Multi-wavelength observations of the black widow pulsar 2FGL J2339.6-0532 with OISTER and Suzaku

1.6 MB·English

by  Yoichi Yatsu

Draftversion January28,2015
PreprinttypesetusingLATEXstyleemulateapjv.01/23/15
MULTI-WAVELENGTH OBSERVATIONS OF THE BLACK WIDOW PULSAR 2FGL J2339.6 0532WITH
−
OISTER AND SUZAKU
Yoichi YATSU1, Jun KATAOKA2, Yosuke TAKAHASHI2, Yutaro TACHIBANA1, Nobuyuki KAWAI1, Shimpei
SHIBATA3, Sean PIKE4, Taketoshi YOSHII1, Makoto ARIMOTO1, Yoshihiko SAITO1, Takeshi NAKAMORI3,
Kazuhiro SEKIGUCHI5, Daisuke KURODA6, Kenshi YANAGISAWA6, Hidekazu HANAYAMA7 Makoto
WATANABE8, Ko HAMAMOTO8, Hikaru NAKAO8, Akihito OZAKI8, Kentaro MOTOHARA9, Masahiro
KONISHI9, Ken TATEUCHI9, Noriyuki MATSUNAGA10, Tomoki MOROKUMA9, Takahiro NAGAYAMA11,12
5 Katsuhiro MURATA11 Hiroshi AKITAYA13, Michitoshi YOSHIDA13, Gamal B. ALI14, A. Essam MOHAMED14,
1 Mizuki ISOGAI5,15 Akira ARAI15 Hidenori TAKAHASHI9,16, Osamu HASHIMOTO16, Ryo MIYANOSHITA12
0 Toshihiro OMODAKA12, Jun TAKAHASHI17, Noritaka TOKIMASA17, Kentaro MATSUDA17, Shin-ichiro
2 OKUMURA18 Kota NISHIYAMA18 Seitaro URAKAWA18 Daisaku NOGAMI19, Yumiko OASA20, on behalf of
OISTER team21
n
a (AcceptedApJ)
Draft version January 28, 2015
J

7 ABSTRACT
2
Multi-wavelength observations of the black-widow binary system 2FGL J2339.6 0532 are reported.
The Fermigamma-raysource 2FGL J2339.6-0532was recently categorizedas a b−lack widow in which
]
E a recycled millisecond pulsar (MSP) is evaporating up the companion star with its powerful pulsar
H wind. Our optical observations show clear sinusoidallight curves due to the asymmetric temperature
distribution of the companion star. Assuming a simple geometry, we constrained the range of the
h. inclination angle of the binary system to 52◦ < i < 59◦, which enables us to discuss the interaction
p betweenthepulsarwindandthecompanionindetail. TheX-rayspectrumconsistsoftwocomponents:
- a soft, steady component that seems to originate from the surface of the MSP, and a hard variable
o
componentfromthewind-terminationshocknearthecompanionstar. ThemeasuredX-rayluminosity
r
t is comparable to the bolometric luminosity of the companion, meaning that the heating efficiency is
s less than 0.5. In the companion orbit, 1011 cm from the pulsar, the pulsar wind is already in particle
a
[ dominant-stage, with a magnetization parameter of σ < 0.1. In addition, we precisely investigated
the time variations of the X-ray periodograms and detected a weakening of orbital modulation. The

1 observed phenomenon may be related to an unstable pulsar-wind activity or a weak mass accretion,
v both of which can result in the temporal extinction of radio-pulse.
9
Keywords: gamma rays: stars — pulsars: general — pulsars: individual (2FGL J2339.6 0532) —
1 −
X-rays: binaries
8
6
0
.
1 [email protected]
0 1Department of Physics, Tokyo Institute of Technology,
5 2-12-1OhokayamaMeguroTokyo152-8551,Japan
1 2Research Institute for Science and Engineering, Waseda
University,3-4-1OokuboShinjukuTokyo169-8555, Japan
v: 3Department of Physics, Yamagata University, 1-4-12 Koji-
rakawamachi, Yamagata,Yamagata 990-8560,Japan
i
X 4Department ofPhysics,BrownUniversity,182HopeStreet,
Providence, RhodeIsland02912, USA
r 5National Astronomical Observatory of Japan, National
a
Institute of Natural Sciences, 2-21-1 Osawa, Mitaka, Tokyo
181.8588, Japan 739-8526, Japan
6OkayamaAstrophysicalObservatory,NationalAstronomical 14NationalResearchInstitute ofAstronomyandGeophysics,
Observatory of Japan, 3037-5 Honjo, Kamogata, Asakuchi, Cairo11722, Egypt
Okayama719-0232 15Kamiyama Astronomical Observatory, Kyoto Sangyo Uni-
7IshigakijimaAstronomicalObservatory,NationalAstronom- versity, Kamigamomotoyama, Kita-ku, Kyoto, Kyoto 603-8555,
icalObservatoryofJapan,Ishigaki,Okinawa907-0024,Japan Japan
8Department of Cosmosciences, Hokkaido University, Kita 16Gunma Astronomical Observatory, 6860-86 Nakayama,
10,Nishi8,Kita-ku,Sapporo,Hokkaido060-0810, Japan Takayama-mura,Agatsuma-gunGunma377-0702, Japan
9Institute of Astronomy, Graduate School of Science, The 17Nishi-Harima Astronomical Observatory, Center for As-
University of Tokyo, 2-21-1 Osawa, Mitaka, Tokyo 181-0015, tronomy, University of Hyogo, 407-2, Nishigaichi, Sayo-cho,
Japan Sayo,Hyogo679-5313, Japan
10Department of Astronomy, Graduate School of Science, 18Bisei Spaceguard Center, Japan Spaceguard Association,
The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 1716-3Okura,Bisei-cho,Ibara,Okayama714-1411, Japan
113-0033, Japan 19Department of Astronomy, Kyoto University, Ki-
11Department of Astrophysics, Nagoya University, Furo-cho, tashirakawa Oiwake-cho, Sakyo-ku, Kyoto, Kyoto 606-8502,
Chikusa-ku,Nagoya464-8602,Japan Japan
12Faculty of Science, Kagoshima University, 1-21-30 Kori- 20Faculty of Education, Saitama University, 255 Shimo-
moto,Kagoshima,Kagoshima890-0065,Japan Okubo,Sakura,Saitama,Saitama388-8570, Japan
13Hiroshima Astrophysical Science Center, Hiroshima 21Optical and Infrared Synergetic Telescopes for Education
University, 1-3-1 Kagamiyama, Higashihiroshima, Hiroshima andResearch
2 Yatsu et al.
1. INTRODUCTION ity is likely related to the orbital motion. The intensity
of the optical counterpart varies from 20 to 17 mag in
Amilli-secondpulsar(MSP)isbelievedtoevolvefrom
R-band(Romani & Shaw2011;Kong et al.2012). More-
a dead pulsar in a binary system, via mass accretion
over the phase-resolved spectroscopy indicated that the
from a companion star that has spun up the pulsar for
billions of years (Alpar et al. 1982). Indeed most MSPs companionmaybeK-classstarwithamassof0.075M⊙.
This means that the pulsar side hemisphere of the com-
arediscoveredinbinarysystems,howeverisolatedMSPs
panion star is drastically heated and the energy might
also exist. The missing link between the isolated MSPs
be supplied from an unknown recycled MSP via pulsar
andbinaryMSPsisblackwidowpulsarsinwhichasuffi-
wind, as seen in PSR B1957+20.
ciently spun up pulsar is evaporating its companionstar
Inthispaper,wereportmulti-wavelengthobservations
with its powerful pulsar wind. The prototype of the
ofthenewlydiscoveredblackwidowbinarysystem2FGL
blackwidowisPSRB1957+20witha1.61msradiopulse
J2339.6-0532, covering near-infrared, optical and X-ray
(Fruchter et al. 1988). So far less than 10 black widow
energy band. In section 2 and 3, the optical and X-
classobjectshavebeendiscoveredhoweverthe evolution
ray observations and the obtained results are described,
process from an accreting MSP to a rotation-powered
respectively. In section 4, we discuss the orbital param-
MSP is still unclear(Roberts 2011, 2013).
eters based onthe obtained phase resolvedSED and the
Moreoverablackwidowmaybeagoodprobefordeep
properties of the pulsar wind just around the pulsar as
investigation of pulsar wind. Currently young rotation
wellasaninterpretationoftheX-raylightcurveshowing
powered pulsars are believed to have pulsar wind neb-
intriguing irregular variability.
ulae (PWNe) made up of relativistic electron-positron
plasma, as seen in the Crab nebula (Weisskopf et al.
2. OPTICALOBSERVATIONS
2000; Hester et al. 2002), however the physical mecha-
nism that generates the pulsar wind is not yet under- 2.1. Observation
stood. Although we do not know whether the descrip- 2FGL J2339.6 0532was observed from September 22
tionsoftheCrabpulsarcanbeappliedtoarecycledMSP toOctober7201−1utilizing the globaltelescopenetwork,
withmagneticfieldfourordersofmagnitudeweakerthan “Optical and Infrared Synergetic Telescopes for Educa-
thatof typicalradiopulsars,if this is the case,the black tionandResearch(OISTER)”22.OISTERconsistsof14
widows canprovide a crucialchance for us to probe pul- independent observatories that are funded by Japanese
sar wind via the direct interaction with the companion universities and research associations. For this work, we
nearby the light cylinder. Theoretically pulsar wind is alsoaskedforphotometryobservationfromtheKottamia
dominated by Poynting flux at the light cylinder, there- Astronomical observatory which has a 188 cm reflector.
fore the magnetic energy in the wind must be converted Thanks to the locations of the observatories distributed
into kinetic energy just after the wind flies out from across the globe, we can continuously trace the variabil-
thelightcylinderasreportedbyAharonian et al.(2012), ity all day long. Moreover the telescopes cover a wide
andwe canapproachthe originof the pulsarwind much range of wavelengths from Ks to B band which can pro-
deeper with black widows for investigating the unknown vides phase resolved spectral energy distribution (SED)
physicalmechanismwhichcanexplaintheσ paradox. In of the target. The conducted photometric observations
order to clarify the history of MSP formation and also aresummarizedinTable4. Thelocationsofthereference
to constrain the physical mechanism of particle acceler- stars and the target are shown in Figure 1.
ation just around the pulsar, a black widow pulsar is an
intriguing target. 2.2. Hα imaging
The Large Area Telescope (LAT) on the Fermi satel-
In the case of PSR B1957+20, the binary system is
lite, with unprecedented sensitivity and angular resolu-
surrounded by an Hα bow-shock due to its super sonic
tion in the energy rangeof 100 MeV to greaterthan 300
proper motion with respect to the surrounding ISM
GeV, has discoveredmore than2000gamma-raysources
(Kulkarni & Hester1988). Tosearchforasimilarfeature
sinceitslaunch,and30%ofthesesourcesarestilluniden-
around2FGLJ2339.6-0532,wetookHαimageswiththe
tified. A bright gamma-ray source discovered at high
KisoSchmidttelescopeattheminimumphasetoprevent
galactic latitude vicinity, 2FGL J2339.6 0532 (1FGL J
− contaminationfromthe companionstar. Figure 1 shows
2339.7 0531), was also listed in the first Fermi source
− the obtained Hα image in the vicinity of 2FGL J2339.6-
catalog as an un-identified source (Abdo et al. 2010;
0532 utilizing an Hα narrow band filter (∆λ = 9 nm),
Ackermann et al. 2012). The gamma-ray flux amounts
in which we could not find the evidence of bow-shock
to 3.0 0.2 10−11 erg s−1 cm−2 with a variable in-
± × nebula.
dexof15.7,indicatingthatthegamma-rayfluxseemsto
Toevaluatethedetectionlimit,wecalculatedthestan-
be steady at the month time scale. While the gamma- dard deviation of the 33′′ 67′′ (50 100pixel) rectangu-
ray spectrum has a cutoff structure at 3 GeV. These × ×
lar region near the target and obtained STDEV =7.82
characteristics lead us to believe the object is a pulsar.
ADU. The sky background of the CCD image shows
A follow-up X-ray observation conducted with Chandra
a weak striped pattern running in the southeast-to-
discovered an X-ray point source within the error circle
northwest direction, with a peak-to-peak value of 8
expected from the gamma-ray image(Kong et al. 2012). ∼
ADU, which seems to be the dominant noise compo-
However a radio pulse was not detected at the position
nent restricting the detection limit. Regardless, if we
at that moment. On the other hand, ground based op-
adopt a conservative threshold with a 3σ confidence
tical observations discovered clear sinusoidal variability
level, the detection limit becomes 23.5 ADU. Based on
with a period of 4.63 hours, which implies the object
is in a binary system and the observed optical variabil- 22 http://oister.oao.nao.ac.jp/
Multi-wavelength observations of the black widow pulsar 2FGL J2339.6 0532 3
−
320
-5:30:00.0
300
31:00.0
Ref2
32:00.0
0) Ref1 280
0
0
2
c (J 33:00.0
e
D
260
34:00.0
35:00.0
240
36:00.0
220
55.0 50.0 45.0 23:39:40.0 35.0 30.0 25.0
R.A. (J2000)
Figure 1. Hα image of the vicinity of 2FGL J2339.6−0532 observed with the Kiso-105cm Schmidt telescope. The circle region with a
radiusof10′′indicatesthelocationofthetarget. Tworectangular regionsindicatethephotometricreferencestarsshowninTable1.
the flux calibrations of the reference stars listed in Ta- references stars were measured and compared to the
ble 1, the conversion coefficient of ADU to energy flux, standard stars in the Landolt catalog (Landolt 1992)
(8.3 0.1) 10−18 erg s−1 cm−2 ADU−1, was observed with MSI of the Pirka telescope in Hokkaido prefecture
± ×
for the Hα filter with a bandwidth of 9 nm. Finally, we (Japan). Since the Pirka employs the Johnson-Cousins
obtained a 3σ detection limit of < 1.9 10−16 erg s−1 filter system, we estimated the photon flux in the SDSS
cm−2 pixel−1, which correspondsto a su×rface brightness g’-bandbasedontheobservedBandVmagnitudesusing
of <8.7 10−17 erg s−1 cm−2 arcsec−2 23. a conversionequation proposed by Smith et al. (2002),
×
The prototype black widow PSR B1957+20, residing g′ =V +0.54(B V) 0.07. (1)
at a distance of 1.6 kpc from the earth, has a bow-shock − −
nebula extending over 30′′ 30′′with an Hα flux of For J, H, and Ks, we employed the photometric data in
3.3 10−14 erg s−1 int∼egrate×d over the entire nebula theTwoMillionAllSkySurvey(2MASS)catalogforflux
×
(Kulkarni & Hester 1988). If our target is accompanied calibration(Skrutskie et al. 2006). The obtained magni-
by an equivalent bow-shock nebula, the Hα flux and the tude of the reference stars are summarized in Table 1.
sizeshouldbe6.1 10−14ergs−1 cm−2 and 41′′ 41′′, Figure2showsthelightcurvesof2FGLJ2339.6 0532
× ∼ × −
respectively. Therefore, the expected surface brightness obtained by OISTER. We re-confirmed the clear sinu-
is 3.6 10−17 erg s−1 cm−2 arcsec−2, which is slightly soidalmodulationasreportedby Kong et al.(2012)and
×
lower than the detection limit. Romani & Shaw(2011). The absoluteflux seemsconsis-
Compared with the other bow-shock PWNe, the size tent with the past observations: the modulation ampli-
andintensitybothseemtobescatteredacrossarangeof tude is about 4.5 magnitude in the R-band and is larger
two orders of magnitude, possibly reflecting their shock at shorter wavelength. In R-band, we successfully ob-
conditions (Chatterjee & Cordes 2002). Therefore, we served the maximum phases of the target 8 times dur-
conclude that this observation provides only a weak up- ing the observation campaign. The photon flux at the
per limit of surface brightness in the Hα band. maximum phase is R = 17.7 mag on average and
max
varies among peaks within a range of 0.1 mag. There
±
2.3. Photometry wasnoirregularactivityliketheflaresobservedin2FGL
J1311.6 3429(Kataoka et al. 2012).
The other telescopes except for the Kiso observa- −
torycarriedoutphotometricobservationscoveringKs B 2.4. Phase-resolved SED
∼
bands from September 9th 2011 to September 30th
To discuss the energetics quantitatively, we estimated
2011. For flux calibration we chose two reference stars
the energy flux based on the photometric data shown
near the target object, U0825 19993817 (R=17.5) and
inFigure 2. Firstwe correctedthe galacticextinctionat
U0825 19993871 (R=16.4) in USNO-2.0A catalog, so
thecoordinateofthetarget(Schlafly & Finkbeiner2011;
thatallofthe observatoriescanemploythe commonref-
Cardelli et al. 1989)24; the employed extinction at vari-
erences without saturation. The photon fluxes of the
24 In the above extinction evaluation we utilized
23 Kiso2kCCD’spixelresolutionis1.5′′(Itoh etal.2001). http://ned.ipac.caltech.edu/forms/calculator.html, and
4 Yatsu et al.
Table 1
SummaryofFieldphotometry
Reference-1 Reference-2 Aλ†
Name(USNO-2.0A) U0825 19993817 U0825 19993871
Coordinate(J2000.0) (23:39:39.487, −05:32:40.56) (23:39:40.366, −05:31:51.89)
B 18.435±0.014 17.603±0.008 0.120
V 17.834±0.018 16.828±0.008 0.091
R 17.484±0.016 16.388±0.007 0.072
I 17.296±0.020 16.105±0.008 0.050
g’ 18.098±0.011a 17.177±0.006a 0.110
J 16.47±0.10b 15.19±0.04b 0.024
H 16.45±0.23b 14.87±0.05b 0.015
Ks 16.12b,c 14.55±0.08b 0.011
Hα. (656nm) 314±46 µJyd 854±170 µJyd ···
Note. —Errorsarewith1σconfidencelevel.
† Galacticextinctionatthetargetcoordinate(Schlafly&Finkbeiner2011;Cardellietal.1989)
a SDSSmagnitudeswerecalculatedfromB-band andV-band magnitudesbasedon Smithetal.
(2002).
bForthefluxcalibrationsofJ,H,andKs,wereferredthe2MASScatalog(Skrutskieetal.2006).
c ThecatalogerroroftheReference-1forKs-bandwasnotavailable.
dThefluxdensitywasevaluatedfromtheabsorptioncorrectedSEDassumingasimplebalckbody
model. TheestimatedtemperatureofthereferencestarswereTBB,ref1=7320±160K,TBB,ref2=
6300±180K.
ous wavelengthare listed in Table 1. Then we converted largas. Panel(b) showsthe spectralenergydistribution
theabsorptioncorrectedmagnitudestoenergyfluxusing during the brightening phase from orbital phase= 0 to
conversionequations presented in Fukugita et al. (1996) 0.5 in panel (a). Clearly the peak frequency increases as
andTokunaga & Vacca(2005)foroptical(B,V,R,I,g’) the orbital phase increases from orbital phase 0 to 0.5,
and IR(J, H, Ks) energy bands, respectively. implying that the temperature of the companion is in-
Figure 3—(a) shows the yielded energy fluxes as func- creasing with the orbital phase. At the maximum phase
tions of the binary orbital phase. Phase=0 was set to the SED is fitted with a black-body model with an ef-
be MJD=55500.0. While the optical light-curves show fective temperature of T = 7540 130 K, which is
eff
±
symmetric structures, IR (H and Ks) light-curves pos- consistent with the past study (Romani & Shaw 2011).
sess somewhat asymmetric shapes. This may be due to In this paper, we assumed a simple emission model
the geometry of the companion star or evaporating stel- from the surface of the companion star to explain the
18 observed optical emission. As described in Figure 4, we
B 20 supposed that a companion star with a radius rcomp is
orbiting around a pulsar at an orbital radius R , and
22 orb
18 the hemisphere of the companion that faces the pulsar
V 20 is heated by the pulsar wind, while the opposite side of
the companionhas ordinaltemperature T . Note that
22 cool
18 thismodeldoesnottakeintoaccounttheeffectofenergy
g’ 20 transferviaconvectionoradvectiononthesurfaceofthe
22 companion,thereforethetemperaturedistributionofthe
e 18 companionissimplydescribedbytheenergyinjectionvia
ud R 20 pulsarwindalthoughtheheatingefficiencyisunclear. In
nit 16 thispaper,weassumedthatthespin-downenergyisper-
g
a fectly converted into the isotropic pulsar wind and the
M I 18
injected energy via the pulsar wind heats the compan-
20
ion up with a heating efficiency of f for simplicity. We
16 also adopted a spin-down luminosity of 2.3 1034 erg
J 18 s−1 based on the timing analysis in radio ban×d recently
20 reported by Ray et al. (2014).
16
H We then fitted the obtained phase-resolved SED with
18
themodelfunctiondescribedabove(Figure5). Sincethe
20 modelseemsto correlateweaklywith the inclinationan-
16
Ks 18 gle,wefixedtheinclinationanglei. Theresultantmodel
parameters are summarized in Figure 6 as functions of
55831 55832 55833 55834 55835 inclination angle. Although the fit seems rather poor —
MJD mainlybecauseofthelargeresidualintheIRband—we
foundthataninclinationanglei=59◦minimizestheχ2.
Figure 2. Multi-colorlightcurveof2FGLJ2339.6−0532obtained
Forcomparison,wealsoattemptedthis analysiswithout
viaOISTER.
IR data and obtained a best-fit parameter of i = 52◦.
These results seem consistent with the inclination angle
http://dogwood.physics.mcmaster.ca/Acurve.html reportedby Romani & Shaw(2011). Theobtainedbest-
Multi-wavelength observations of the black widow pulsar 2FGL J2339.6 0532 5
−
(a) Phase folded light-curves (b) Spectral energy distributions at increasing phase
300
R-band phase = 0.4~0.5
V-band phase = 0.3~0.4
250 B-band phase = 0.2~0.3
H-band phase = 0.1~0.2
y] Ks-band phase = 0.0~0.1
J
µ 200
[
x
u
Fl 150
y
g
r
e
n 100
E
50
0
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 1 2 3 4 5 6 7 8 9 10
Orbital Phase Frequency [1014 Hz]
Figure 3. (a)Phasefoldedmulti-colorlightcurves. Theassumedorbitalperiodis4.63435hrandPhase=0issettobeMJD=55500day.
Marker colors represent the energy bands: red= R, green= V, blue=B, magenta=H, and black=Ks. (b)Spectral energy distributions at
increasingphasefromtheminimumtothemaximum. Colorscorrespondtotheorbitalphase: black=0-0.1,magenta=0.1-0.2,red=0.2-0.3,
green=0.3-0.4,andblue=0.4-0.5.
T r
cool comp
i
10-4
y]
J
Companion R ux [
orb Fl 10-5
Observer gy
Pulsar with LDS ner
E 10-6
Figure 4. Schematic image of the assumed emission model in
which a companion star with a radius rcomp is orbiting around a
pulsaratanorbitalradiusRorb. Thehemisphereofthecompanion
starfacingthepulsarisheated. Inthispaper,theinclinationangle 10-7
i is defined as the angle between the lineof sight and the normal 0
linetotheorbitalplane. 0.2 6 8 10
0.4 3 4
fithtepoabratmaineteedrsteamrepseurmatmuraeridziesdtriinbuTtaiobnles 2o.f Athdedictoiomnpaallny-, Orbital Pha0s.e6 0.8 1 1 Frequ2ency [1014 Hz]
ion surface are plotted in Figure 7 as functions of the
Figure 5. Phase-resolvedSEDfittedwithamodelfunction.
zenith angle of the pulsar from the measured point on
the companion surface. single power-law function however the resultant fit was
poor(χ2/ν =67.71/55)duetoresidualsbelow1keV,im-
3. X-RAYOBSERVATION plyingthepresenceofasoftexcesscomponent. Wethere-
3.1. Observation fore added a black-body to the model function for the
soft excess, and obtained a better fit (χ2/ν =51.82/55).
2FGL J2339.6 05312 was observed with the Suzaku
DottedlinesonFigure8describethebest-fitmodelfunc-
−
satellite onJun. 29th, 2011. The targetwas observedat
tions. The resultant parameters of the model fitting
the XIS nominal point with the normal imaging mode.
are summarized in Table 3. In both case, the column
Thenetexposuretimeafterthestandarddatareduction
density converged on zero. The resultant upper limit is
process was 96 ks. The accumulated photons are 2555 <1.2 1021cm−2 with90%confidencelevel,andiscon-
counts with the front-illuminated CCDs (XIS0 + XIS3) sisten×twiththeGalacticabsorptionof3.23 1020cm2for
and 1874 counts with the back-illuminated CCD (XIS1) ×
this direction (Dickey & Lockman 1990; Kalberla et al.
in the 0.4 8 keV energy band. During the observation,
the event−rates were 8.25 0.28 10−3 (XIS0, XIS3) 2005).
In order to clarify the origin of these two components,
photons s−1 and 8.89∼ 0.50± 10−3×photons s−1 (XIS1) phase-foldedX-raylightcurvesweregeneratedas shown
± ×
in the energy band.
in Figure 9. In this figure, Phase=0 was set to be
MJD=55500.0inthesamewayastheopticallightcurves
3.2. Spectral analysis
shown in Figure 3 (a). Panel (a) (c) in Figure 9 rep-
− ∼
Figure 8 shows the X-ray spectrum obtained with the resent the energy bands 0.4 1.0 keV, 2.0 4.0 keV and
− −
Suzaku. Atfirst, we triedto fit the X-rayspectra with a 4.0 8.0 keV, respectively. Clearly, the hard X-ray light
−
6 Yatsu et al.
Table 2
Obtainedparametersofthebinarysystemfromthephase-resolvedSED
Dataset Optical+IRa Opticala
Orbitalradius(Rorb)b 1.08±0.02 ×1011 cm 1.18±0.02 ×1011 cm
Companionradius(rcomp) 1.26±0.02 ×1010d1.1 cmc 1.53±0.02 ×1010d1.1 cmc
CompanionTemperature(Tcool) 3170±70 K 2740±120 K
HeatingEfficiency(f)d 1.42±0.05 1.16±0.04
Inclinationangle(i) 59.◦0±1.◦5 52.◦1±1.◦0
BolometricLuminositye 1.09±0.17 ×1032 d21.1 ergs−1 1.12±0.16 ×1032d21.1 ergs−1
χ2 1392.6 637.6
Note. —Errorsarewith1σconfidencelevel.
a “Optical”and“IR”correspondto[B,V,R,I,g’]data-setand[J,H,Ks]data-set,respectively.
b Rorb arecalculatedbasedonradialvelocityofthecompanionstarreportedbyRomani&Shaw(2011).
c d1.1 isthedistanceinunitof1.1kpc.
d Assuminganisotropicpulsarwindwithaluminosityof2.3×1034 ergs−1 basedonRayetal.(2014).
e Bolometricluminosityofthecompanion’sheatedhemisphere.
Table 3
ResultsofmodelfittingoftheaveragedX-rayspectra
Model wabs*(bbody +power-law) wabs*power-law
NH (1021 cm−2) <1.2(0) <0.13(0)
BlackbodyTemperature(keV) 0.15±0.06 ···
BlackbodyRadius(km)† 0.28+−00..9136 ···
Power-lawPhotonIndex 1.14+−00..1145 1.32±0.08
Power-lawFlux0.5−10keV (10−13 ergcm−2 s−1) 2.50±0.16 2.50±0.15
Totalflux0.5−10keV (10−13ergcm−2 s−1) 2.67+−01..0215 2.46+−00..0743
χ2 (dof) 51.82(51) 67.71(53)
Note. —Errorsarewith90%confidencelevel.
† Blackbodyradiusassumingadistanceof1.1kpc.
curvesshowclearmodulationcoincidingwiththeorbital stant component originates in the blackbody, the modu-
motion. Thistimingcoincidencemayimplythatthenon- lationfactorisexpectedtobe(PL)/(BB+PL) 50%,
∼
thermalemissionoriginatesfromthecompanionsurface. which is about twice of the measured value. Taking into
Howevertheshapeofthelightcurvesissomewhatdiffer- account the large errors arising from the uncertainties
ent from the sinusoidal shapes observed in optical, i.e., of the blackbody components (see Table 3), the mea-
a double-peaked shape at 2.0 4.0 keV band, and a flat sured modulation factor should be larger because of the
−
top shape at 4.0 8.0 keV. On the other hand, the soft tight upper limits to the blackbody component. This
−
X-rays seem to be steady. This may imply that the soft may indicate that the constant emission may consist of
component has a different emitting region, namely the the blackbody from the NS surface and the non-thermal
pulsar. component originating in the pulsar magnetosphere.
To evaluate the upper limit of the modulation fac-
3.3. Temporal analysis
tor in the soft X-ray energy band, we tried to fit the
X-ray light curve with a sinusoidal curve and a con- The upper panel of Figure 10 shows the obtained X-
stant component, Asin(2πx)+B, where x is the orbital ray light curve with a bin size of 600 s for the energy
phase. The obtained amplitude of the sinusoidal curve range of 0.4 8.0 keV. Note that the whole length of
is A = (0.80 0.34) 10−3 counts s−1 against the con- the light cur−ve is 190 ks, containing occultation peri-
stantcompon±entofB×=(3.95 0.23) 10−3 countss−1. ods of earth, and corresponding to the twice of the net
Therefore, the upper limit of±the mo×dulation factor is exposure time of 96 ks. Although the photon statistics
A/B < 28.6% with 1 σ confidence level. Note that the seem poor, the X-rays show periodic variability coinci-
maximumpointwasnotfixedtobephase=0.5inthises- dent with an orbital period of 16683 s. We expected to
timation;thishelpedtomaximizethemodulationfactor. observetheflare-likefeaturesreportedinpreviousstudies
The best-fit model curve is shown in Figure 9 (a). (Kataoka et al. 2012; Papitto et al. 2013; Ferrigno et al.
If we adopt the two-component model consisting of a 2014; Linares et al. 2014), which may be related to the
blackbodyandapower-lawfunctionfromtheX-rayspec- accretion activity; however, no significant evidence for
troscopy,wecanroughlyevaluatethecontributionsfrom this was discovered. Nonetheless, the shapes of the light
these two components to the constant emission. The curve seem to change over time.
phase-averaged absorbed-photon flux for the blackbody In order to clarify the temporal variation in the wave-
and the power law in the 0.4 1.0 keV energy band are form of the X-ray light curve, we traced the temporal
both 2.0 10−5 photons s−1−. Assuming that the con- changes of the power spectrum with the Lomb-Scargle
× algorithm,whichcanbe applied to anunevenly sampled
Multi-wavelength observations of the black widow pulsar 2FGL J2339.6 0532 7
−
m] 12xx1100 1111 a1 from companion’s radial velocity (a) n [K] 89000000 OPTO +P TIR
Radius [c 2345xxxx11110000 11110000 OOOppptttOiiicccpaaatllli c+++a IIIlRRR ((((dddd1111....1111 ==== 1011....0802)))) Companio 7000
e
K] 14x01000 10 (b) e of th 6000
mp [ atur 5000
e 3000 er
T p
n m 4000
o e
ompani 2000 urface T 3000
C S
1000
(c) 2000
cy 2.5 0 20 40 60 80 100 120 140 160 180
n
e Zenith angle of the pulsar [degree]
ci 2.0
Effi Figure 7. Simulated temperature of the companion surfaceas a
g 1.5 function of zenith angle of the pulsar from the position for which
n the calculation is performed. The simulations were performedfor
ati 1.0 thebest-fitparameterslistedinTable2.
e
H
0.5 (d) –1V 0.01 XIS1
e
1500 k
Chi-square 1000 –1ed counts s 1100––43 XIS0+3 Blac Power law
z k
500 mali body
30 40 50 60 70 80 90 or
N
Inclination Angle [degree] 10–5
2×10–3
Figure 6. The obtained parameters from model-fitting of the –1V]
phase-resolved SED as functions of inclination anglei. Panel ke 0
(a)∼(d)showtheorbitalradiusRorbandcompanionradiusrcomp, –1s
fth,eacnodmtphaenrieosnultteamntpeχr2at(udr.eo.aft=th9e0c1o),olressidpee,cttihveelhy.eaTtihneggerffiayc-iebnocldy nts −2×10–3 Model = “ wabs *(bbody + powerlaw) ”
u
line on panel (a) represents the semi-major axis of the compan- o
ioncalculatedfromtheradialvelocityof350kms−1 withrespect s [c 2×10–3
ttLohSiDsthc=ealc2mu.3alas×tsio1cn0e3nw4tr,eobeidamspeodlfoyotehndeRasaysyspteientm-adlo(.wR(n2o0ml1ua4mn).iin&osSithyaowft2h0e1p1)u.lsaIrn, Residual −2×10–03
Model = “ wabs * powerlaw ”
data set. In this analysis, we employed a rectangular 0.5 1 2 5 10
Energy [keV]
function as a window function to detect small changes
from a short data sequence. The width of window func- Figure 8. X-rayspectraobservedbytheXISaboardtheSuzaku
satellite. Black and red data points corresponds to the spectra
tion was set to 50 ks in order to cover three orbital laps
observed with the front illuminated CCDs (XIS0 and XIS3), and
and to detect the light-curve variations for the total ex- withthebackilluminatedCCD(XIS1), respectively. Dotted-lines
posure time of 190 ks. We then scanned the light curve show the best-fit model functions consistingof a low-temperature
black-body and a power-law function. The model parameters are
bysweepingthewindowfunction,andgeneratedapower
foundinTable3.
spectrumforeverytimespaninthelightcurve. Notethat
this method cannot detect rapid variations of the power
spectra (faster than a few tens of kilo-seconds), because is much greater than the timescale that can be detected
the width of the time window has a finite timespan. bythismethod. Nevertheless,aclearpeakstructurewas
The lower panel of Figure 10 shows the obtained pe- detected at a frequency of around f1 = 0.6 10−5 Hz,
×
riodogram as a function of time, which represents the whichcorrespondstotheorbitalfrequency. Ontheother
central time of the sampled time window. Therefore, hand, f2 = 1.2 10−5 Hz appears to denote the second
×
the periodogram at t=25 ks consists of the X-ray light harmonic of the orbital frequency, which may be related
curve in the time span of 0 t 50 ks. Both ends of tothedipstructureofthedouble-peakedlightcurve. In-
the grayscalemap are omitte≤d be≤cause the time window terestingly, these two components in the power spectra
can sweep in the range of the obtained light curve. The decrease sequentially after t = 1.2 104 s.
×
sweeping step size was set to 600 s. Therefore, the time
resolutionoftherunningperiodogramisabout230which 4. DISCUSSION
8 Yatsu et al.
Folded period: 16683.6585600000 s
0.015 (a) 0.4−1.0 keV
]s−1
s [ 0.01
nt
u
Co 5×10−3
0
0.015 (b) 2.0−4.0 keV
s]−1
s [ 0.01
nt
u
Co 5×10−3
0
0.015 (c) 4.0−8.0 keV
s]−1
s [ 0.01
nt
u
Co 5×10−3
0
0 0.5 1 1.5 2
Orbital Phase
Start Time 15741 14:28:32:096 Stop Time 15743 19:28:01:748
Figure 9. Phase-folded X-ray light curves for (a)0.4−1.0 keV (b)2.0−4.0 keV and (c)4.0−8.0 keV energy bands. Phase=0 is set to be
MJD=55500.0 in the same way as the optical light curves. Dotted line in panel (a) is the best-fit sinusoidal curve for evaluating the
modulation factor. Dashed line in panel (c) shows a model function for synchrotron emission from a thin layer covering the companion
hemisphere.
4.1. Constraints on the inclination angle of the binary 4.2. Condition for mass Accretion
orbit
In the case of neutron star (NS) binary systems, the
The model fitting of the phase-resolved SED resulted mass accretion process is thought to be controlled by
in inclination angles of i = 59◦ from the entire data the propeller effect (Stella et al. 1986), e.i., the pressure
set (covering optical + IR bands), and i = 52◦ from balance between the ram pressure of accreting material
the optical-only data. This discrepancy seems to arise p and the magnetic pressure of the pulsar p . Here
ram B
from the systematic errors of the reference stars in the we define r where p and p are in balance,
m B ram
IR band (J, H, Ks), resulting in an underestimate of the
target luminosity in the IR band. Therefore, the SED B∗2 r∗ 6 = m˙ 2GM∗ 1/2,
of the optical+IR data set tended to return higher com- 8π rm 4πrm2 rm
panion temperatures than the optical-only data. On the r (cid:16)= (cid:17) B∗4r∗12 1(cid:16)/7 (cid:17)
other hand, it is also difficult to constrain the tempera- m 8GM∗m˙2
turewiththeoptical-onlydata(B,V,g’,R,andI),which (cid:16) (cid:17)4/7 12/7
may then result in an underestimate of the companion =2.5 106 B∗ r∗
× 108 G 10 km
temperature. Therefore, the true inclination angle must
be in the range of 52◦ < i < 59◦. Hereafter, we take M∗ −1(cid:16)/7 (cid:17)m˙ (cid:16) −2/7(cid:17)cm, (3)
i 55◦ 5◦. 1.4M⊙ 1016 g s−1
∼We sh±ould also summarize the pulsar parameters in whereGist(cid:16)hegrav(cid:17)itation(cid:16)alconstant(cid:17),r∗istheNSradius,
preparation for the discussion below. Ray et al. (2014) M∗ istheNSmass,m˙ isthemassaccretionrate. Nearby
recently reported the detection of radio and gamma-ray the pulsar, charged particles are trapped by the pulsar
pulse emission from the central MSP with a period of magnetosphere and are co-rotating with the pulsar. If
2.88mswithaspin-downluminosityofLSD =2.3 1034 the accreting gas approaches the neutron star within a
erg s−1. Based on these parameters, the pulsar’×s sur- radius,
face magnetic field can be estimated, assuming that the
pulsar’srotationenergyis dissipated by dipole radiation r = GM∗ 1/3
(Longair 1994), c ω∗2
B∗ =3.0×1019(PP˙)1/2 =1.9×108I4−51/2 G, (2) =1.7×106 1.(cid:16)M4M∗⊙ 1(cid:17)/3 2.88Pms 2/3 cm (4)
whereI isthepulsar’smomentofinertiainunitsof1045 at which the centri(cid:16)fugal fo(cid:17)rce a(cid:0)nd the (cid:1)gravitational at-
45
g cm2, which is a typical magnetic field for a standard traction are equal, the gas will accrete on the neutron
MSP. star (where ω∗ is the angular velocity of the NS).
Multi-wavelength observations of the black widow pulsar 2FGL J2339.6 0532 9
−
Figure 10. Upper panel shows the X-ray light curve for energy band of 0.4−8.0 keV with a bin size of 600 s. Lower panel shows the
running periodogram generated from the X-ray light curve shown in the upper panel with the Lomb-Scargle algorithm. We employed a
50ksrectangularwindowfunctionthatscansovertheentireX-raylightcurveof190ks. Thehorizontallineinthelowerpanelcorresponds
to the center time of the sampledtime spanof 50ks. The grayscale shows arbitraryamplitude of modulation. Note that the amplitude
of6.3correspondto90%confidence level.
Utilizing B∗ from the pulse profile, the lower limit of erg s−1, which is comparable to the observed X-ray lu-
theaccretionratem˙ thatfulfillstheconditionofr >r minosity of 2FGL J2339.6 0532. Ferrigno et al. (2014)
c m
can be calculated25, claimed that r reachedth−e light-cylinder radius in this
m
regime. This condition would allow for the existence
m˙ > B∗2r∗6ω∗7/3 of pulsar wind and would also prevent mass accretion.
23/2G5/3M∗5/3 Thus, we conclude that the binary system can be in the
=1.3×1016 10rk∗m 8 1.M4M∗⊙ −2/3g s−1. (5) sthame elatceonsdtaitgieonofsetehne einvoIlGutRionJ1p8r2o4c5e−ss24w5i2thionut20m02asosraicn-
(cid:16) (cid:17) (cid:16) (cid:17) cretion. In the latter case, where the mass accretion
Recently, Ray et al. (2014) reported temporal extinc-
alreadyended, the radio-pulseextinction canalso be ex-
tions of the pulse emission in radio implying that the
plained by the unstable activity of the pulsar’s magne-
accretion on the MSP is ongoing. If that is the case,
tosphere itself, as with “nulling” pulsars (Gajjar et al.
the mass accretion rate must be higher than Eq. 5
2012, see also references therein.).
and the expected luminosity during the mass accretion
will amount to 2.4 1036 erg s−1 which is about 5 or-
ders of magnitude h×igher than the X-ray luminosity at 4.3. Origin of the X-ray emission
the 0.5 10.0 keV energy band. Papitto et al. (2013) re- As shown in the spectral analysis, the soft X-rays be-
−
ported intriguing activities of IGR J18245 2452 in the low 1 keV seem to originate from a black-body. The
−
intermediate stage between the rotation and accretion obtained radius of 0.28 km for d = 1.1 kpc is unusually
power. Linares et al. (2014) and Ferrigno et al. (2014) smallfor anaccretiondisc andthus the radiationcanbe
reported weak accretion flows (below m˙ < 106 g s−1) interpretedas the X-ray radiationfrom the neutron star
in IGR J18245 2452; these exhibit a variety of activi- surface. A radius of 0.28 km is still small for a neutron
−
ties possibly reflecting accretion conditions. In case of star, therefore the emitting region could be a hot spot
r r , namely the weak-propeller regime, accretion around the polar cap area. In order to constrain the ge-
m c
≈
flow is only partly inhibited and the X-ray luminosity ometry of the polar cap, a high time resolution X-ray
is in the range of 1035 1037 erg s−1. For cases of observationisrequired. Inaddition,thesmallvariability
∼
r > r , namely the strong propeller regime, the X-ray andthe lackofeclipseinX-raylightcurveareconsistent
m c
luminosityisinthe rangeof1033 1034 withaveryfast with the estimated inclination angle of i 55◦.
and striking variability. Regardle∼ss, the X-ray luminos- In contrast, the hard X-ray light curve∼s vary with the
ityof2FGLJ2339.6 0532isstilllowerthanthatofIGR orbitalmotion. TheX-rayluminosityofthenon-thermal
−
J18245 2452in the strong-propellerregime by an order component from the averagedX-ray spectra amounts to
of magn−itude. L = 3.6 1031 erg s−1 for a distance of 1.1 kpc at
pow
In addition IGR J18245 2452 exhibited another qui- 0.5 10.0 keV×. Therefore the ratio of L against the
escent state in 2002 with a−n X-ray luminosity of 1032 spin−-downluminosity is about 0.15%, wphoiwchis the typi-
∼
calvalueforastandardradiopulsar(Becker & Truemper
25 InwhichweassumedarelationI45∝M∗r∗2. 1997; Kargaltsev & Pavlov 2008).
10 Yatsu et al.
To explain the flat top modulation of the 4 8 keV Termination shock
−
light curve, two possible emitting regions can be pro-
posed. One is an extended emitting region just around
Contact Discontinuity
the pulsar like a halo. In this hypothesis the minimum
phase at the inferior conjunction can be explained by
an eclipse of the halo. The observed smooth modula-
tion requires the emitting region to be larger than the
companion, 1010cm , which is still small for a standard
∼
pulsar wind nebula. In many case the non-thermal X-
rays from a pulsar wind nebula is from the downstream
of the termination shock and the radius of the termina- u u
2 1
tionshockisconstrainedbythepressurebalance. Inthis
Companion
case,the upper limit ofthe emitter size is the orbitalra-
dius of R 1011 cm. At this distance however the
orb
∼
wind pressure is still too strong to be terminated.
The other hypothesis for the originof the hard X-rays
isthatofashockedpulsarwindinteractingwiththecom-
panion star. The companion star orbiting nearby the
pulsar can partially terminate the strong pulsar wind.
Shock-heated pulsar wind
In this case the flat-top light curve can be explained by
the Lorentz boost of shocked plasma. As shown in Fig-
ure 11, we assumed a thin emitting layer covering the
hemisphere of the companion star. The surface of the Figure 11. SchematicviewoftheHardX-rayemissionfromthe
surfaceofthecompanion.
emitting region is the wind termination shock and the
injected electron plasma decelerated at the shock to a
bulk velocity β reflecting the magnetization parameter where ǫ is the synchrotron photon energy. The ob-
2 X
upstreamoftheshock. Atthesuperiorconjunctionphase tained gyration radius is far smaller than the compan-
the shocked plasma is flowing in a direction away from ion, r 1010 cm, therefore the shock heated elec-
comp
∼
us, therefore the synchrotron luminosity may be weak- trons cannot escape from the downstream immediately.
ened(Pelling et al.1987). Thedashedlineinthebottom Intermsoftheenegetics,thephaseaveragedX-raylumi-
panelofFigure9showsthecalculatedphotonfluxforthe nosityofthepower-lawcomponentwasLpow,0.5−10keV =
thin layer of the shocked pulsar wind on the surface of 3.6 1031 erg s−1 from the spectral fitting. Taking into
the companion with a photon index of p = 1.14 and an acco×untthe Lorentz boost effect of 1/0.42,and the ratio
inclinationangleofi=55◦. The best-fitbulk velocityof of the maximum flux against the phase averaged flux of
the shockedwind is β2 =0.43 0.15 inunits of the light 3.0,theintrinsicsynchrotronluminosityfromthecom-
speed. If that is the case, the±X-ray flux is attenuated ∼panionbecomesL′ =1.1 1032whichiscom-
by a factor of pow,0.5−10keV ×
parabletothebolometricluminosityoftheheatedhemi-
sphereofthecompanion,L 1.1 1032ergss−1. This
1+p BB
1 β2 briefestimationalsoindicates∼thatt×he heatingefficiency
− 2 0.42. (6)
1−βp2cos(π/2+i)! ∼ sehnotufrldombethsme halelaetrinthgaenffi0c.i5e.ncTyhoisbtraeisnueltdofrbovmioutshleypdhiffaeser--
resolvedSED (see Figure 5) in which anisotropic pulsar
Here we employed a magnetization parameter σ
wind is assumed. This discrepancy may indicate that
for the pulsar wind upstream of the shock, σ =
the pulsar wind fromthe centralMSP is notisotropic as
B2/(4πn u γ ,m c2) (Kennel & Coroniti 1984), where
1 1 1 1 e expected in the crab nebula(Bogovalov & Khangoulyan
B is the magnetic field just upstream of the shock, n
1 1 2002). The wind pressure at the companion is then
is the number density of the pulsar wind, u is the flow
fvaeclotocrityofdtehfienewdinbdyu1p+strue21am=oΓf21t(hΓe1sihsotchke),b1aunlkd Lcoirsetnhtez pthwainnd t=haLtSoDf/4thπeRo2crobrcon∼al4.g7asdy(nT w=hic1h06mKay, bnep g=rea1t0e8r
light velocity. Assuming that half of the wind energy is cm−3 for the sun) but much smaller than the photo-
carried as Poynting flux (σ =1), B amounts to sphere. Therefore the thickness of the emitting region
1
should be not higher than the corona of the companion.
σ L 1/2 Ontheotherhandtheradiationlengthoftherelativistic
SD
B = =5.5 G. (7)
1 1+σR2 c electronsis quite long: 716A/(Z(Z+1)ln(278/ (Z))
(cid:18) orb (cid:19) 4 1017 cmforthe coronalgasofn 108cm−3. Ther∼e-
Furthermore,thepulsarwindiscompressedattheshock, fo×re these electrons can radiate synpch∼rotronX-rpays even
therefore the downstream magnetic field B2 is higher in the immediate area of the photosphere.
than B1. Adopting B2 > 5.5 G as a lower limit, we can If the hard X-rays originated in the shocked pulsar
constrainthegyrationradiusofthesynchrotronelectrons windonthesurfaceofthecompanionstarasisdiscussed,
emitting 1 keV X-rays, the composition of the pulsar wind can be constrained
based on the downstream flow velocity. Considering
γ m c2 ǫ 1/2 B −1
r = 2 e <3.5 106 X 2 cm, the Rankin-Hugoniot relation for magnetized electron-
gyro
eB × 1 keV 5.5 G positronplasmaforsmallσ,wecanfindthedownstream
(cid:16) (cid:17) (cid:18) (cid:19) (8) flow velocity as a function of the magnetizationparame-