PSR J1840-1419: A very cool neutron star

0.36 MB·English

by  E. F. Keane

Draft version January 15, 2013
PreprinttypesetusingLATEXstyleemulateapjv.5/2/11
PSR J1840−1419: A VERY COOL NEUTRON STAR
E. F. Keane1, M. A. McLaughlin2, M. Kramer1,3, B. W. Stappers3, C. G. Bassa3, M. B. Purver3 &
P. Weltevrede3
1 MaxPlanckInstitutfu¨rRadioastronomie,AufdemHu¨gel69,53121Bonn,Germany.
2 DepartmentofPhysics,WestVirginiaUniversity,Morgantown,WV26506,USA.
3 UniversityofManchester,JodrellBankCentreforAstrophysics,SchoolofPhysics&Astronomy,ManchesterM139PL,UK.
Draft version January 15, 2013
ABSTRACT
WepresentupperlimitsontheX-rayemissionforthreeneutronstars. ForPSRJ1840−1419,witha
3
characteristic age of16.5 Myr, we calculatea blackbodytemperature upper limit (at99% confidence)
1
of kT∞ < 24+17 eV, making this one of the coolest neutron stars known. PSRs J1814−1744 and
0 bb −10
J1847−0130arebothhighmagneticfieldpulsars,withinferredsurfacedipolemagneticfieldstrengths
2
of 5.5×1013 and 9.4×1013 G, respectively. Our temperature upper limits for these stars are kT∞ <
bb
n 123+20 eVandkT∞ <115+16 eV,showingthatthesehighmagneticfieldpulsarsarenotsignificantly
a −33 bb −33
hotterthanthosewithlowermagneticfields. Finally,weputtheselimitsintocontextbysummarizing
J
all temperature measurements and limits for rotation-driven neutron stars.
3
Subject headings: pulsar: general — stars: magnetic fields — X-rays: stars
1

]
R 1. INTRODUCTION be expected to have somewhat similar X-ray proper-
ties. This, plus its relative proximity at 850 pc, make
S Measurements of the temperatures of neutron stars
PSR J1840−1419 an excellent target for adding to our
. (NSs) are few and far between. They are often impos-
h sible as, for ages (cid:38) 1 Myr, the star will have experi- knowledge of the X-ray emission and temperature of old
p NSs. In this paper we present the result of a Chandra
encedsignificantcoolingsinceitsformation(Yakovlev&
- observation of PSR J1840−1419.
o Pethick 2004). The thermal emission from NSs peaks
One factor which has been suggested to be a major
r in soft X-rays, below ∼ 1 keV, and the luminosities are
st such that sources even a few kiloparsecs away are diffi- factor in NS cooling is the magnetic field strength. Sev-
eral authors have suggested that NSs with higher mag-
a cult to detect. Currently, a total of 37 measurements,
[ and 9 upper-limit measurements, of rotation-driven NS neticfieldstrengthsarehotterthanthosewithlowerfield
strengths, for the same age (Gonzalez et al. 2007; Zhu
temperatures have been published (Li et al. 2005; Car-
1 et al. 2009; Olausen et al. 2010; Zhu et al. 2011). Such
aveo et al. 2010; Abdo 2010; Speagle et al. 2011; Chang
v an effect is theoretically predicted (Yakovlev & Pethick
et al. 2011; Zhu et al. 2011; Marelli et al. 2011; Guver
4
2004) and the work of Geppert et al. (2004) and P´erez-
1 et al. 2012; Pires et al. 2012). Of these, only 11 are for
Azor´ın et al. (2006) shows that high magnetic fields su-
8 NSs with characteristic ages older than 1 Myr (exclud-
press heat conductivity perpendicular to the field lines,
2 ing 5 millisecond pulsars for whom we have no meaning-
naturally producing an anisotropic temperature distri-
. fulestimateofage), andonlythreeofthosegreaterthan
1 bution on the stellar surface with small hot regions at
10Myr: PSRsB0950+08,J2144−3933andJ0108−1431.
0 the magnetic poles. We examine this claim with XMM-
TheemissionfromPSRB0950+08,a17.5Myrpulsar,is
3 Newton observations of two pulsars: J1814−1744 and
non-thermal, with a power-law spectrum, and an upper
1 limit of kT∞ <41 eV, or T <0.48 MK, on the thermal J1847−0130, both of which have high magnetic field
: bb strengths, of 5.5×1013 and 9.4×1013 G, respectively.
v contribution(Beckeretal.2004). Toputthisincontext,
i it is at the level of the lowest measured temperature for Finally, wepresent anupdated plotof kTb∞b vs charac-
X anyNS:kT∞ =43eVforGeminga(DeLucaetal.2005). teristicageforallX-raymeasurementsof,andupperlim-
r PSR J2144b−b3933 is the slowest spinning pulsar known its on, rotation-powered NS temperature made thus far,
a and re-examine the temperature-magnetic field strength
(P = 8.5 s) and an order of magnitude older (272 Myr)
relationship.
than PSR B0950+08 so that it is perhaps not surpris-
ing that no X-ray emission has been seen from it, with
an upper limit of kT∞ < 20 eV (Tiengo et al. 2011). 2. OBSERVATIONS&RESULTS
bb
Posseltetal.(2012)haverecentlyreportedthedetection
2.1. Calculating Limits
of PSR J0108−1431 (170 Myr) with kT∞ = 110 eV, a
bb For a non-detection, one could determine a count rate
source which seems to be undergoing a heating process.
limit by following the procedure of Pivovaroff et al.
Inarecentre-analysisoftheParkesMulti-beamPulsar
(2000): (i) define an aperture on the image about the
Survey, PSR J1840−1419 was discovered (Keane et al.
position of the source, and add up the counts in this
2010). This 6.6-s pulsar shows sporadic radio emission
region, N; (ii) define a background region of the same
with strong single pulses detected, on average, every
area (or appropriately scale to the same area) elsewhere
∼ 10 rotation periods, at flux densities up to 1.7 Jy.
on the image and add up the counts in the background
At 16.5 Myr, PSR J1840−1419 has a very similar char-
region, B. The signal from the source is N − B and
acteristic age to PSR B0950+08, and therefore might √
the noise is N +B. The signal-to-noise ratio is then
2 E. F. Keane et al.
√
(N −B)/ N +B. To set a ‘3−σ limit’ count rate we density of N = 6×1020 cm−2 which is derived from
H
can then solve for N in 9(N +B) = (N −B)2, where the dispersion measure, assuming 10 neutral hydrogen
B is known. We can see that in the N (cid:29) B ‘photon- atoms per free electron (Seon & Edelstein 1998). Pre-
rich’ case this tends to the expected Poisson value in dicted count levels were calculated using the standard
the absence of background sky noise. However, in the PIMMS (Portable, Interactive Multi-Mission Simulator)
‘photon-poor’ case we clearly can not use this expres- tool1.
sion, e.g. for B =0.35, N =10 na¨ıvely suggests a 99.7%
2.3. PSRs J1814−1744 & J1847−0130
detection/limit,howeverweknowthatthePoissonprob-
abilityfor10countsduetonoiseis≈10−13%; forsucha We also present the results of two XMM-Newton
background rate 3 counts represents a 99.8% limit. The observations using the PN detector and medium filter in
observations presented here are in the photon-poor sce- the energy range 0.15−15.0 keV. PSR J1814−1744 was
nario,sothatusingthePivovaroffetal.(2000)procedure observed for 6.1 ks on 2004-10-21, and PSR J1847−0130
would result in a much poorer limit than what has truly was observed for 17.0 ks on 2004-09-14. The observa-
been obtained. tions of these young, high-B pulsars also resulted in
To convert count rate limits to flux limits we need ei- non-detections. AsaboveforPSRJ1840−1419wederive
ther some kind of absolute scale, e.g. from observing temperature upper limits for these sources of: kT∞ <
bb
a standard source of known flux, or a model for our 123+20eV(R∞/10 km)−0.23, for PSR J1814−0130,
−34 bb
source,e.g. anassumedspectralform(McLaughlinetal. and kT∞ < 115+16eV(R∞/10 km)−0.21, for
2003). Below we calculate one set of limits for a black- bb −33 bb
PSR J1847−0130 (see Figure 1). These lim-
body model (whence we obtain temperature limits as a
its are much less constraining given the much
functionoftheemittingradius),andanothersetoflimits
larger estimated distances (7.7 and 9.8 kpc re-
for a non-thermal model with a power law with negative
spectively). The associated flux and luminos-
index of 2.0.
ity limits for the blackbody scenario are: f <
bb
2.6 × 10−13(R∞/10 km)1.77 erg s−1 cm−2 and L∞ <
2.2. PSR J1840−1419 bb bb
3.0×1033(R∞/10km)1.77 ergs−1 for PSR J1814−1744;
PSR J1840−1419 is an old pulsar (16.5 Myr) with a and f < 3b.b2 × 10−13(R∞/10 km)1.79 erg s−1 cm−2
6.6-s spin period. Due to its sporadic radio emission it bb bb
and L∞ < 2.3 × 1033(R∞/10 km)1.79 erg s−1 for
wasdetectedinasearchforsinglepulses,ratherthanina bb bb
PSR J1847−0130.
periodicitysearch(Keaneetal.2010). Pulsarsdiscovered
The non-thermal limits are: f < 8.9 ×
in this way are often referred to as “RRATs” (Keane & nt
10−15 erg s−1 cm−2 and L∞ < 1.0×1032 erg s−1 for
McLaughlin 2011). The properties of PSR J1840−1419, nt
PSR J1814−1744; and: f < 5.1×10−15 erg s−1 cm−2
both measured and derived, are given in Table 1. nt
and L∞ < 3.6 × 1031 erg s−1 for PSR J1847−0130.
On 2011-02-20, we performed a 10-ks observation of nt
For PSR J1814−1744 the value of N derived from the
PSRJ1840−1419,usingtheACIS-SdetectoronChandra H
dispersion measure was ∼ 50% higher than the maxi-
intheenergyrange0.2−10.0keV.Wedetectedonlyone
mum Galactic value for this line of sight, as calculated
photon within the ∼1 arcsec (3 pixel) error circle of the
by the standard COLDEN tool2, so the COLDEN value
position derived from pulsar timing (Keane et al. 2011).
of N = 1.6×1022 cm−2 was used. For this reason N
Assumingthatthis1countisconsistentwithbackground H H
is unlikely to be underestimated, but we have allowed
noise(thebackgroundratebeing0.24counts/pixel),and
for the possibility that it may be overestimated by as
ignoring the Poisson nature of the source, the count rate
limit is 3×10−4 counts/s (at 99% confidence). much as a factor of two. Likewise, for PSR J1847−0130,
the neutral hydrogen column density was derived to be
In the case of a blackbody model this count
rate limit implies a temperature limit of kT∞ < NH =2.1×1022 cm−2. This value of NH is very close to
bb the maximum value for this line of sight, so that it too
24+17eV(R∞/10 km)−0.39, where the dependence on
−10 bb is unlikely to be an overestimate.
R∞ is a purely empirical fit to a simple power law of
bb
the form T Rα , where R is the blackbody radius 3. DISCUSSION&CONCLUSIONS
10 10 10
in units of 10 km and T10 is the temperature when Althoughthispaperreportsthreenullresultswedeem
R10 = 1: see top left panel of Figure 1. The error it to be of the utmost importance to report such inves-
bars reflect both NE2001 ‘worst case’ errors of a fac- tigations to avoid duplicated efforts, wasted telescope
tor of two in the distance (Cordes & Lazio 2002) as resources, etc. Table 1 summarises the temperature,
well as factor of two uncertainties in the neutral hy- flux and luminosity limits for the sources reported here.
drgoen column density (see below). The bottom right From the non-thermal luminosity limits we can deter-
panel of Figure 1 shows all measured neutron star tem- mine upper-limits on the non-thermal X-ray efficiency,
peratures as a function of age. The PSR J1840−1419 η = L /E˙, where E˙ is the spin-down energy loss rate.
nt
limit is amongst the coolest of all published limits.
AlthoughtheluminositylimitsfortheXMM-Newton ob-
The corresponding flux and luminosity limits are f <
bb servations are ∼ 2 orders of magnitude poorer than for
5.0 × 10−14(Rb∞b/10 km)1.61 erg s−1 cm−2 and L∞bb < the Chandra observation of J1840−1419, the E˙ values
4.3×1030(R∞/10 km)1.61 erg s−1, respectively. In the
bb arecorrespondinglyhigher,suchthatforall3pulsarsthe
caseofthenon-thermalmodel(powerlaw,withnegative limitturnsouttobeη (cid:46)0.2. Possentietal.(2002)deter-
index of 2.0), the flux and luminosity limits are f <
nt mined,fortheirstudyof39pulsars,thatallsourceshada
3.0×10−15 erg s−1 cm−2 and L∞ < 2.6×1029 erg s−1,
nt
respectively. 1 http://asc.harvard.edu/toolkit/pimms.jsp
The above estimates use a neutral hydrogen column 2 http://asc.harvard.edu/toolkit/colden.jsp
A very cool neutron star 3
value of η less than η = 10−18.5(E˙/erg s−1)0.48. Our for Scientific Advancement.
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4 E. F. Keane et al.
Table 1
Measuredandderivedproperties,andlimitsfor3pulsars.
Quantity J1840−1419 J1814−1744 J1847−0130
P (s) 6.6 4.0 6.7
P˙ (fs/s) 6.3 745 1270
τc (kyr) 16500 85 83
B (1012 G) 6.5 55.1 93.6
E˙ (1030ergs−1) 1.0 468 167
DM (cm−3pc) 19 792 667
D (kpc) 0.9 9.8 7.7
Telescope Chandra XMM XMM
Instrument ACIS-S PN PN
Date 2011-02-20 2004-10-21 2004-09-14
EnergyRange(keV) 0.2−10 0.15−15 0.15−15
T (ks) 10.0 6.1 17.0
obs
NH (1020cm−2) 6 157 205
kT∞ (eV)† <24+17 <123+20 <115+16
bb −10 −34 −33
T∞ (MK)† <0.28+0.19 <1.42+0.22 <1.33+0.19
bb −0.12 −0.39 −0.38
f (10−14ergcm−2s−1)‡ <5.0 <6.1 <8.9
bb
L (1030ergs−1)‡ <4.3 <692 <640
bb
fnt (1015ergcm−2s−1) <3.0 <8.9 <5.1
Lnt (1030ergs−1) <0.26 <102 <36
η=Lnt/E˙ <0.26 <0.22 <0.22
Note. — The measured and derived properties for the three pulsars discussed in this paper. The spin period, P, and its derivative,
P˙, are derived from pulsar timing techniques, and τ, B and E˙, the characteristic age, magnetic field strength and spin-down energy
respectively,areinferredfromthese(Keaneetal.2011). Thedistance,D,isderivedfromthemeasuredDM usingtheNE2001modelfor
theGalacticfreeelectrondistribution(Cordes&Lazio2002). The99%-confidencetemperaturelimitsarederivedasoutlinedinthetext.
Non-thermalfluxlimitsaregivenforapowerlawwithnegativeindexof2.0. †theselimitsareforanemittingradiusof10km,andscale
as (R∞/10km)α, where α equals −0.39, −0.23 and −0.21, respectively for the three sources (see main text). ‡ The blackbody flux and
bb
luminositylimitssimilarlyscaleas(R∞/10km)2−α.
bb
A very cool neutron star 5
80 260
PSR J1840-1419 10..05 DDDDMM 240 PSR J1814-1744 10..05 DDDDMM
70 2.0 DDM 2.0 DDM
V) producing 3e-4 cps 456000 V) producing 5e-4 cps 112268020000
∞T (ebb 30 ∞T (ebb 140
k k 120
20
100
10 80
0 2 4 6 ∞ 8 10 12 14 16 0 2 4 6 ∞ 8 10 12 14 16
Rbb (km) Rbb (km)
240 500
PSR J1847-0130 0.5 DDM
220 12..00 DDDDMM 450
400
V) producing 3e-4 cps 111246800000 kT (eV) 223305050000
∞kT (ebb 120 150
100
100
50
80 0 2 4 6 Rb∞b 8(km) 10 12 14 16 0 103 104 Ch1a0r5acteristic 1A0g6e (yr) 107 108
Figure 1. The first three plots show the temperature limit (i.e. which produces the observed count rate limit for an assumed
blackbody model) as a function of the assumed emitting radius, for the Chandra observation of PSR J1840−1419 (10 ks) and
theXMM-Newton observationsofPSRsJ1814−1744(6ks)andJ1847−0130(17ks). Thepointsarefitwithasimplepower-law
of the form T (R )α. In each case the middle curve is for the nominal dispersion-measure-derived distance. The cooler and
10 10
hotter curves are for ‘worst-case’ distance errors of a factor of 2 (Cordes & Lazio 2002). For clarity of presentation we show
here only the curves for the nominal N values quoted in Table 1. The fourth plot shows all known temperature values as a
H
function of characteristic age. Sources with inferred magnetic field strengths ≥1013 G are marked in blue, whereas those with
lower values are marked in black. Open circles denote upper limits. The three upper limit presented in this paper are marked
withlargecirclesforclarity. NotethatthesymbolsforPSRsJ1814−1744andJ1847−0130arelargelyoverlappingastheirages
and derived limits are very similar.