Table Of ContentPASJ:Publ.Astron.Soc.Japan,1–??, hpublicationdatei
(cid:13)c 2008.AstronomicalSocietyofJapan.
Suzaku Results on Cygnus X-1 in the Low/Hard State
Kazuo Makishima,1,2 Hiromitsu Takahashi,3 Shin’ya Yamada,1 Chris Done,4
Aya Kubota,5 TadayasuDotani,6 Ken Ebisawa,6 Takeshi Itoh,1 Shunji Kitamoto,7
Hitoshi Negoro,8 Yoshihiro Ueda,9 and Kazutaka Yamaoka,10
1 Department of Physics, The University of Tokyo, 7-3-1, Hongo, Bunkyo-ku, Tokyo, 113-0033, Japan
2 Cosmic Radiation Laboratory, Institute of Physical and Chemical Research (RIKEN)
2-1 Hirosawa, Wako-shi, Saitama, 351-0198, Japan
8 3 Department of Physical Science, School of Science, Hiroshima University
0 1-3-1 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8526, Japan
0 4 Durham University, United Kingdom
2 5 Department of Electronic Information Systems, Shibaura Institute of Technology
n 307 Fukasaku, Minuma-ku, Saitama-shi, Saitama, 337-8570, Japan
a 6 Institute of Space and Astronautical Science, JAXA,
J
3–1–1, Yoshinodai, Sagamihara, Kanagawa, 229-8510, Japan
2 7 Department of Physics, Rikkyo University,
2
3-34-1, Nishi-Ikebukuro, Toshima–ku, Tokyo 171–8501, Japan
8 Department of Physics, College of Science and Technology, Nihon University
]
h 1-8, Kanda-Surugadai, Chiyoda-ku, Tokyo, 101-8308, Japan
p 9 Department of Astronomy, Kyoto University
- Kitashirakawa-Oiwake-cho, Sakyo-ku, Kyoto, 606-8502, Japan
o
r 10 Department of Physics & Mathematics, Aoyama Gakuin University
t 5-10-1 Sagamihara, Kanagawa, 229-8558, Japan
s
a [email protected]
[
(Received hreception datei; accepted hacception datei)
1
v Abstract
5
1 The black-hole binary Cygnus X-1 was observed for 17 ks with the Suzaku X-ray observatory in 2005
3 October,whileitwasinalow/hardstatewitha0.7–300keVluminosityof4.6 1037ergs−1. TheXISand
3 ×
HXD spectra, spanning 0.7–400keV,were reproducedsuccessfully incorporatinga coolaccretiondisk and
.
1 ahotComptonizingcorona. Thecoronaischaracterizedbyanelectrontemperatureof 100keV,andtwo
∼
0 opticaldepthsof 0.4and 1.5whichaccountfortheharderandsoftercontinua,respectively. Thediskhas
8 the innermosttem∼perature∼of 0.2keV,andis thoughto protrudehalfwayinto the corona. The disk not
0 ∼
onlyprovidesseedphotonstotheComptoncloud,butalsoproducesasoftspectralexcess,amildreflection
:
v hump, and a weakly broadened iron line. A comparison with the Suzaku data on GRO J1655 40 reveals
−
i severalinterestingspectraldifferences,whichcanmostly be attributedto inclinationeffects assumingthat
X
thediskhasaflatgeometrywhilethecoronaisgrosslyspherical. Anintensity-sortedspectroscopyindicates
r
a that the continuum becomes less Comptonized when the source flares up on times scales of 1–200 s, while
the underlying disk remains unchanged.
Key words: accretiondisks — black hole physics — stars: individual (Cygnus X-1)—X-ray: binaries
1. Introduction Indeed, BHBs often emit a major fraction of their ra-
diative luminosity in the hard X-ray band, in the form
Luminous softX-rayradiationofaccretingstellar-mass of spectral hard-tail component if they are in so-called
blackholes(BHs)hasgenerallybeenexplainedasthermal high/soft state, or as the entire power-law (hereafter PL)
emission from optically-thick (in particular “standard”) like continuaif they areinso-calledlow/hardstate (here-
accretion disks (Shakura & Sunyaev 1973; Makishima after LHS) which appears under relatively low accretion
et al. 1986; Dotani et al. 1997; Remillard & McClintock rates. Furthermore, the hard X-ray emission (partic-
2006), which are expected to form around them under ularly in the LHS) involves another interesting aspect,
rather high accretion rates. In contrast, their hard X-ray namely the long-known aperiodic variation over a wide
production process is much less understood, even though frequency range (e.g., Oda et al. 1971; Oda 1977; Nolan
intense hardX-ray emissioncharacterizesblack-hoebina- et al. 1981; Miyamoto et al. 1991; Pottschmidt et al.
ries(BHBs)amongvarioustypesofcompactX-raysources 2003;Remillard&McClintock 2006). Thesespectraland
in the Milky Way and Magellanic clouds. timingstudiesarenotlimitedtostellar-massBHs,sincea
2 [Vol. ,
fairfractionofactivegalacticnuclei(AGNs)arealsocon- &Zycki 1999;Zdziarskietal.2004),oritpenetratesdeep
sidered to be in a state analogous to the LHS of BHBs, intothehotcorona(Youngetal.2001;Milleretal.2006b).
emitting luminous hard X-rays (e.g., Mushotzky, Done & Yet another fundamental issue is how to understand the
Pounds 1993). violent hard X-ray variability in the context of thermal
In hard X-rays, a BHB in the LHS emits a roughly Comptonization scenario (e.g., Miyamoto et al. 1987).
PL shaped continuum with a photon index Γ 1.7 (e.g., To address these issues associated with the LHS of
∼
Remillard & McClintock 2006), but the spectrum grad- accreting BHs, a number of spectral studies have been
ually steepens toward higher energies. This has been ex- conducted so far, by fitting observed broad-band spec-
plained in terms of unsaturated Comptonization of some tra withtheoreticalspectralmodels (e.g., Gi´erlinskietal.
soft seed photons, in a cloud of hot Maxwellianelectrons, 1997; Zdziarski et al. 1998; Dove et al. 1998; Frontera
or a “corona”, with a temperature of T =50 100 keV etal.2001a). Similarly,extensivestudiesinthefrequency
e
−
andanopticaldepthofτ 1(Shapiroetal.1976;Sunyaev domain have been conducted to characterize the random
∼
& Tru¨mper 1979;Sunyaev & Titarchuk 1980;Gi´erlinski variations (e.g., Miyamoto and Kitamoto 1989; Negoro
et al. 1997; Di Salvo et al. 2001; Frontera et al. 2001a; et al. 1994; Nowak et al. 1999b; Gilfanov et al. 2000;
Zdziarski & Gierlin´ski 2004; Ibragimov et al. 2005). As Revnivtsev et al. 2000; Remillard & McClintock 2006).
a result,the Comtpony-parameter,y 4τkT /m c2, also However, the former approach is limited in that differ-
e e
≡
take a value of 1, where k is the Boltzmann constant, ent modelings often degenerate. The latter approach has
∼
m is the electron mass and c is the light velocity. The also a limitation, in that it may not constrain details of
e
electrons will be cooled by transferring their energies to the Comptonization scenario without invoking particular
thephotons,whileheatedviaCoulombiccollisionsbyions models (Kazanas et al. 1997;Hua et al. 1997).
thatacquirenearlyafree-falltemperature(e.g.,Zdziarski We analyze broad-band (0.7–400 keV) Suzaku data of
et al. 1999). The Comptonizing cloud may be identi- Cygnus X-1 (Cyg X-1; Oda 1977; Lian and Nolan 1984)
fied with radiatively inefficient inner accretion flow (e.g., acquired in 2005 October. With the wide-band sensitiv-
Ichimaru 1977; Narayan & Yi 1995). ity,Suzakuisexpectedtoprovidenoveldiagnosticsofthis
Superposed on the dominant continuum characteriz- BHB, the first celestial object suspected to be a black
ing the LHS, we often observe a spectral hump at en- hole (Oda et al. 1971). To overcome the above difficul-
ergies of 20–40 keV due to “reflection” of the primary ties, we compare the Suzaku spectra of Cyg X-1 with
X-rays by some cool materials (e.g., Lightman & White those of GRO 1655 40,searchingthe twospectra for dif-
−
1988;Inoue 1989),afluorescentFe-Kemissionline(Basko ferences attributable to their different system inclination,
1978;Kitamotoetal.1990),andanapparently“smeared” i 45◦ of Cyg X-1 (Abubekerov et al. 2004) and i 70◦
∼ ∼
Fe-K edge (Ebisawa et al. 1994). These features can be of GRO 1655 40 (Green et al. 2001). We hence refer to
−
explainedbyassumingthatafractionoftheprimaryhard the Suzaku results on GRO 1655 40 by Takahashi et al.
−
X-rays are reprocessedby cool materials located near the (2007a)asPaperI.Wealsoanalyzespectralchangesasso-
BH, presumably an optically-thick part of the accretion ciatedwiththe randomvariationsonatime scaleof 1s
∼
disk(Doneetal.1992;Marshalletal.1993;Ebisawaetal. (section4),toseehowtheComptonparametersarechang-
1996;Gi´erlinskiet al.1997). This partofthe disk may at ing; this will be complementaryto the study of long-term
thesametimeproduceanX-rayexcesswhichweoftenob- spectral changes (e.g.,Ibragimov et al. 2005). We employ
serve at the softest end of the spectrum (e.g., Balucinska a distance of 2.5 kpc (Margonet al. 1973;Bregmanet al.
& Hasinger 1991), and furthermore, may provide the 1973)to CygX-1. The errorsrefertostatistical90%con-
Comptonizingcoronawiththeseedphotons(e.g.,Shapiro fidence limits.
et al. 1976).
In spite of general success of this currently popular in- 2. Observation and Data Processing
terpretation of the LHS in terms of Comptonizing hot
2.1. Observation
clouds and cool materials, details of the scenario still
need to be elucidated. It is yet to be confirmed that the The present Suzaku observation of Cyg X-1 was per-
seed photons are provided by a cool accretion disk (e.g., formed on 2005 October 5, from UT 04:34 through 15:11
Zdziarski et al. 1999), rather than by thermal cyclotron for a net exposure of 17.4 ks, as a part of the ini-
emissionintheComptoncoronaitself(e.g.,Kantachetal. tialperformanceverificationprogramofSuzaku(Mitsuda
2001). The geometry and size of the Comptonizing cloud et al. 2007). Suzaku carries 4 sets of X-ray telescopes
is not well known, except for loose constraints, includ- (Serlemitsoset al.2007),eachcoupledtoa focal-planeX-
ing; that the cloud should be large enough for the com- rayCCDcameracalledXIS(X-rayImagingSpectrometer;
pactness parameter to remain within a reasonable range Koyamaetal.2007)operatinginthe energyrangeof0.2–
(Lightman&Zdziarski 1987),butshouldnotbetoolarge 12keV.Three(XIS0,XIS2,andXIS3)ofthefourXISsen-
to be able to produce the hard X-ray variation on a 1 sors use front-illuminated (FI) CCDs, while XIS1 utilizes
∼
s time scale, and that it should have a sufficient verti- a back-illuminated (BI) one thus achieving an improved
cal height so as to efficiently illuminate the reprocessing soft X-ray response. The Hard X-ray Detector (HXD;
matter. Still unsettled is the relation between the hot Takahashi et al. 2007c; Kokubun et al. 2007) covers the
corona and the putative cool disk; whether the cool disk 10–70 keV energy band with Si PIN photo-diodes (HXD-
is truncatedat a largeradius (Zdziarskiet al. 1998;Done PIN), and the 50–600 keV range with GSO scintillation
No. ] 3
counters (HXD-GSO). rated, and discarded this and the subsequent exposures
Because the optical axes of the XIS and the HXD are in the same window. This typically occurred in the last
slightly ( 3′.5) offset, the observation was carried out one or two exposures in the 8-s window. Because these
∼
with the source placed at the center of the XIS field telemetry-saturatedtimebinsareregardedasdeadtimes,
of view. The HXD was operated in the nominal mode the source count rate remains approximately intact, al-
throughout the observation. Because the source was very though the exposure is reduced.
bright, the XIS was operated with the “1/8 window op- Afterfilteringoutthetelemetry-saturateddata,wefur-
tion”,inwhichasmallerfieldofviewof17′.8 2′.2isread ther removed the effects of photon pile up. Although the
×
out every 1 s. The editing mode of the FI CCDs was set SuzakuX-raytelescopeshaverelativelywidepointspread
to 2 2, while that of the BI CCD (XIS1) was 3 3. In functions, photon pile up becomes noticeable at the im-
× ×
the present observation, however,XIS1 with the BI CCD age center when observing a point source brighter than
chip suffered severe telemetry saturation, for more than 100 cts per exposure. The average count rate of Cyg
∼
60% of the total exposure. X-1 during the present observation was 3 times higher
∼
Duringtheobservation,theaveragedsourcecountrates than this limit. We therefore accumulated XIS events
(excluding backgrounds) of XIS0, HXD-PIN, and HXD- by excluding circular regions of various radii at the im-
GSOwere 300, 47,and 20cs−1,intheenergybands age center, and examined the obtained series of spectra
∼ ∼ ∼
of 0.5–10 keV, 10–70 keV, and 70–400 keV, respectively. for shape changes. As a result, we have decided to ex-
However, the true XIS0 counts would be higher, if the clude an image-center region with a radius of 1′.0 (60
data were free fromtelemetry overflowsandeventpile up pixels). Although this means that the event extraction
(subsection 2.2). region has a complicated shape (intersection of an annu-
lus and a rectangle), we use standard ancillary response
2.2. XIS Data Selection
files (ae xi0 xisnom4 20060615.arf, etc.) defined for a
WeretrievedtheXISdatawhichwerepreparedviaver- circular region of 4′.0 radius. This is because the point
sion 1.2 processing. The XIS data were further screened spread function of the telescope has little energy depen-
basedonthefollowingstandardcriteria: theXISGRADE dence,andhencethespectrumwouldnotchangeexceptin
should be 0, 2, 3, 4, or 6; the time interval after an exit normalization. Weconfirmedthisinferenceusingthedata
from the South Atlantic Anomaly should be longer than of a moderately bright point source, SS Cyg observed on
500 s; and the object should be at least 5◦ and 20◦ above 2005November2, andfound that the spectral normaliza-
the dark and sunlit Earth rim, respectively. tionshouldbe multipliedbyafactorof 0.092compared
Inordertoconductspectralandtimingstudies,weuti- with that of PIN, when excluding the∼central 1′ of the
lize XIS events extracted from a region which is defined image; this is taken into account in the subsequent spec-
to fit in the “1/8 window”. Since the Suzaku attitude tral analysis. We also use the standardresponse matrices
is known to wobble on a time scale of several tens min- (ae xi0 20060213.rmf,etc.) for the XIS.
utes, the region needs to be a little smaller than the win-
2.3. HXD Data Selection and Background Subtraction
dow size. Considering the magnitude of the wobbling, we
defined the region as an intersection between a rectan- The HXD data utilized here were prepared via ver-
gle of 70 1024 pixels and a circle with a radius of 3′.3 sion 1.2 processing. We further screened the HXD events
×
(200pixels),bothcenteredalwaysontheimagepeak. For by imposing conditions that more than 500 s should be
this purpose, we calculated the image center every 200 s elapsed from an exit out of the South Atlantic Anomaly,
and re-defined the event extraction region also every 200 that the target should be >5◦ above the Earth rim, and
s. Thus, the location of the region on the CCD chip is thatthegeomagneticcutoffrigidityshouldbehigherthan
time dependent. The background was entirely negligible 8 GV.
throughout the observation (less than 0.1% of the signal Since the HXD has neither imaging capability nor an
at 8 keV considering the Galactic ridge emission), so was offset counter to measure instantaneous non X-ray back-
not subtracted from the data. ground(NXB),weneedtoestimatetheNXBandsubtract
Because Cyg X-1 was so bright that we need to care- it from the raw data. Accordingly, we employed the PIN
fullyremovetheeffectsoftelemetrysaturationandphoton background modeling by Watanabe et al. (2007), known
pile up. When the number of detected events exceeds the as “PINUDLC (bgd a)” model 1, which combines actual
telemetry allocationto the XIS, the excessevents aredis- HXD-PIN data acquired while pointing to night Earth
carded without being transferred to telemetry. This phe- (Kokubun et al. 2007). Thus, we produced a large num-
nomenon is called telemetry saturation. Since the event ber (10.0 times as large as the actual) of fake non X-ray
buffer is cleared every 8 s, and the XIS exposure was 1 s background events, which would just be detected during
in the present observation, we excluded the telemetry- thepresentobservationifwehadanimaginaryNXBmon-
saturated data in the following way. For each 8-s win- itor detector. By processing this fake event files exactly
dow, we calculated an average count rate over the first 4 in the same manner as the actual data, we can estimate
s which are usually free from the telemetry saturation. If NXB contributions in the PIN data, and subtract them
a subsequent exposure in the same 8-s window showed a fromPINlightcurvesaswellasfromPINspectra,witha
count rate lower than this average by more than 2 stan-
1 http://www.astro.isas.jaxa.jp/suzaku/analysis/hxd/hxdnxb/
dard deviations, we judged that the telemetry was satu-
4 [Vol. ,
typicalaccuracyof5%or better. The cosmic X-rayback- backgrounds (PIN and GSO) were subtracted in the way
ground entering through the PIN field of view is entirely as described insection 2.3,andthe results were corrected
negligible in the present study. fordeadtimes. Incontrast,theXISbackground,whichis
Subtraction of the HXD-GSO background employs a completely negligible, is inclusive. The count rates indi-
differentmethod,developedbyFukazawaetal.(2007)and cated by these XIS light curves are by an order of mang-
called “LCFIT (bgd d)” model 2. In this case, the whole nitude lower than those actually observed,because of the
GSO energy band is divided into 32 energy bins, and a image-centerexclusionwhich was needed to avoidpile-up
light curve of each energy bin is produced over a long events (subsection 2.2). On time scales of a few minutes
period. Because each light curve has characteristic repe- to a day, the source thus varied randomly typically by
tition patterns with periods ofone orbitalrevolution,one 10%.
∼±
day(duetotherelationwithrespecttotheSouthAtlantic
2.5. Time-Averaged Spectra
Anomaly), and 51 days (due to orbital precession),it can
be expressedasafunction oftime usingarelativelysmall Weshowinfigure2thetime-averagedSuzakuspectraof
number of adjustable parameters. The parameters can CygX-1. Likeinsection2.4,wecorrectedthe HXD spec-
be determined by analyzingthese light curves overa long tra for the dead times, and subtracted the backgrounds
period, typically one month. We can thus derive the ex- using the method described in section 2.3. In contrast,
pected NXB counts in individual energy bins at a given the XIS2 spectrumis here andhereafter notcorrectedfor
time during the present observation, and subtract them thenormalizationreduction(downto0.092oftheoriginal
either from light curves or spectra. The GSO data are level), due to the exclusionof the central1′ of the images
hence analyzedusingthe sameenergybinnings asusedin (subsection 2.2). For comparison, we superpose the mod-
the NXB modeling. When integrated over 1 day, this eled backgroundspectra of HXD-PIN and HXD-GSO.
∼
method reproduces the GSO background within a sys- As seen in figure 2, we have significantly detected Cyg
tematic error of 2% (Takahashi et al. 2007b). Since X-1 nearly across the full energy band of Suzaku. In par-
∼
the PIN and GSO background models both utilize PIN ticular, the source signals in HXD-PIN exceed the NXB
upper-discriminator hit rates, their basic time resolution by more than a factor of 5, even at the highest energy
∼
istypically4sorlonger,dependingonthetelemetryrate. of 70keV.Therefore,thesystematicbackgrounduncer-
∼
tainty of HXD-PIN is completely negligible compared to
photon counting statistics. Even in the GSO range, the
source signal is so strong that it can be detectable up to
400keV,abovewhichthesystematicbackgrounderrors
∼
start dominating.
Fig. 1. LightcurvesofCygX-1binnedinto64s,obtainedin
the present Suzaku observation performed in 2005 October.
The ordinates are in units of ct s−1. From top to bottom,
panels refer to those obtained with XIS2 (0.7–2 keV), also Fig. 2. Time-averaged XIS2 (black), HXD-PIN (red), and
withXIS2(2–10keV),withHXD-PIN(10–60keV),andwith HXD-GSO(green)spectraofCygX-1,shownaftersubtract-
HXD-GSO (60–200 keV). The HXD backgrounds were sub- ing the background but not removing the instrumental re-
tracted. Theperiodicdatagapsareduetospacecraftpassages sponses. The XIS2 spectrum is accumulated after removing
throughtheSouth AtlanticAnomalyandtheEarthocculta- the telemetry overflow and and cutting off the image center
tion. (seetext). ThemodelednonX-raybackgroundofHXD-PIN
isindicatedinmagenta. ThatofHXD-GSOisshownincyan,
together withits5%and3%levels.
2.4. Light Curves
Figure 1 shows XIS2, HXD-PIN, and HXD-GSO light
curvesofCygX-1fromthepresentobservation. TheHXD
2 http://www.astro.isas.jaxa.jp/suzaku/analysis/hxd/gsonxb/
No. ] 5
3. Analysis of the Time Averaged Spectra leavesseveralnoticeablediscrepancies,including apromi-
nent data deficit above 100 keV, and a data excess in
∼
This section is devoted to the study of time-averaged 0.9–1.5 keV. Obviously, the former is due to the high-
SuzakuspectraofCygX-1. Becauseofthestrongteleme- energy cutoff, while the latter is due to the presence of
try saturation (section 2.1), we do not use the data from soft excess, both of which have been established through
XIS1 (BI). Of the three FI CCD cameras comprising the previous observations.
XIS,onlythedatafromXIS2areemployedinourspectral
analyses, while those from XIS0 and XIS3 are utilized in
section 4 for a different purpose; even if we incorporated
data from the other XIS sensors, systematic errors would
dominatebeforedatastatisticsareimproved,becauseCyg
X-1 is so bright for the XIS.
3.1. Fitting with conventional models
In order to roughly characterize the time-averaged
broad-band spectra derived in subsection 2.5, we simul-
taneously fitted simple empirical models to the XIS2,
HXD-PIN, and HXD-GSO spectra, so that the results
may be compared with other observations of Cyg X-1, as
well as those of other similar objects. Here andhereafter,
the overall model normalization is allowed to differ be-
tween the XIS and HXD (mainly to absorb uncertainties
introduced by the removal of telemetry saturated data
and the image-center exclusion), but constrained to be
the same between HXD-PIN and HXD-GSO. As the
detector responses, we use ae xi2 20060213.rmf
and ae xi2 xisnom4 20060615.arf for XIS2,
ae hxd pinxinom 20060814.rsp for PIN,
while ae hxd gsoxinom 20060321.rsp and
ae hxd gsoxinom 20070424.arf for GSO. The last Fig. 3. Simultaneous fittings to the time-averaged XIS2,
GSO file is the correction factor described in Paper I, HXD-PIN, and HXD-GSO spectra of Cyg X-1, us-
introduced so as to make the Crab Nebula spectrum ing various combinations of conventional models. (a)
described by a single PL model with Γ = 2.1 3. We Background-subtracted and response-inclusive spectra (the
same as in figure 2), compared with the best-fit
discard the XIS data in the 1.7–1.9 keV range to avoid
model consisting of a disk-blackbody, a cutoff pow-
complexity of the Si-K edge modeling, and those in the er-law, a Gaussian, and a reflection component; i.e.,
>8keVbandwhichispossiblyaffectedstillbythephoton wabs*(diskbb+cutoffpl+pexrav+gau). Individual model
pile-upeffects. TheHXD-PINdatabelow12keVarealso components are separately drawn. (b) Data-to-model ratios
for an absorbed single-PL fit. (c) The same as panel b, but
discarded in the fitting, to avoid response uncertainties
a disk-blackbody component is added, and the power-law is
there (Kokubun et al. 2007). We utilize the HXD-GSO replacedbyacutoffpower-lawmodel. (d)Thedata-to-model
data in the 70–400keV energy band, because of the poor ratios corresponding to the fit in panel a, namely, obtained
signal significance (figure 2) and relatively large response by further adding to the model of panel c a Gaussian and a
reflectioncomponent.
uncertainties, in >400 keV and <70 keV, respectively.
We assign a systematic error of 1% to all the spectral
To express the high-energy turn over, we next re-
bins, in order to absorb uncertainties in the instrumental
placed the PL component with a cutoff power-law model
calibration, including in particular those associated with
(cuoffpl; a power-law times an exponential factor). To
the detector responses and background estimation. The
accountforthesoftexcess,wealsoaddedadisk-blackbody
effect of contamination on the optical blocking filters of
component(diskbb: Mitsudaetal.1986;Makishimaetal.
the XIS is modeled as additional absorption (Koyama
1986)to our fitting model. The diskbbmodel is a simple
et al. 2007). We also trimmed the energy offset by
∼ non-relativisitic approximationto the X-rayspectrum in-
17 eV, considering the current uncertainty with the XIS
tegratedovera standardaccretiondisk,andis considered
energy scale in the 2 2 mode.
× to provide an approximately (within 10%; Watarai et
Asthesimplestattempt,wefirstfittedanabsorbedsin- ∼
al. 2000)correctinnermostdiskradiuswhenanappropri-
gle PL model simultaneously to the three sets of spectra.
atecorrectionfactorisincorporated(Kubotaetal.1998).
ThefitindicatedΓ 1.77asthebestestimate,andanab-
sorbinghydrogenco∼lumndensityofN =3.5 1021cm−2. Thus,themodeliswabs*(diskbb+cutoffpl)inthexspec
H
However,thefitwasfarfromacceptable,with×χ2/ν=14.3 terminology;here,we utilize version11ofthe xspec spec-
tral fitting package 4, together with the photoelectric ab-
forν=360degreesoffreedom. Asseeninfigure3b,thefit
sorptionmodel,wabs,byMorrison&McCammon (1983).
3 http://www.astro.isas.jaxa.jp/suzaku/analysis/hxd/gsoarf/
4 ttp://www.heasarc.gsfc.gov/docs/xanadu/
6 [Vol. ,
Then, the fit was improved drastically to χ2/ν =3.7 for to use a diskbb model as the seed photon source; and
ν=357,though not yet acceptable. The exponential cut- it has a built-in function to allow a part of the initial
offenergywasobtainedasT 170keV,andthenominal Compton-producedphotonstogetintoacoldmatterand
cut
∼
PL photon index decreased to Γ 1.45. The innermost reflected.
∼
disk temperature of the diskbb component was obtained We thus fitted the XIS2, HXD-PIN, and HXD-
as T 0.5 keV, and its innermost radius as r 13 GSOspectrasimultaneouslybyawabs*(diskbb+compPS)
in in
∼ ∼
km at a distance of 2.5 kpc (section 1), without any cor- model, in the same manner as before. At this stage,
rection for the color hardening factor, or inner boundary the reflection option in compPS was suppressed, and the
condition, or inclination (see Makishima et al. 2000). We Gaussian component was not included. Here and here-
regard these diskbb parameters still unphysical. after, we assume that the Compton cloud has a spherical
In figure 3c, the data to model ratios from the geometry(parameter=4)withanelectrontemperatureT
e
wabs*(diskbb+cutoffpl) fit show a slight broad hump and an optical depth τ, and that the seed photons to
over 20–40 keV (though not very clear in this presen- the compPS component are supplied by a cool standard
tation), indicative of reflection from a cool thick ma- disk having the same T (but a separate normalization)
in
terial. In addition, there is clear evidence of Fe-K as the diskbb component. The implied picture is that
line at about 6.4 keV. Accordingly, we added a re- some fraction of photons from a standard disk is fed to
flection component (pexrav; Magdziarz & Zdziarski theComptonizingcorona,whiletherestisdirectlyvisible
1995) and a Gaussian to our model, thus constructing as the spectral soft excess. This picture is supported by
a model wabs*(diskbb+cutoffpl+pexrav+gau),and re- the presenceofatightcorrelationbetweenthe continuum
peated the fitting. In using pexrav, the reflector was as- slopeandthereflectionstrengthinBHBs(Zdziarskietal.
sumed to have an inclination i = 45◦, and its incident 1999; Gilfanov et al. 1999; Revnivtsev et al. 2001) and
photons to have the same spectral shape and normaliza- in AGNs, which indicates that the optically-thick disk,
tion as those of the cutoffpl continuum (rel refl<0 producing the reflection, is also causally related to the
technically). The Gaussian was allowed to have a free Comptonized emission.
centroid and a free width. Then, the fit further improved
to χ2/ν = 2.4 for ν = 353, implying that the reflection
is significantly present as reported by many previous ob-
servations. The model gave the cuoffpl parameters as
Γ 1.74 and T >1000 keV, and the diskbb parame-
cut
∼
ters as T 0.3 keV and r 45 km. The reflector was
in in
∼ ∼
found to have a moderate solid angle as Ω/2π 0.5, and
∼
theGaussiancentroidanditswidthwereobtainedas6.43
keV and σ<0.2 keV respectively. In figure 3a, we show
this fit where contributions of the individual model com-
ponents are separately drawn. The data-to-model ratios
are given in figure 3d. Thus, the reflection signal is in-
ferred to carry 30% of the continuum at 30 keV.
∼
In this way, the empirical model fittings have roughly
quantified the time-averaged Suzaku spectra of Cyg X-1,
and extracted their basic features. The fit implies a 0.7–
300 keV flux (after absorption by N ) of 5.4 10−8 erg
H
cm−2 s−1. Assuming an isotropic emission, an×d employ-
ing the distance of 2.5 kpc (section 1), the absorption-
removed 0.7–300 keV luminosity becomes 4.6 1037 erg
s−1, which is typical of this object. Although t×he fit still
remains unacceptable, and hence errors associated with
the model parameters cannot be evaluated, further im-
Fig. 4. Simultaneous fittings to the time-averaged XIS2,
proving the model would be of little meaning, because
HXD-PN,andHXD-GSOspectraofCygX-1,employingther-
cutoffpl is a mere empirical approximation to the spec- malComptonizationmodels. Thederivedbest-fitparameters
traexpectedfromunsaturatedComptonizationprocesses. aregivenintable1. (a)Thesamespectraasinfigure3a,com-
Instead, we proceed to more physical modelings, using pared with the best-fit wabs*(diskbb+compPS+compPS+gau)
model. The two compPS components areconstrained to have
theoretical Comptonization codes.
thesameTe,andacommonsetofreflectionparameters. (b)
3.2. Fitting with Comptonization models FitresidualsfromamodelincorporatingasinglecompPScon-
tinuum without reflection, together with a diskbb. (c) The
Among various Comptonization codes currently avail- sameaspanelb,butreflectionisincorporated,andaGaussian
for the Fe-K line is added. (d) The residuals correspond to
able,hereweemploythatofPoutanen&Svensson(1996),
the fit shown in panel a. Compared to panel c, the second
calledcompPSin xspec, because of the following three ad-
compPScomponent isadded.
vantages: it correctly takes into account the relativistic
effects in the Compton scattering kinematics; it allows us As shown in figure 4b, this model gave a poor fit with
No. ] 7
χ2/ν=3.9 for ν=357. Since figure 4b reveals the reflec- and reduced a residual structure near 2 keV where the
tionhumpat 30keV,we activatedthe reflectionoption twocompPScomponentsnowcrossover. Furthermore,the
∼
in compPS.This has nearly the same effect as the pexrav XIS vs. HXD normalization has became 0.088, which is
model, but with two additional free parameters; the ion- close to the value of 0.092 calibrated using SS Cyg (sub-
izationparameterξ inunits of ergcm s−1, andthe inner- section 2.2). We are therefore confident that the Suzaku
most reflector radius Rref. The latter controls relativistic datarequiretwo(orpossiblymore)Comptoncomponents.
in
blurring of spectral features, and is expressed in units of Although the fit is not yetformallyacceptable, we regard
the gravitational radius R =GM /c2, with M the itassatisfactory,becausethedatavs. modeldiscrepancy,
g BH BH
BHmass andGthe gravitationalconstant. The outer ra- typically within 4% over the entire 0.7–400 keV range,
dius was fixed at 106R and the radial emissivity profile is comparable to those obtained when we fit the Suzaku
g
was assumed to depend on the radius r as r−3 (Fabian spectra of bright objects, such as the Crab spectra, with
et al. 1989). The inclination angle, abundance, and tem- simple models.
perature of the reflector were also fixed at 45◦, 1 solar, Our final model obtained in this way is shown in fig-
and 106 K, respectively. We retained the Gaussian com- ure 4a in the convolvedform, and in figure 5a in the inci-
ponent, because the compPS reflection does not include dentνFν form. Itsmodelparametersaregivenintable1.
anyemissionline. Then,asshowninfigure4c,thefitwas Hereafter,we callthe compPScomponents with the larger
much improved (χ2/ν=2.6 with ν =351); the reflection and smaller y-parameters compPSh and compPSs, respec-
hump is significantly present with Ω/2π 0.2 and ξ 0, tively, withthe suffix “h” standing for“hard”and“s”for
although Rref was not constrained in thi∼s and all su∼bse- “soft”. In figure 5, the compPSs component includes not
in
quent fits. The fit goodness is comparable to that ob- onlythescatteredphotons[ 1 exp( τ)],butalsothose
∝ − −
tained with the wabs*(diskbb+cutoffpl+pexrav+gau) seedphotonswhichtraversedthe Comptoncloudwithout
model(figure3d). While the conventionalmodelfailedto scattered [ exp( τ)].
∝ −
constrain T , the present model gave an electron tem-
cut
perature of T =115 keV.
e
In figure 4c, the fit still leaves a prominent data deficit
in the highest XIS range. In addition, the XIS vs. HXD
normalization ratio was obtained at 0.108, which exceeds
the nominal value of 0.092 (subsection 2.2); this is too
∼
large,evenconsideringuncertaintiesinthecorrectionfac-
tors involved in the XIS and HXD data processing (sec-
tion 2). We hence suspect that the actual source contin-
uum is rising from 10 keV toward lower energies, with
∼
a steeper slope than indicated by the model, so that the
fitting algorithm tried to absorb this mismatch by arti-
ficially raising the model prediction to the XIS data. In
other words, the broad-band continuum has a more con-
cave shape over 2–20 keV (see figure 3b), than would be
explained by the compPScomponentwith reflection. This
effect was previously noticed by Di Salvo et al. (2001) as
a soft excess that cannot be accounted for by the cool
disk emission. The same effect was noticed in the Suzaku
data of GRO J1655 40 (Paper I), and led Takahashi
−
et al. (2007a) to incorporate another compPS component.
Such a “double-Compton” modeling was previously ap-
pliedsuccessfullytothe LHSofCygX-1inseveralworks;
by Gi´erlinskiet al.(1997)andIbragimovetal. (2005)us-
ing simultaneous Ginga, RXTE, and CGOR/OSSE data,
and by Frontera et al. (2001a) to the BeppoSAX data.
WehencetriedamodelincorporatingtwoComptonized
continua, which are constrained to have the same seed-
Fig. 5. (a) The inferred best-fit νFν spectrum of Cyg X-1,
photon temperature (which in turn is the same as that
shownwiththeabsorptionremoved. Itcorrespondstopanels
of diskbb), the same hot-electron temperature, and the
a and e of figure 4, with the model parameters detailed in
same reflection parameters, but are allowed to differ in the column labeled“Average” of table 1. Red, blue, orange,
theirnormalizationandopticaldepth. Themodelishence green,andpurplespecifycompPSh,compPSs,diskbb,reflection
wabs*(diskbb+compPS+compPS+gau). As shown in fig- components, and a Gaussian for the Fe-K line, respectively.
ure4d,thefithasbeenimproveddrasticallytoχ2/ν=1.13 (b) The same as panel a, but for GRO J1655−40 observed
withSuzaku on2005 September 22and23. Itisidentical to
(ν=349);theXIS,PIN,andGSOspectraarereproduced
figure7ofPaperI,except fortheremovaloftheabsorption.
with χ2/ν = 1.3, 0.9, and 0.9, respectively. The model
has removed the data deficit in the highest XIS range,
8 [Vol. ,
3.3. Examination of the Comptonization model results Rseed=1.18√A . (2)
in
Although the analysis conducted in the preceding sub- Though similar to R of equation (1), Rseed is not
in in
section has provided a plausible representation of the corrected for the inclination, since we presume the
time-averagedspectra, different spectral models could of- Comptonized emission to be approximately isotropic. As
tendegenerate. Therefore,letusbrieflyexamineourfinal seenintable1,thevalueofRseed associatedwithcompPSs
in
spectral model for its implications and physical consis- is comparable to R , while that of compPSh is consider-
in
tency, leaving detailed discussion to section 5. ably smaller. We hence obtain an implication that 7
Theobtainedabsorption,(6.6+0.8) 1021cm−2,isclose times more seed photons are fed to compPSs than∼to
−0.3 ×
to the value of (5.3 0.2) 1021 cm−2 derived by Dotani compPSh: afurtherdiscussioncontinuesinsubsection5.4.
± ×
etal.(1997)withASCA.ThecompPShandcompPSscom- Although the final model is generally successful, the
ponents,crossingat 4keV,arecharacterizedbyphoton obtained XIS vs. HXD normalization, 0.088, is 5% too
∼ ∼
indices of 1.6 and 2.4, respectively, with the former small. When this ratio is fixed to the calibration value
∼ ∼
consistent with the approximate X-ray slope of Cyg X- of0.092,the PINdata exhibitsomepositive andnegative
1 measured in energies of a few to a few tens keV. The residuals at 15 keV and 40 keV, respectively. These
∼ ∼
underlying parameters, T 100 keV, τ of order unity, can be eliminated by allowing compPSs and compPSh to
e
∼
and Ω/2π 0.4, are consistent with a large number of have a larger ( 1.0) and a slightly smaller ( 0.3) value
∼ ∼ ∼
past measurements (e.g., Gi´erlinski et al. 1997; Di Salvo ofΩ/2π,respectively,withtherawchi-squarereducingby
et al. 2001; Frontera et al. 2001a; Zdziarski & Gierlin´ski 13 (for ν=349). This is because the reflection associated
2004; Ibragimov et al. 2005). with compPSs appears at lower energies than those with
The diskbb component has a raw innermost radius as compPSh (figure 5a). There is thus some indication that
r =180+150 km, which yields the best-estimate actual the softer Comptonsignals aremore stronglyreflectedby
in −50
inner radius of R =250+210 km; this is obtained by ap- cold materials than the harder one.
in −60
plying the nominal correction factors in Makishima et al. When the two compPS components are allowed to take
(2000)(includingacolorhardeningfactorof1.7;Shimura different values of Te, tolerances of the model parameters
& Takahara 1995) to r , and correcting for the inclina- increase, but the fit goodness remains the same. This is
in
tion assuming i=45◦, as because the high-energy roll over of compPSs is masked
by compPSh. Therefore, we do not adopt this modeling,
Rin=1.18rin/√cosi . (1) althoughthe datadonotnecessarilyexclude the presence
of multiple electron temperatures.
This R is larger than the value of R 100 km mea-
in in
∼
sured in the high/soft state with ASCA (Dotani et al.
4. Analysis of Spectral Changes Associated with
1997; Makishima et al. 2000), suggesting a disk trunca-
the Random Variation
tion at a larger radius than in the high/soft state (see
also subsection 5.4). The innermost disk temperature, 4.1. Methods
0.19 keV, is a factor 2.3 lower than that measured in the
Now that the time-averaged spectra have been quanti-
high/soft state (Dotani et al. 1997), consistent with a re-
fied in section 3, we proceed to the latter half of our data
duced accretionrate and possibly a larger innermost disk
analysis, namely attempts to understand how the spec-
radius. Theseconsistenciessupport(thoughnotnecessar-
trum changes as the source varies randomly; a similar at-
ily prove) our basic assumption that the seed photons for
tempt was carried out previously by Takahashi, Inoue &
Comptonization are provided by the optically-thick disk,
Dotani (2001). For this purpose, it is essential to sort
as proposed previously (Zdziarski et al. 1999; Gilfanov
the broad-band spectra, particularly those of the HXD,
et al. 1999; Revnivtsev et al. 2001). Although the bolo-
into two subsets, according to whether the instantaneous
metric luminosity of the directly-visible disk emission is
19+24% of the overall0.7–300 keV luminosity, the energy source count rate is higher or lower than a certain mean
−6 value. However, the HXD has a relatively small effec-
generationin the cooldisk is approximatelytwice higher,
tive area ( 230 cm2 at 100 keV; Takahashi et al. 2007c),
when the seed photon luminosity is added. Then, the en-
so the ave∼rage count rates of Cyg X-1 are 47 c s−1
ergy generation in the the cool disk becomes comparable
and 20 c s−1 in the 10–70 keV (with PIN) ∼and 70–400
to that in the hot corona.
∼
keV (with GSO) bands, respectively (subsection 2.1; fig-
While the compPSh component carries the dominant
ure 1c,d). Then, eveninthe PINrangewhere the NXB is
hard continuum, compPSs successfully explains the con-
relativelyminor,Poissonfluctuationsofsignalcountswith
tinuum steepening in energies below a few keV, or equiv-
1 s integration would become comparable to the intrin-
alently, the soft excess that is too hot to be explained
sic variations of Cyg X-1 ( 20% in root-mean-square
by the cool disk emission (Di Salvo et al. 2001). These ∼±
amplitude). Making the integration time longer would
twoComptoncontinuaareinferredtohaveopticaldepths
not help, because Cyg X-1 varies with an approximately
whichdifferbyafactorof3.9 0.5. Theirnormalizations,
± “white” power spectrum on time scales longer than a few
originallyexpressedasthe disk areaAsupplying the seed
seconds(e.g.,Belloni&Hasinger 1990),andhenceampli-
photons, have been converted in table 1 to its innermost
radius Rseed as tudes of the intrinsic and Poisson variations will scale in
in a similar manner when the integration time is increased.
No. ] 9
Fortunately, we can utilize the XIS for our purpose, statistical fluctuations, which amount to 8% (1 sigma)
because oneXIS sensorreceived 60 c s−1 evenafter the at the 1-s integration. The intensity-sorte∼d spectra from
∼
image center has been removed to avoid photon pile up XIS0 and XIS3, if to be utilized in the spectral fitting,
(subsection2.2;figure1a,b),andthevariationofCygX-1 wouldneedtobecorrectedforthiseffect. TheXIS2data,
isknownto be crudelysynchronizedacrossthe softX-ray in contrast, are free from this problem, because Poisson
to hard X-ray energies (Negoro et al. 1994). Since the errors from different detectors are independent.
present XIS data have a 1 s time resolution (subsection
4.2. Auto-correlation and cross-correlation functions
2.1),wecanjudgeeverysecond,referringtotheXISdata,
whetherthesourceisbrighterorfainterthanamean,and Before actually conducting the analysis described in
sorttheXISandHXDdataintotherequiredtwosubsets. section 4.1, we briefly perform auto-correlation function
In practice, we have constructed the following procedure (ACF) and cross-correlation function (CCF) analyses, in
to do this. order to examine whether the GTIs sorted in reference to
the energy band below 10 keV is really applicable to the
1. In the same way as in section 2.4, a 0.3–10 keV
extractionofvaryingspectralcomponents inharderener-
XIS0+XIS3 light curve C (with i the bin num-
{ i} gies. We utilize raw light curves with neither background
ber)isproducedthroughouttheobservation,witha
subtraction nor dead-time correction.
bin width of t 1 s.
b≥ Panels (a) and (b) of figure 6 show ACFs and CCFs
2. The calculationof C excludesthose1-stimebins
{ i} fromthepresentSuzakudata,respectively,calculatedem-
in which either XIS0 or XIS3 (or both) suffered
ploying a bin width of 1 s. They were calculated using
telemetry saturation.
data intervals each 256 s long, and then averaged over
3. Every T s (T t ), a mean XIS0+XIS3 count rate
C¯ is calculated≫frobm C . ensemble. The XIS telemetry saturation was treated in
{ i} the same way as item 2 of the previous subsection (using
4. Twosetsofgoodtimeintervals(GTIs),called“high-
t =1 s). Thus, the XIS vs. PIN andXIS vs. GSO CCFs
phase” (HP) and “low-phase” (LP), are defined, by b
both exhibit clear peaks at zero time lag, with correla-
judgingeveryt swhetherC ishigherthanC¯+nσ,
b i tion coefficients of 0.75. This ensures that the intrinsic
or lower than C¯ nσ, respectively. Here, σ is 1- ∼
− variation on a time scale of 1 s is well correlated over the
sigma Poisson error and n 0 is a discriminator
≥ entire energy range in which we are conducting our anal-
level.
ysis, and validate our method described in the previous
5. Using the same two GTIs, the XIS2 data are ac-
subsection.
cumulated into two spectra, called HP (high phase)
Although the XIS data do not have time resolution
andLP(lowphase)spectra. Thedataarediscarded
higher than 1 s, the HXD data afford it down to 61 µs
if C¯ nσ<C <C¯+nσ.
− i (Takahashi et al. 2007c; Terada et al. 2007). We calcu-
6. Employing the same two GTIs, the HXD-PIN data
lated ACFs and CCFs from the PIN (10–60 keV) and
aresortedintoHPandLPspectra;soaretheHXD-
GSO (60–200 keV) light curves accordingly, with 0.1 s
GSO data.
resolution and 409.6 s data intervals. The results, shown
7. Using the two sets of GTIs, two background data
in panels (c) and (d) of figure 6, reveal that the varia-
sets are accumulated for HP and LP, and are
tions are detected on shorter time scales, where the PIN
subtracted from the corresponding on-source HXD
vs GSO correlation is still peaked at zero-lag. The varia-
spectra.
tionsinthetwoenergybandsareagainwellsynchronized,
Thus, the procedure involves three adjustable parame- with the correlation coefficient of 1.
∼
ters,t ,T,andn. Amongthem,T shouldbelongenough In addition to the basic properties described above,
b
to suppress Poisson errors, but short enough to compen- these ACFs and CCFs reveal one interesting property:
sateforbackgroundchanges. Thethresholdnshouldalso namely, the left-to-right asymmetry in the CCFs, seen
be optimized, because a larger n leads to a decrease of on both time scales, in such a way that the correlation
live time, while a smaller n means that the data sorting is stronger toward hard-lag sense. This reconfirms the
is diluted by the Poissonfluctuation in C which is not previous report of the same phenomenon in early days
i
{ }
relatedtotheintrinsicsourcevariation. Here,forsimplic- by Nolan et al. (1981), then using Ginga by Miyamoto
ity, we have chosen t =1 s (the highest time resolution et al. (1991), and further with RXTE (e.g., Nowak et al.
b
of the XIS), T =200 s, and n=0 as our baseline condi- 1999a; Maccarone et al. 2000). On the other hand, the
tions. This is because a larger value of n, such as n=1, presentresultsdonotclearlyreconformanotherproperty,
was found to enhances the HP vs. LP ratio, but also to reported previously (e.g., Negoro et al. 1994; Feng, Li &
degradethedatastatistics,thushamperingustomoreac- Chen 1999; Maccarone et al. 2000), that ACFs in softer
curatelyseparatetheHPandLPmodelparameters. Since energies have wider correlation peaks, and hence longer
we have chosento use the XIS0+XIS3 data for the inten- correlation lengths. We leave this issue to future Suzaku
sityjudgement,wederivetheintensity-sortedXISspectra observationsusing higher XIS time resolution(employing
only from the other FI sensor, namely XIS2. This divi- so-calledPSUMmode);then,wewillalsobeabletostudy
sionoftaskisneededbecausethe intensity-sortedspectra how the spectrum differs between the rising and falling
from XIS0 and XIS3 would include those time bins when phases of “shots”, as suggested previously (Negoro et al.
their summed counts are higher (or lower) due simply to 1994).
10 [Vol. ,
Fig. 6. Auto-correlationfunctions(ACFs)andcross-correlationfunctions(CCFs)ofthepresentSuzakudata. TheeffectsofPoisson
errorswereremoved,andtheCCFswerenormalizedsothatthevalueatzerolagdirectlyrepresentthecorrelationcoefficient. Positive
time differences in CCFs mean that softer signals lead the harder ones. (a) ACFs from the XIS2 (0.7–10 keV; black), HXD-PIN
(10–60 keV; red), and HXD-GSO (60–200 keV; green) data, calculated using 1-s binned light curves. (b) CCFs of the PIN (red)
and GSO (green) data, calculated against the 0.7–10 keV XIS2 lightcurve inthe sameway as panel a. (c) ACFs calculated using
0.1-sbinnedPIN(10–60keV;red)andGSO(60–200keV;green)lightcurves. (d)CCFsoftheGSOlightcurveagainstthatofPIN,
calculatedinthesamewayaspanelc.
Fig. 7. Background-inclusive XIS (top: 0.7–10 keV), HXD-PIN (middle: 10–60 keV), and HXD-GSO (bottm: 60–200 keV) light
curves of Cyg X-1 with 1-s binning, sorted according to the instantaneous XIS0+XIS3 counts. Red and blue data bins represent
thosewhentheXIS0+XIS3counts,Ci,arehigherandlowerthanthelocal200saverage,C¯,respectively. Nocorrectionismadefor
theinstrumentaldeadtimes.
