on November
pulsarinTerzan5from2003(black)and2009(grey)data,along 19.1 The outburst thus has a duration of > 6 weeks, so
withaneutronstaratmospheremodelfit(solidlines).
theestimated dutycycle would imply that the source must
spend & 100 yr in quiescence. This increases further if the
outburst is observed to continuefor a longer time.
twoarecompletelydominatedbyahardspectralcomponent
Althoughitisplausiblethatthe2010Terzan5transient
(Campana et al. 2002; Wijnands et al. 2005b).
hasaverylowdutycycle,analternativeexplanationisthat
Our limited understanding of the origin of hard qui-
enhancedneutrinoemissionmechanismsareoperatinginthe
escent X-ray emission makes it difficult to interpret the
core,whichcooltheneutronstardowntotheobservedqui-
difference between the 2010 Terzan 5 transient and other
escent luminosity. Within our current understanding, this
X-ray pulsars. However, if the powerlaw spectral compo-
would suggest that the neutron star in this X-ray binary is
nent is related to the magnetic field of the neutron star
relatively massive (e.g., Colpi et al. 2001), or that the core
(Campana et al. 1998), we would have expected to detect
composition includes different forms of matter (e.g., pions
a hard X-ray spectrum for the 11 Hz pulsar in Terzan 5,
or kaons; Yakovlev& Pethick 2004). It is not obvious that
since this source must have a substantial magnetic field
the neutron star in this transient LMXB would be partic-
(Papitto et al. 2010, Cavecchi et al. in prep.). A possible
ularly massive, since both its high inferred magnetic field
explanation for thesoft X-rayspectrum of the 2010 Terzan (B ∼ 109 −1010 G or possibly even higher; Papitto et al.
5 transient is that little matter is available in quiescence to
2010,Cavecchietal.inprep.)andrelativelyslowspinperiod
interact with the neutron star magnetic field, so that the
(Ps=11 Hz) are suggestive of a relatively youngsystem.
powerlawemissioncomponentthatmayarisefromthispro-
From modelling the quiescent spectral data we find a
cess is strongly reduced. The effects of the strong magnetic
neutronstareffectivetemperature,asobservedbyadistant
field on the neutron star spectrum might also play a role observer, of kT∞ ∼ 73 eV. In the neutron star frame this
(e.g., Zavlin & Pavlov 2002). translates into a value of kT ∼ 95 eV (T ∼ 1×106 K) for
MNS =1.4M⊙ andRNS =10km(correspondingtoagravi-
tationalredshiftof1+z=1.3).Usingthemodelcalculations
3.2 The quiescent thermal emission
of Brown & Cumming (2009) we can obtain a rough esti-
The observed thermal quiescent luminosity of the 11 Hz mate of the neutron star core temperature of ∼3×107 K.
pulsar in Terzan 5 can be used to estimate the duty cy- For standard (i.e., slow) core neutrino emission processes,
cle of the system, by equating this value to the luminos- suchacoretemperatureyieldsaneutrinoluminosityonthe
ity that is expected to be radiated due to heating of the order of Lν ∼ 1028 erg s−1 (cf. figure 2 of Schaab et al.
neutron star. Assuming that the observed accretion lumi- 1999). This is negligible compared to the observed photon
nosity provides a measure for the amount of matter that emissions of Lq ∼6×1032 (D/5.5 kpc)2 erg s−1. However,
is accreted onto the neutron star, we can deduce the mass- forasampleenhancedcoolingmodel(directUrca),theneu-
accretion rate during outburst from the average outburst trinoluminosityisontheorderofLν ∼1035 erg s−1 (cf.fig-
flux. Papitto et al. (2010) infer a 0.1–100 keV unabsorbed ure2ofSchaab et al.1999),and thusthedominantcooling
flux of ∼ 1×10−8 erg cm−2 s−1 from fitting RXTE/PCA mechanism.Forsuchneutrinolossesadutycycleofroughly
and HEXTE spectral data obtained during the 2010 out- ∼25%isrequiredtoexplaintheobservedquiescentthermal
burst of Terzan 5. This yields an estimated bolometric ac- luminosity. The time-averaged mass-accretion rate for this
cretion luminosity of Lacc ∼4×1037 (D/5.5 kpc)2 erg s−1. scenario is hM˙i∼8×10−10 M⊙ yr−1.
For an accretion luminosity given by Lacc =
(GMNS/RNS)hM˙obi,theaverageaccretion rateduringout-
burst is hM˙obi ∼ 2×1017 g s−1 ∼ 3×10−9 M⊙ yr−1, for 1 Terzan5isunobservablewithRXTE between2010November
canonical neutron star parameters of MNS = 1.4 M⊙ and 19and2011January17duetoSun-angleconstraints,norcanit
RNS =10 km. The nuclear reactions induced by the accre- beobservedwithotherX-raysatellitesduringthatepoch.
The Terzan 5 X-ray pulsar in quiescence 5
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