LETTER

PASJ: Publ. Astron. Soc. Japan 62, L27–L32, 2010 October 25
c 2010. Astronomical Society of Japan.


MAXI GSC Observations of a Spectral State Transitionin the Black Hole Candidate XTE J1752223
Satoshi NAKAHIRA,1 Kazutaka YAMAOKA,1 Mutsumi S UGIZAKI,2 Yoshihiro U EDA,3 Hitoshi N EGORO,4 Ken E BISAWA,5
Nobuyuki K AWAI,6,2 Masaru M ATSUOKA,2,7 Hiroshi T SUNEMI,8 Arata DAIKYUJI,9 Satoshi E GUCHI,3 Kazuo H IROI,3
Masaki I SHIKAWA,10 Ryoji I SHIWATA,4 Naoki I SOBE,3 Kazuyoshi K AWASAKI,7 Masashi K IMURA,8 Mitsuhiro KOHAMA,2,7
Tatehiro M IHARA,2 Sho M IYOSHI,4 Mikio M ORII,6 Yujin E. NAKAGAWA,11 Motoki NAKAJIMA,12 Hiroshi O ZAWA,4
Tetsuya S OOTOME,2 Kousuke S UGIMORI,6 Motoko S UZUKI,2 Hiroshi T OMIDA,7 Shiro U ENO,7 Takayuki YAMAMOTO,2
Atsumasa YOSHIDA,1,2 and the MAXI team
1 Department of Physics and Mathematics, Aoyama Gakuin University, 5-10-1 Fuchinobe, Chuo-ku Sagamihara 252-5258

nakahira@phys.aoyama.ac.jp
2 Coordinated Space Observation and Experiment Research Group, Institute of Physical and Chemical Research (RIKEN),

2-1 Hirosawa, Wako, Saitama 351-0198
3 Department of Astronomy, Kyoto University, Oiwake-cho, Sakyo-ku, Kyoto 606-8502
4 Department of Physics, Nihon University, 1-8-14 Kanda-Surugadai, Chiyoda-ku, Tokyo 101-8308
5 Department of Space Science Information Analysis, Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency,

3-1-1 Yoshinodai, Chuo-ku, Sagamihara 252-5210
6 Department of Physics, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8551
7 ISS Science Project Office, Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency,

2-1-1 Sengen, Tsukuba, Ibaraki 305-8505
8 Department of Earth and Space Science, Osaka University, 1-1 Machikaneyama, Toyonaka, Osaka 560-0043
9 Department of Applied Physics, University of Miyazaki, 1-1 Gakuen Kibanadai-nishi, Miyazaki, Miyazaki 889-2192
10 School of Physical Science, Space and Astronautical Science, The Graduate University for Advanced Studies (Sokendai),

3-1-1 Yoshinodai, Chuo-ku, Sagamihara 252-5210
11 High Energy Astrophysics Laboratory, Institute of Physical and Chemical Research (RIKEN), 2-1 Hirosawa, Wako, Saitama 351-0198
12 School of Dentistry at Matsudo, Nihon University, 2-870-1 Sakaecho-nishi, Matsudo, Chiba 101-8308(Received 2010 March 31; accepted 2010 June 28)

Abstract
We present the first results on the black-hole candidate XTE J1752223 from the Gas Slit Camera (GSC)
aboard the Monitor of All-sky X-ray Image (MAXI) on the International Space Station. Including the onset of an
outburst reported by the Proportional Counter Array aboard the Rossi X-ray Timing Explorer on 2009 October 23,
MAXI / GSC has been monitoring this source approximately 10 times per day with high sensitivity in the 2–20 keVband. XTE J1752223 was initially in a low / hard state during the first 3 months. An anti-correlated behavior
between the 2–4 keV and 4–20 keV bands was observed around 2010 January 20, indicating that the source exhibited a spectral transition to the high / soft state. A transient radio jet may have been ejected when the source was in
the intermediate state where the spectrum was roughly explained by a power-law with a photon index of 2.5–3.0. The
unusually long period in the initial low / hard state implies a slow variation in the mass-accretion rate, and a dramatic
soft X-ray increase may be explained by a sudden appearance of the accretion disk component with a relatively low
innermost temperature (0.4–0.7 keV). Such a low temperature might suggest that the maximum accretion rate wasjust above the critical gas-evaporation rate required for the state transition.
Key words: accretion, accretion disks — black hole physics — stars: individual (XTE J1752223) —X-rays: stars
1. Introduction
Galactic Black hole candidates (BHCs) are ideal objects
for studying how accretion disks and coronae evolve due tochanges in the accretion rate, thanks to their high photon statistics and frequent state transitions (see, e.g., McClintock &Remillard 2006 for a review). They are characterized by transient behavior, such as sudden X-ray brightening caused byinstabilities originating in the outer accretion disk. During
an outburst, which typically lasts a few months, they may go
through several distinct spectral states, initial low / hard state(LHS), intermediate or very high state (IMS / VHS), high / soft
state (HSS), LHS, and then go back to quiescence. These states
are believed to reflect changes in the geometry of the accretion
disk and corona (Esin et al. 1997), as well as jets (Markoff et al.
2001; Yuan et al. 2005). BHCs sometimes show powerful radio
jets in their outbursts in association with state transitions fromthe LHS to the HSS. Hence, studying the spectral transition
and the disk-jet interaction are important for clarifying the jet
production mechanisms (see, e.g., Fender 2006 for a review).
The new X-ray transient XTE J1752223 was first detected
at the 30 mCrab flux level in the 2–10 keV range on

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S. Nakahira et al.

[Vol. 62,

Fig. 1. MAXI / GSC false-color images during the period of December 20 to January 19 (upper panel) and January 20 to February 28 (lower panel).
Red: 2–4 keV, Green: 4–10 keV, Blue: 10–20 keV. 10ı radius area from XTE J1752223 is extracted, and the source clearly reddens because of thespectral transition. There are many bright X-ray sources near XTE J1752223 with a few hundreds of mCrab such as GX 9+1 and GX 3+1. Source andbackground regions used for the light curve analysis are shown by green circles on both images.

2009 October 23 during galactic bulge monitoring with the
Proportional Counter Array (PCA) of the Rossi X-ray Timing
Explorer (RXTE) (Markwardt et al. 2009a). On October 24,
the source also triggered the Swift Burst Alert Telescope (BAT)
at a 100 mCrab flux in the 15–50 keV range (Markwardt et al.
2009a). The position was determined to be (RA, Dec) =(17h52m 15:s 14, 22:ı 200 33:008) with a 500 uncertainty by followup Swift X-Ray Telescope (XRT) localization (Markwardt
et al. 2009b). The energy spectrum is consistent with a powerlaw with a photon index of 1.38˙0.01, and the power spectrum
shows band-limited noise with a total rms of 30%, suggestingthat the source was in a typical LHS (Shaposhnikov et al.
2009; Muñoz-Darias et al. 2010). The optical (Torres et al.
2009a), near-infrared (Torres et al. 2009b), and radio counterparts (Brocksopp et al. 2009) were also discovered within the
X-ray error circle during the initial phase. Possible doublesided jets were detected on 2010 February 11 (Brocksopp et al.
2010a, 2010b); however, the BH mass, inclination angle andthe distance have not yet been determined.
Since 2009 August 3, the highly sensitive all-sky
monitor, Monitor of All-sky X-ray Image, MAXI (Matsuoka
et al. 2009), has been activated on the Exposed Facilityof the Japanese Experimental Module “Kibo” of theInternational Space Station (ISS). Soon after the discovery of
XTE J1752223, when the MAXI was still in the commissioning phase, the source was found at the 30 mCrab level in an
X-ray image of the MAXI Gas Slit Camera (GSC) (Nakahira
et al. 2009). MAXI / GSC also observed a rapid spectral softening on 2010 January 22 due to a spectral transition from the
LHS to the HSS (Negoro et al. 2010). The MAXI initial results
are described in Matsuoka et al. (2010) and Ueno et al. (2010).

In this paper, we report on MAXI GSC observations of
XTE J1752223 during its outburst since 2009 October. In theobservation and result section, we demonstrate MAXI’s spectral performance for such a bright source. Finally, we discuss
the origin of the rather unusual behavior of XTE J1752223 in
the light curve and spectral hardness.
2. Observations and Results
MAXI carries two scientific instruments: the GSC andthe Solid State Camera (SSC). GSC consists of twelve onedimensional position-sensitive proportional counters operated
in the 2–20 keV range, while SSC is composed of 32 X-ray
CCD cameras with an energy range of 0.5–12 keV. GSC
observes two different directions (horizontal and zenithal direction) with an instantaneous field of view of 3ı  160ı, eachcovered by six cameras. It covers 70% of the whole sky inevery orbit 32 times per day at maximum. Both instruments
have been working properly in orbit, but four out of the twelve
GSC cameras have been turned off due to discharges in the
proportional counters. See Matsuoka et al. (2009) for moredetails concerning MAXI. In this paper, we use only the GSC
data, which offer a larger sky coverage and effective area thanthose of SSC.
Figure 1 shows two 1-month integrated X-ray sky images
with a radius of 10ı centered on XTE J1752223 in the 2–
20 keV band. These false-color images are produced by superposing red (2–4 keV), green (4–10 keV), and blue (10–20 keV)
images. Data from all of the counters were combined, andtwo images were made for two separate periods (from 2009
December 20, to 2010 January 19, and 2010 January 20 toMAXI/GSC Observations of XTE J1752223

L29

Fig. 2. MAXI light curve of XTE J1752223 in four energy ranges (2–20, 2–4, 4–10, and 10–20 keV from the top to the fourth panel) in comparison
with Swift / BAT (15–50 keV: the fifth panel). The hardness ratio between the 2–4 keV and the 4–10 keV bands is shown in the bottom panel. Right panel:
The period in the left panel when the plasma ejection was detected (January 21 as shown by the dashed line) is expanded.

2010 February 28). XTE J1752223 is clearly detected inboth GSC images. Because of its location near the Galactic
Center (l, b) = (6:ı 42, 2:ı 11), many bright X-ray sources, suchas GX 9+1, GX 51, and GX 3+1, are visible in this field.
The point spread function (PSF) of the MAXI / GSC is estimated to be about 2:ı 0 (FWHM) in this energy range. The
position of XTE J1752223, determined by GSC, is consistent
with the Swift / XRT localization.
We calculated the net source counts by subtracting the background from the on-source data. To avoid source contamination from the nearby bright sources GX 9+1 (2:ı 82 away)
and SAX J1748.92021 in the globular cluster NGC 6440
(Patruno et al. 2010; 2:ı 13 away), we carefully selected thesource and background regions as circles of 1:ı 17 and 2:ı 48
radius, respectively (see figure 1). The extracted source counts
were normalized by dividing by the total exposure (in units of
cm2 s) obtained with a time integral of the collimator effective area. The Crab Nebula was analyzed in the same way,
and the count rates of XTE J1752223 were normallized by

the Crab unit (1.30, 1.25, and 0.32 counts cm2 s1 in the 2–4,
4–10, and 10–20 keV range). The thus-obtained 1-day averaged light curve of XTE J1752223 is shown in figure 2. GSC
scans were performed approximately 10 times per day. A data
gap from December 7th to 23th was due to a solar avoidance limitation (the current solar protection is set at above 4ı
from the Sun).
The X-ray light curve shows the following notable features.
There are two initial plateau phases that lasted for  25 d and 40 d, respectively, and the spectral hardness between them isslightly different: the second phase (Phase D in figure 2) hasa softer spectrum than the first one (Phase B). The source could
not be observed during phase D by any other X-ray instruments, except for RXTE / ASM and Swift / BAT due to the Sun
constraints. The time scales of rising phases A and C wereabout 5 d and 15 d, respectively. Another notable feature isan anti-correlated behavior around the peak. The 2–4 keV flux
rapidly increased after 2010 January 20 (MJD: 55216), while
fluxes in the other two bands and the Swift / BAT 15–50 keV

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S. Nakahira et al.

Fig. 3. Hardness–intensity diagram during the outburst of XTE
J1752223. Data points during the six phases (A to H) are shownby the different symbols; 6 hr of integrations were used in Phase E,
F, and G, while one-day integrations were used for the other phases.
The spectral evolution generally moves in the direction shown by thearrows. Filled diamonds show the data when increased radio emissions,presumably jets, were detected on 2010 January 21.

flux decreased (Phase E for  9 d). The anti-correlation
between the soft and hard bands indicates a spectral transition. This fact means that it took a very long time of  90 dfor the source to complete the transition after the onset of an
outburst. The 2–20 keV flux reached a peak on 2010 January 23(MJD: 55219) at about the 430 mCrab level, and then gradually decreased (Phase F). The emissions during Phase F were
dominated by the soft bands (<4 keV) (see also the lower panelof figure 1). Around March 28 (MJD: 55283), hard X-rays
and the hardness ratio again increased (Phase G and H). The
outburst had continued at the time of writing this paper (theend of 2010 May), and the current duration for the outburst isabout 210 d.
Figure 3 shows a hardness–intensity diagram for thecurrently on-going outburst in XTE J1752223. Because of
the anti-correlated behavior between the soft and hard X-rays
during Phases E and G, the data points track the Q-shaped
curve, i.e., hysteresis behavior, as seen in other BHCs (Homan& Belloni 2005). The two plateau phases B and D correspond to the LHS, while the softer phase F corresponds with
the HSS. Thus, the state transition from the LHS to the HSS
can be clearly identified with the MAXI / GSC data alone.
A color–color diagram is also shown in figure 4. A line isplotted for a power-law spectral model with a photon index (Γ)
ranging from 1.0 to 5.0 modified by the absorption (equivalenthydrogen column density NH = 0.72  1022 cm2 ; MuñozDarias et al. 2010). Note that the current GSC energy response
gives systematic uncertainties of about 0.5  1022 cm2 and
0.1 in NH and Γ, respectively. As the outburst proceeds, the Γcontinuously varies from  1.5 to  5.0. The spectral shapes
during Phases B + D and F is approximated by a power-law
with photon indices of 1.5–2.0 and above 3.5, respectively.
These estimated photon indices are roughly consistent withtypical values found from BHCs in the LHS (Phase B + D)
and the HSS (Phase F). The power-law index in Phase E[Vol. 62,

Fig. 4. Color–color diagram during the outburst of XTE J1752223.
The same symbols are used in figure 3. The dashed line represents the
spectral shapes of an absorbed power-law with a photon index ranging
from 1.0 to 5.0. The open triangles correspond to photon indices from
1.0 to 5.0 (0.5 step), from upper-right to lower-left.

shows an intermediate value (2.0–3.0) between those seen in
LHS and HSS. An RXTE-pointed observation carried out on2010 January 19 (MJD: 55215) during Phase E shows that thesource was at least in the IMS (Shaposhnikov 2010). Detailed
spectral parameters and their evolution in this outburst will
be reported in a forthcoming paper (K. Yamaoka et al. 2010,
in preparation).
3. Discussions
We have presented the first results on the black-holecandidate XTE J1752223 in the current outburst from the
MAXI / GSC on ISS. Judging from the light curve and spectral hardness, we have found that the source was initially in
the LHS with two long plateau phases (B and D). After a longstay ( 90 d) in the LHS, it exhibited a spectral transition via
the IMS (Phase E) into the HSS at around 2010 January 22(Phase F). The source came back to the LHS at around 2010
April 3 (Phase H), and the source flux is gradually declining
toward the quiescent state.
It should be noted that a relatively high level of radio emission was detected by the Australia Telescope Compact Array(ATCA) on 2010 January 21 (Brocksopp et al. 2010a), justcorresponding to the beginning of the spectral transition fromthe LHS to the HSS. The GSC spectrum on this date is roughly
represented by a power-law with a photon index of 2.5–3.0 (seefigure 4), suggesting that the source was probably in the IMS.
Later, two separate radio sources were identified with highresolution imaging by the European VLBI Network (e-EVN)(Brocksopp et al. 2010b). If we interpret it as being plasmaejection, it is consistent with the scenario that large-scale jets
are produced in association with the spectral transition fromthe LHS to the HSS. Another important result is that the luminosities during the transition from the HSS to the LHS, andfrom the LHS to the HSS were not the same: the hard-to-soft

MAXI/GSC Observations of XTE J1752223

transition occurred at a 2–20 keV flux higher by a factor of
about 7 than that at the soft-to-hard transition. Such hysteresis
behavior was first noticed for GS 112468 and GX 3394(Miyamoto et al. 1995), and has been well established formany BHCs. This means that the spectral transition is notcontrolled by the mass-accretion rate alone, but depends on
its history. Most of the BHCs show a rapid flux rise withina few days, which causes a near-instantaneous spectral transition (Gierliński & Newton 2006), and then their fluxes decay
with an e-folding time of several ten days into quiescence(some BHCs show a secondary flare). It is known that the spectral state begins with the initial LHS, and then evolves throughthe VHS / IMS, HSS, and LHS during an outburst. However,about 8 BHCs, such as XTE J1118+480 and GRO J0422+32,
remained in the LHS throughout their previous outbursts(Brocksopp et al. 2001). The very long duration of theinitial LHS in XTE J1752223 and the two long plateau
phases are rather uncommon for the BHC outbursts. MeyerHofmeister (Meyer-Hofmeister 2004) points out that systems
with short orbital periods, like XTE J1118+480 (4.1 hr) andGRO J0422+32 (5.1 hr), tend to have low peak luminosities
in their outbursts (and hence do not show state transition)
because smaller accretion disks have less accumulated matter.
The long LHS observed in XTE J1752223 might be related
to a short orbital period. From our observations, however,
XTE J1752223 also resembles GX 3394, which remainedin the LHS for a long period before a transition into the HSS(McClintock & Remillard 2006). GX 3394 has a relatively
long orbital period (1.7 d) among BHCs categorized in lowmass X-ray binaries, indicating a larger accretion-disk size. In
this case, the unusually long duration of the LHS before thestate transition would result from a gradual change in the accretion rate. It is still unclear what causes the slow increase of
the accretion rate, in contrast to other X-ray novae. Therefore,
determining the orbital period of XTE J1752223 should be
important to clarify the nature of the long LHS period.
Let us explain the overall picture of the light curve of the
XTE J1752223 outburst. We never assume the generally
accepted disk-truncation scenario in the LHS, though someother scenarios have been proposed for the LHS, e.g., Liu et al.
2007, where the innermost radius of the disk (Rin ) is basicallydetermined by the balance between the accretion rate and thegas evaporation rate (Meyer et al. 2000; Liu et al. 2002; Doneet al. 2007). In this scenario, the LHS is the state where theaccretion disk is truncated at a larger radius than the Innermost
Stable Circular Orbit (ISCO = 3 Schwarzschild radii for a nonrotating BH), due to a low accretion rate. As the accretion rateincreases (then begins in an outer part of the accretion disk),
Rin decreases. When the radius decreases, the corona aroundthe disk cools down via efficient inverse Compton scattering of
the soft disk photons, and exhibits softer X-ray spectra. Thus,
the two phases in the LHS (B and D) can be interpreted witha slightly higher accretion rate in Phase D than that in Phase B.

L31

Furthermore, when the accretion rate eventually exceeds a critical rate required for the state transition, the gas evaporation
will not be strong enough to truncate the disk. Thus, Rinreaches the ISCO, and intense thermal radiation from the accretion disk, i.e., an ultra-soft component, appears in soft X-rays(Phase F in the HSS). The disk truncation scenario does notpredict any time evolution of Rin during the HSS. As can beseen in the light curve with a very smooth decay during the
HSS, the local accretion rate smoothly decreases with time.
When it falls below the critical rate again, the source comesback to the LHS (Phase H).
The significant increase mainly in the 2–4 keV band suggeststhat the disk innermost temperature (kTin ) is relatively low.
Preliminary fits of the MAXI / GSC data with the disk blackbody model (Mitsuda et al. 1984) during Phase F revealed that
kTin was 0.4–0.7 keV, including a systematic uncertainty of
0.1 keV. The Swift / XRT observation taken on 2010 February 4
also showed that the blackbody temperature is about 0.5 keV(Curran et al. 2010). This temperature is lower than those ofother BHCs, such as GRO J165540 and XTE J1550564,
whose kTin in the HSS exceeds 1 keV (Gierliński & Done
2004). The variation of kTin is determined by how much theaccretion rate exceeds the critical rate, and hence a low kTin
indicates that the accretion rate in the HSS is just above thecritical rate. It is known that the spectral transition from the
LHS to the HSS will occur at around 10%–20% Eddington
luminosity, but with large uncertainties (Wu et al. 2010).
The kTin for the Schwarzschild BH can be estimated to be 1.2 keV (M=10 Mˇ)1=4 (=1.0)1=4(h=1.7) (e.g., Makishima
et al. 2000), where h is the spectral hardening factor (Shimura& Takahara 1995),  is the Eddington ratio (L=LE ), and M
is the mass of the central object. Assuming that the maximum
kTin is 0.6–0.7 keV and  is 0.1–0.2, the black-hole mass canbe roughly estimated at 8.6–32 Mˇ . The GSC flux in the
2–10 keV range at the outburst peak (2010 January 23) was
9.7  109 erg cm2 s1 with a  20% absolute flux uncertainty. Assuming that (1) the 2–10 keV flux is 21%–28% of thebolometric flux based on the disk blackbody model, and that(2) the Eddington luminosity is 1.5  1038 (M=Mˇ) erg s1 ,the distance to the source can be estimated as being 5–10 kpc.
The authors are indebted to many other members of the
MAXI team and the ISS / MAXI operation team for theirefforts during the data acquisition and the daily MAXI operation. This research was partially supported by the Ministryof Education, Culture, Sports, Science and Technology(MEXT), Grant-in-Aid No. 19047001, 20041008, 20540230,
20244015, 21340043, 21740140, 22740120, and GlobalCOE from MEXT “The Next Generation of Physics, Spunfrom Universality and Emergence” and “Nanoscience andQuantum Physics”. One of the authors (K. Y.) is grateful to
Dr. T. Kawaguchi and Dr. P. Gandhi for valuable discussions.
The authors also acknowledge the Swift / BAT team for making
the hard X-ray transient monitor results publicly available.1

1

hhttp://heasarc.gsfc.nasa.gov/docs/swift/results/transients/i.

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