Gas Slit Camera Research Articles

A soft X-ray lag detected in Centaurus A

Abstract We performed time-lag analysis on the X-ray light curves of Centaurus A (Cen A) obtained by the Gas Slit Camera (GSC) aboard the Monitor of All-sky X-ray Image (MAXI) in three energy bands (2–4 keV, 4–10 keV, and 10–20 keV). We discovered a soft X-ray lag relative to higher energies (soft lag) on a timescale of days in a flaring episode by employing the discrete correlation function (DCF) and the z-transformed discrete correlation function (ZDCF) method. In the episode, a peak and a centroid in the DCF and the ZDCF was observed at a soft lag of ∼ 5 d in 2–4 keV versus 4–10 keV and in 4–10 keV versus 10–20 keV, and ∼ 10 d in 2–4 keV versus 10–20 keV. We found it difficult to explain the observed X-ray variation by a single energy injection with the one-zone synchrotron self-Compton (SSC) model, in which the soft lags in these three energy bands reflect the different cooling times of the relativistic electrons, by assuming the magnetic field and minimum Lorentz factor estimated from a broad-band spectral energy distribution. Alternatively, if the phenomenon is interpreted as cooling of Comptonizing electrons in a corona covering the accretion disk, the temperature of the corona producing the variable X-rays should be ∼ 10 keV for reconciliation with the soft lag in the energy range of 2–20 keV.

Swift/BAT and MAXI/GSC broadband transient monitor

Abstract We present a newly developed broadband transient monitor using the Swift Burst Alert Telescope (BAT) and the MAXI Gas Slit Camera (GSC) data. Our broadband transient monitor keeps vigil for high-energy transient sources from 2 keV to 200 keV in seven energy bands by combining the BAT (15–200 keV) and the GSC (2–20 keV) data. Currently, daily and 90-minute (one orbit) averaged light curves are available for 106 high-energy transient sources. This transient monitor is available to the public through our web server, http://yoshidalab.mydns.jp/bat_gsc_trans_mon/, for wider use by the community. We discuss a daily sensitivity of our monitor and possible future improvements on our pipeline.

X-ray variability with spectral state transitions in NS-LMXBs observed with MAXI/GSC and Swift/BAT

Abstract X-ray variabilities with spectral state transitions in bright low-mass X-ray binaries containing a neutron star are investigated by using the one-day bin light curves of MAXI/GSC (Gas Slit Camera) and Swift/BAT (Burst Alert Telescope). Four sources (4U 1636−536, 4U 1705−44, 4U 1608−52, and GS 1826−238) exhibited small-amplitude X-ray variabilities with spectral state transitions. Such “mini-outbursts” were characterized by smaller amplitudes (several times) and shorter duration (less than several tens of days) than those of “normal outbursts.” A theoretical model of disk instability by Mineshige and Osaki (PASJ, 37, 1, 1985) predicts both large-amplitude outbursts and small-amplitude variabilities. We interpret the normal outbursts as the former prediction of this model, and the mini-outbursts as the latter. Here, we can also call the mini-outburst a “purr-type outburst” referring to the theoretical work. We suggest that similar variabilities lasting for several tens of days without spectral state transitions, which are often observed in the hard state, may be repeats of mini-outbursts.

Orbital phase-resolved spectroscopy of 4U 1538−52 with MAXI

4U 1538-52, an absorbed high mass X-ray binary with an orbital period of 3.73 days, shows moderate orbital intensity modulations with a low level of counts during the eclipse. Several models have been proposed to explain the accretion at different orbital phases by a spherically symmetric stellar wind from the companion. The aim of this work is to study both the light curve and orbital phase spectroscopy of this source in the long term. Particularly, the folded light curve and the changes of the spectral parameters with orbital phase to analyse the stellar wind of QV Nor, the mass donor of this binary system. We used all the observations made from the Gas Slit Camera on board MAXI of 4U 1538-52 covering many orbits continuously. We obtained the good interval times for every orbital phase range which were the input to extract our data. We estimated the orbital period of the system and then folded the light curves and we fitted the X-ray spectra with the same model for every orbital phase spectrum. We also extracted the averaged spectrum of all the MAXI data available. The MAXI spectra in the 2-20 keV energy range were fitted with an absorbed Comptonization of cool photons on hot electrons. We found a strong orbital dependence of the absorption column density but neither the fluorescence iron emission line nor low energy excess were needed to fit the MAXI spectra. The variation of the spectral parameters over the binary orbit were used to examine the mode of accretion onto the neutron star in 4U 1538-52. We deduce a best value of $\dot{M}/v_\infty=0.65\times 10^{-9}$ $M_{\odot} \, yr^{-1}/(km \, s^{-1})$ for QV Nor.

Low-mass X-ray binary MAXI J1421−613 observed by MAXI GSC and Swift XRT

Monitor of All sky X-ray Image (MAXI) discovered a new outburst of an X-ray transient source named MAXI J1421−613. Because of the detection of three X-ray bursts from the source, it was identified as a neutron star low-mass X-ray binary. The results of data analyses of the MAXI GSC (Gas Slit Camera) and the Swift XRT (X-Ray Telescope) follow-up observations suggest that the spectral hardness remained unchanged during the first two weeks of the outburst. All the XRT spectra in the 0.5–10 keV band can be well explained by thermal Comptonization of multi-color disk blackbody emission. The photon index of the Comptonized component is ≈ 2, which is typical of low-mass X-ray binaries in the low/hard state. Since X-ray bursts have a maximum peak luminosity, it is possible to estimate the (maximum) distance from its observed peak flux. The peak flux of the second X-ray burst, which was observed by the GSC, is about 5 photons cm−2 s−1. By assuming a blackbody spectrum of 2.5 keV, the maximum distance to the source is estimated as 7 kpc. The position of this source is contained by the large error regions of two bright X-ray sources detected with Orbiting Solar Observatory-7 (OSO-7) in the 1970s. Besides this, no past activities at the XRT position are reported in the literature. If MAXI J1421−613 is the same source as (one of) these, the outburst observed with MAXI may have occurred after a quiescence of 30–40 years.

MAXIINVESTIGATION INTO THE LONG-TERM X-RAY VARIABILITY FROM THE VERY-HIGH-ENERGY γ-RAY BLAZAR Mrk 421

The archetypical very-high-energy gamma-ray blazar Mrk 421 was monitored for more than 3 years with the Gas Slit Camera onboard Monitor of All Sky X-ray Image (MAXI), and its longterm X-ray variability was investigated. The MAXI lightcurve in the 3 -- 10 keV range was transformed to the periodogram in the frequency range $f = 1 \times 10^{-8}$ -- $2 \times 10^{-6}$ Hz. The artifacts on the periodogram, resulting from data gaps in the observed lightcurve, were extensively simulated for variations with a power-law like Power Spectrum Density (PSD). By comparing the observed and simulated periodograms, the PSD index was evaluated as $\alpha = 1.60 \pm 0.25$. This index is smaller than that obtained in the higher frequency range ($f > 1 \times 10^{-5}$ Hz), namely, $\alpha = 2.14 \pm 0.06$ in the 1998 ASCA observation of the object. The MAXI data impose a lower limit on the PSD break at $f_{\rm b} = 5 \times 10^{-6}$ Hz, consistent with the break of $f_{\rm b} = 9.5 \times 10^{-6}$ Hz, suggested from the ASCA data. The low frequency PSD index of Mrk 421 derived with MAXI falls well within the range of the typical value among nearby Seyfert galaxies ($\alpha = 1$ -- $2$). The physical implications from these results are briefly discussed.

A new X-ray nova MAXI J1910−057 (= Swift J1910.2−0546) and mass accretion inflow

Abstract We report on long-term monitoring of a newly discovered X-ray nova, MAXI J1910−057 (= Swift J1910.2−0546) by MAXI and Swift. The new X-ray transient was first detected on 2012 May 31 by the MAXI Gas Slit Camera (GSC) and the Swift Burst Alert Telescope (BAT) almost simultaneously. We analyzed X-ray and UV data for 270 d following the outburst onset taken by repeated MAXI scans and Swift pointing observations. The obtained X-ray light curve for the inital 90 d is roughly represented by a fast-rise and exponential-decay profile. However, it re-brightened at ∼ 110 d after the onset and finally went down below both GSC and BAT detection limits after 240 d. All the X-ray energy spectra are fitted well with a model consisting of a multi-color-disk blackbody and its Comptonized hard tail. During the soft-state periods, the inner-disk radius of the best-fit model was almost constant. If the radius represents the innermost stable circular orbit of a non-spinning black hole and the soft-to-hard transitions occur at 1%–4% of the Eddington luminosity, the mass of the compact object is estimated to be > 2.9 M⊙ and the distance to be > 1.70 kpc. The inner-disk radius became larger in the hard/hard-intermediate state. This suggests that the accretion disk would be truncated. We detected an excess of the UV flux over the disk blackbody component extrapolated from the X-ray data, which can be modeled as reprocessed emission irradiated by the inner disk. We also found that the UV light curve mostly traced the X-ray curve, but a short dipping event was observed in both the UV and the X-ray bands with a 3.5 d X-ray time lag. This can be interpreted as the radial inflow of accreting matter from the outer UV region to the inner X-ray region.

MAXI observations of gamma-ray bursts

Abstract The Monitor of All-sky X-ray Image (MAXI) Gas Slit Camera (GSC) detects gamma-ray bursts (GRBs), including bursts with soft spectra, such as X-ray flashes (XRFs). MAXI/GSC is sensitive to the energy range from 2 to 30 keV. This energy range is lower than other currently operating instruments which are capable of detecting GRBs. Since the beginning of the MAXI operation on 2009 August 15, GSC observed 35 GRBs up to the middle of 2013. One third of them were also observed by other satellites. The rest of them show a trend to have soft spectra and low fluxes. Because of the contribution of those XRFs, the MAXI GRB rate is about three times higher than those expected from the BATSE log N–log P distribution. When we compare it to the observational results of the Wide-field X-ray Monitor on the High Energy Transient Explorer 2, which covers the the same energy range as that of MAXI/GSC, we find the possibility that many of the MAXI bursts are XRFs with Epeak lower than 20 keV. We discuss the source of soft GRBs observed only by MAXI. The MAXI log N–log S distribution suggests that the MAXI XRFs are distributed over a closer distance than hard GRBs. Since the distributions of the hardness of galactic stellar flares and X-ray bursts overlap with those of MAXI GRBs, we discuss the possibility of confusion of such galactic transients with the MAXI GRB samples.

Firm detection of a cyclotron resonance feature with Suzaku in the X-ray spectrum of GRO J1008−57 during a giant outburst in 2012

Abstract We report on the firm detection of a cyclotron resonance scattering feature (CRSF) in the X-ray spectrum of the Be X-ray binary pulsar, GRO J1008−57, achieved by the Suzaku Hard X-ray Detector during a giant outburst which was detected by the MAXI Gas Slit Camera in 2012 November. The Suzaku observation was carried out on 2012 November 20, outburst maximum when the X-ray flux reached ∼ 0.45 Crab in 4–10 keV, which corresponds to a luminosity of 1.1 × 1038 erg s−1 in 0.5–100 keV at 5.8 kpc. The obtained broadband X-ray spectrum from 0.5 keV to 118 keV revealed a significant absorption feature, considered as the fundamental CRSF, at ∼ 76 keV. This unambiguously reconfirms the previously suggested ∼ 80 keV spectral feature in GRO J1008−57. The implied surface magnetic field, 6.6 × 1012 G, is the highest among binary X-ray pulsars from which CRSFs have ever been detected.

Orbital phase resolved spectroscopy of GX 301-2 with MAXI

GX 301-2, a bright HMXB with an orbital period of 41.5 days, exhibits stable periodic orbital intensity modulations with a strong pre-periastron X-ray flare. Several models have been proposed to explain the accretion at different orbital phases, invoking accretion via stellar wind, equatorial disk, and accretion stream from the companion star. We present results from exhaustive orbital phase resolved spectroscopic measurements of GX 301-2 using data from the Gas Slit Camera onboard MAXI. Using spectroscopic analysis of the MAXI data with unprecendented orbital coverage for many orbits continuously, we have found a strong orbital dependence of the absorption column density and equivalent width of the iron emission line. A very large equivalent width of the iron line along with a small value of the column density in the orbital phase range 0.10-0.30 after the periastron passage indicates presence of high density absorbing matter behind the neutron star in these orbital phase range. A low energy excess is also found in the spectrum at orbital phases around the pre-periastron X-ray flare. The orbital dependence of these parameters are then used to examine the various models about mode of accretion onto the neutron star in GX 301-2.

THE 37 MONTH MAXI/GSC SOURCE CATALOG OF THE HIGH GALACTIC-LATITUDE SKY

We present the catalog of high Galactic-latitude ($|b|>10^{\circ}$) X-ray sources detected in the first 37-month data of Monitor of All-sky X-ray Image (MAXI) / Gas Slit Camera (GSC). To achieve the best sensitivity, we develop a background model of the GSC that well reproduces the data based on the detailed on-board calibration. Source detection is performed through image fit with the Poisson likelihood algorithm. The catalog contains 500 objects detected in the 4--10 keV band with significance of $s_{\rm D, 4-10 keV} \geq 7$. The limiting sensitivity is $\approx 7.5\times10^{-12}$ ergs cm$^{-2}$ s$^{-1}$ ($\approx 0.6$ mCrab) in the 4--10 keV band for 50% of the survey area, which is the highest ever achieved as an all-sky survey mission covering this energy band. We summarize the statistical properties of the catalog and results from cross matching with the Swift/BAT 70-month catalog, the meta-catalog of X-ray detected clusters of galaxies, and the MAXI/GSC 7-month catalog. Our catalog lists the source name (2MAXI), position and its error, detection significances and fluxes in the 4--10 keV and 3--4 keV bands, their hardness ratio, and basic information of the likely counterpart available for 296 sources.

Spectral Evolution of a New X-Ray Transient MAXI J0556−332 Observed by MAXI, Swift, and RXTE

Abstract We report on the spectral evolution of a new X-ray transient, MAXI J0556$-$ 332, observed by MAXI, Swift, and RXTE. The source was discovered on 2011 January 11 (MJD $ =$ 55572) by the MAXI Gas Slit Camera all-sky survey at ($ l$, $ b$) $ =$ (238.$ ^{\!\!\!\circ}$ 9, $-$ 25.$ ^{\!\!\!\circ}$ 2), relatively away from the Galactic plane. Swift/XRT follow-up observations identified it with a previously uncatalogued bright X-ray source, which led to optical identification. For more than one year since its appearance, MAXI J0556$-$ 332 has been X-ray active, with a 2–10 keV intensity above 30 mCrab. The MAXI/GSC data revealed rapid X-ray brightening in the first five days, and a hard-to-soft transition in the meantime. For the following $ \sim$ 70 days, the 0.5–30 keV spectra, obtained by the Swift/XRT and the RXTE/PCA on an almost daily basis, show a gradual hardening, with large flux variability. These spectra are approximated by a cutoff power-law with a photon index of 0.4–1 and a high-energy exponential cutoff at 1.5–5 keV, throughout the initial 10 months where the spectral evolution is mainly represented by a change of the cutoff energy. To be more physical, the spectra are consistently explained by thermal emission from an accretion disk plus Comptonized emission from a boundary layer around a neutron star. This supports the source identification as being a neutron-star X-ray binary. The obtained spectral parameters agree with those of neutron-star X-ray binaries in the soft state, whose luminosity is higher than 1.8 $ \times$ 10$ ^{37}$ erg s$ ^{-1}$. This suggests a source distance of $ >$ 17 kpc.

Observation of a muon excess following a gamma-ray burst event detected at the International Space Station

On April 24, 2012, at 16:47:14 UT, the Gas Slit Camera (GSC) of the Japanese Monitor of All-sky X-ray Image (MAXI) instrument on the International Space Station detected a short x-ray transient lasting about 34 seconds. The MAXI/GSC transient was most likely a gamma-ray burst (GRB), because of the high Galactic latitude, spectral hardness ratio, and the absence of known bright x-ray sources at the detected position. In addition, the MAXI/GSC transient GRB 120424A coordinates were in the field of view of the inclined Tupi muon telescope located at ground level (3 m above sea level) at ($22.9\ifmmode^\circ\else\textdegree\fi{}\text{ }\mathrm{W}$, $43.2\ifmmode^\circ\else\textdegree\fi{}\text{ }\mathrm{S}$) in the South Atlantic Anomaly region. We report here that the Tupi telescope registered a muon excess with a signal significance $6.2\ensuremath{\sigma}$ within the MAXI/GSC transient time period. Assuming a power law function with a spectral index of $\ensuremath{\gamma}=\ensuremath{-}1.54$ in the tail of the primary gamma-ray energy spectrum, we can conclude that the fluence obtained from the muon excess detected by the Tupi telescope is consistent with the preliminary value obtained by the MAXI team. This result agrees with an assumption that the muons were produced in photonuclear reactions in the Earth's atmosphere. In addition, we show also that the South Atlantic Anomaly region can be a favorable place at ground for the detection of the tail of the energy spectrum (the GeV counterpart) of some GRBs.

A Large X-Ray Flare from a Single Weak-Lined T Tauri Star TWA-7 Detected with MAXI GSC

Abstract We present a large X-ray flare from a nearby weak-lined T Tauri star TWA-7 detected with the Gas Slit Camera (GSC) on the Monitor of All-sky X-ray Image (MAXI). The GSC captured X-ray flaring from TWA-7 with a flux of 3 $\times$ 10 $^{-9}\ $ erg cm $^{-2}\ $ s $^{-1}$ in 2–20 keV band during the scan transit starting at 2010-09-07 18:24:30 (UT). The estimated X-ray luminosity at the scan in the energy band is 3 $\times$ 10 $^{32}\ $ erg s $^{-1}$ , indicating that the event is among the largest X-ray flares from T Tauri stars. Since MAXI GSC monitors a target only during a scan transit of about a minute per 92 min orbital cycle, the luminosity at the flare peak might have been higher than that detected. At the scan transit, we observed a high X-ray-to-bolometric luminosity ratio, log $\ L_{\rm X}/L_{\rm bol}$$=$$-$ 0.1 $^{+0.2}_{-0.3}$ ; i.e., the X-ray luminosity is comparable to the bolometric luminosity. Since TWA-7 has neither an accreting disk nor a binary companion, the observed event implies that none of those are essential to generate such big flares in T Tauri stars.

Peculiarly Narrow SED of GRB 090926B with MAXI and Fermi/GBM

Abstract The monitor of the all-sky X-ray image (MAXI) Gas Slit Camera (GSC) on the International Space Station (ISS) detected a gamma-ray burst (GRB) on 2009, September 26, GRB 090926B. This GRB had extremely hard spectra in the X-ray energy range. Joint spectral fitting with the Gamma-ray Burst Monitor on the Fermi Gamma-ray Space Telescope shows that this burst had a peculiarly narrow spectral energy distribution, which can be represented by a Comptonized blackbody model. This spectrum can be interpreted as photospheric emission from a low baryon-load GRB fireball. Calculating the parameter of the fireball, we found the size of the base of the flow to be $r_0$$=$ (4.3 $\ \pm\ $ 0.9) $\times$ 10 $^{9}\ Y'^{-3/2}\ $ cm, the Lorentz factor of the plasma is $\Gamma$$=$ (110 $\ \pm\ $ 10) $\ Y'^{\ 1/4}$ , where $Y'$ is a ratio between the total fireball energy and the energy in the blackbody component of the gamma-ray emission. This $r_0$ is a factor of a few times larger, and the Lorentz factor of 110 is smaller by also factor of a few than other bursts that have blackbody components in the spectra.

Long-Term Monitoring of the Black Hole Binary GX 339−4 in the High/Soft State during the 2010 Outburst with MAXI/GSC

We present the results of monitoring the galactic black hole candidate GX 339 $-$ 4 with the Monitor of All-sky X-ray Image (MAXI) / Gas Slit Camera in the high/soft state during an outburst in 2010. All of the spectra throughout the 8-month period were well reproduced with a model consisting of multi-color disk emission and its Comptonization component, whose fraction is $\leq\ $ 25% in the total flux. In spite of the flux variability over a factor of 3, the innermost disk radius is constant at $R_{\rm in}$$=$ 61 $\ \pm\ $ 2 km for an inclination angle of $i$$=$ 46 $^\circ$ and a distance of $d$$=$ 8 kpc. This $R_{\rm in}$ value is consistent with those of past measurements with Tenma in the high/soft state. Assuming that the disk extends to the innermost stable circular orbit of a non-spinning black hole, we estimate the black hole mass to be $M$$=$ 6.8 $\ \pm\ $ 0.2 $\ M_{\odot}$ for $i$$=$ 46 $^\circ$ and $d$$=$ 8 kpc, which is consistent with that estimated from the Suzaku observation of the previous low/hard state. Further combined with the mass function, we obtained a mass constraint of 4.3 $\ M_\odot$$<$$M$$<$ 13.3 $\ M_\odot$ for the allowed range of $d$$=$ 6–15 kpc and $i$$<$ 60 $^\circ$ . We also discuss the spin parameter of the black hole in GX 339 $-$ 4 by applying relativistic accretion disk models to the Swift/XRT data.

In-Orbit Performance of MAXI Gas Slit Camera (GSC) on ISS

Abstract We report on the in-orbit performance of the Gas Slit Camera (GSC) on the MAXI (Monitor of All-sky X-ray Image) mission carried on the International Space Station (ISS). Its commissioning operation, which started on 2009 August 8, confirmed the basic performances of the effective area in the energy band of 2–30 keV, the spatial resolution of the slit-and-slat collimator and detector with 1 $^\circ\!\!\!.$ 5 FWHM, the source visibility of 40–150 seconds for each scan cycle, and the sky coverage of 85% per 92-minute orbital period and 95% per day. The gas gains and read-out amplifier gains have been stable within 1%. The background rate is consistent with the past X-ray experiments operated at a similar low-earth orbit if its relation with the geomagnetic cutoff rigidity is extrapolated to high latitude. We also present the status of the in-orbit operation and a calibration of the effective area and the energy response matrix using Crab-nebula data.

Gas Slit Camera (GSC) onboard MAXI on ISS

Abstract The Gas Slit Camera (GSC) is an X-ray instrument on the MAXI (Monitor of All-sky X-ray Image) mission aboard the International Space Station. It is designed to scan the entire sky every 92-minute orbital period in the 2–30 keV band and to achieve the highest sensitivity among the X-ray all-sky monitors ever flown so far. The GSC employs large-area position-sensitive proportional counters with a total detector area of 5350 cm $^2$. The on-board data processor has functions to format telemetry data as well as to control the high voltage of the proportional counters so as to protect them from particle irradiation. This paper describes the instruments, on-board data processing, telemetry data formats, and performance specifications expected from ground calibration tests.

The First MAXI/GSC Catalog in the High Galactic-Latitude Sky

Abstract We present the first unbiased source catalog of the Monitor of All-sky X-ray Image (MAXI) mission at high Galactic latitudes ( $\vert b\vert$$>$ 10 $^{\circ}$ ), produced from the first 7-month data (2009 September 1 to 2010 March 31) of the Gas Slit Camera in the 4–10 keV band. We have developed an analysis procedure to detect faint sources from the MAXI data, utilizing a maximum likelihood image fitting method, where the image response, background, and detailed observational conditions are taken into account. The catalog consists of 143 X-ray sources above the 7 $\sigma$ significance level with a limiting sensitivity of $\sim\ $ 1.5 $\times$ 10 $^{-11}\ $ erg cm $^{-2}\ $ s $^{-1}$ (1.2 mCrab) in the 4–10 keV band. Among them, we have identified 38 Galactic/LMC/SMC objects, 48 galaxy clusters, 39 Seyfert galaxies, 12 blazars, and 1 galaxy. Four other sources are confused with multiple objects, and one remains unidentified. The log $\ N$ –log $\ S$ relation of extragalactic objects is in good agreement with the HEAO-1 A-2 result, although the list of the brightest AGNs in the entire sky has significantly changed since that produced 30 yr ago.

MAXI GSC Observations of a Spectral State Transition in the Black Hole Candidate XTE J1752–223

Abstract We present the first results on the black-hole candidate XTE J1752$-$223 from the Gas Slit Camera (GSC) aboard the Monitor of All-sky X-ray Image (MAXI) on the International Space Station. Including the onset of an outburst reported by the Proportional Counter Array aboard the Rossi X-ray Timing Explorer on 2009 October 23, MAXI / GSC has been monitoring this source approximately 10 times per day with high sensitivity in the 2–20 keV band. XTE J1752$-$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 was just above the critical gas-evaporation rate required for the state transition.