arXiv subjects

Kazutaka Yamaoka

Publications and source records attributed to Kazutaka Yamaoka.

At least 55 records · Page 3Linked to original sources

arXiv2014

Spectral and Timing Properties of the Black Hole X-ray Binary H 1743-322 in the Low/hard State Studied with Suzaku

We report on the results from Suzaku observations of the Galactic black hole X-ray binary H 1743-322 in the low/hard state during its outburst in 2012 October. We appropriately take into account the effects of dust-scattering to accurately analyze the X-ray spectra. The time-averaged spectra in the 1-200 keV band are dominated by a hard power-law component of a photon index of \approx 1.6 with a high-energy cutoff at \approx 60 keV, which is well described with Comptonization of the disk emission by hot corona. We estimate the inner disk radius from the multi-color disk component, and find that it is 1.3-2.3 times larger than the radius in the high/soft state. This suggests that the standard disk was not extended to the innermost stable circular orbit. A reflection component from the disk is detected with R = Ω/2π\approx 0.6 (Ωis the solid angle). We also successfully estimate the stable disk component in a way independent of the time-averaged spectral modeling, by analyzing short-term spectral variability on the \sim 1-sec timescale. A weak low-frequency quasi-periodic oscillation (LF QPO) at 0.1-0.2 Hz is detected, whose frequency is found to correlate with the X-ray luminosity and photon index. This result may be explained by the evolution of the disk truncation radius.

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arXiv2014

Synchrotron Self-inverse Compton Radiation From Reverse-shock on GRB120326A

We present multi-wavelength observations of a typical long duration GRB 120326A at $z=1.798$, including rapid observations using a submillimeter array (SMA), and a comprehensive monitoring in X-ray and optical. The SMA observation provided the fastest detection to date among seven submillimeter afterglows at 230 GHz. The prompt spectral analysis, using Swift and Suzaku yielded a spectral peak energy of $E^{\rm src}_{\rm peak}=107.8^{+15.3}_{-15.3}$ keV and equivalent isotropic energy of $E_{\rm iso}$ as $3.18^{+0.40}_{-0.32}\times 10^{52}$ erg. The temporal evolution and spectral properties in the optical were consistent with the standard forward shock synchrotron with jet collimation ($6^{\circ}.69\pm0^{\circ}.16$). The forward shock modeling using a 2D relativistic hydrodynamic jet simulation also determined the reasonable burst explosion and the synchrotron radiation parameters for the optical afterglow. The X-ray light curve showed no apparent jet break and the temporal decay index relation between the X-ray and optical ($α{\rm o}-α_{X}=-1.45\pm0.10$) indicated different radiation processes in the X-ray and optical. Introducing synchrotron self-inverse Compton radiation from reverse shock is a possible solution, and the detection and the slow decay of the afterglow in submillimeter supports that this is a plausible idea. The observed temporal evolution and spectral properties as well as forward shock modeling parameters, enabled to determine reasonable functions to describe the afterglow properties. Because half of events share similar properties in the X-ray and optical to the current event, GRB120326A will be a benchmarks with further rapid follow-ups, using submillimeter instruments such as SMA and ALMA.

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arXiv2013

The CALET Gamma-ray Burst Monitor (CGBM)

The CALET Gamma-ray Burst Monitor (CGBM) is the secondary scientific instrument of the CALET mission on the International Space Station (ISS), which is scheduled for launch by H-IIB/HTV in 2014. The CGBM provides a broadband energy coverage from 7 keV to 20 MeV, and simultaneous observations with the primary instrument Calorimeter (CAL) in the GeV - TeV gamma-ray range and Advanced Star Camera (ASC) in the optical for gamma-ray bursts (GRBs) and other X-gamma-ray transients. The CGBM consists of two kinds of scintillators: two LaBr$_3$(Ce) (7 keV - 1 MeV) and one BGO (100 keV - 20 MeV) each read by a single photomultiplier. The LaBr$_3$(Ce) crystal, used in space for the first time here for celestial gamma-ray observations, enables GRB observations over a broad energy range from low energy X-ray emissions to gamma rays. The detector performance and structures have been verified using the bread-board model (BBM) via vibration and thermal vacuum tests. The CALET is currently in the development phase of the proto-flight model (PFM) and the pre-flight calibration of the CGBM is planned for August 2013. In this paper, we report on the current status and expected performance of CALET for GRB observations.

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arXiv2013

Accretion disk and ionized absorber of the 9.7-hour dipping black hole binary MAXI J1305-704

We report the results from X-ray studies of the newly discovered black hole candidate MAXI J1305-704 based on Suzaku and Swift observations in the low/hard and high/soft states, respectively. The long Suzaku observation shows two types of clear absorption dips, both of which recur on a dip interval of 9.74 +- 0.04 hours, which we identify with the orbital period. There is also partially ionized absorption in the non-dip (persistent) emission in both the high/soft state and, very unusually, the low/hard state. However, this absorption (in both states) has substantially lower ionization than that seen in other high inclination systems, where the material forms a homogeneous disk wind. Here instead the absorption is most probably associated with clumpy, compact structures associated with the dipping material, which we see uniquely in this source likely because we view it at a very large inclination angle. A large inclination angle is also favored, together with a low black hole mass, to explain the high disk temperature seen in the fairly low luminosity high/soft state, as Doppler boosting enhances the disk temperature at high inclination. The disk radius inferred from these data is significantly smaller than that of the soft component seen in the low/hard state, supporting models where the disk is truncated at low luminosities. We find, however, that the lack of variability power on time scales of ~50 sec in the Suzaku low/hard state data is difficult to explain, even with a low mass black hole.

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arXiv2013

Swift and RXTE observations of the black hole transient GX 339-4 in the hard state between outbursts

We use simultaneous Swift and RXTE observations of the black hole binary GX 339-4 to measure the inner radius of its accretion disk in the hard state down to 0.4% L_{Edd} via modeling of the thermal disk emission and the relativistically broadened iron line. For the luminosity range covered in this work, our results rule out a significantly truncated disk at 100-1000 R_g as predicted by the advection-dominated accretion flow paradigm. The measurements depend strongly on the assumed emission geometry, with most results providing no clear picture of radius evolution. If the inclination is constrained to roughly 20 degrees, however, the measurements based on the thermal disk emission suggest a mildly receding disk at a luminosity of 0.4% L_{Edd}. The iron abundance varies between roughly 1-2 solar abundances, with the i=20 degrees results indicating a negative correlation with luminosity, though this is likely due to a change in disk illumination geometry.

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arXiv2013

The 37-month MAXI/GSC source catalog in 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.

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arXiv2013

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

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 MAXI Gas Slit Camera all-sky survey at (l,b)=(238.9deg, -25.2deg), relatively away from the Galactic plane. Swift/XRT follow-up observations identified it with a previously uncatalogued bright X-ray source and 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 ~ 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 a Comptonized emission from a boundary layer around a neutron star. This supports the source identification as 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 1.8x10^37 erg s^-1. This suggests a source distance of >17 kpc.

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arXiv2012

Slow and Fast Transitions in the Rising Phase of Outbursts from NS-LMXB transients, AqlX-1 and 4U1608-52

We analyzed the initial rising behaviors of X-ray outbursts from two transient low-mass X-ray binaries (LMXBs) containing a neutron-star (NS), Aql X-1 and 4U 1608-52, which are continuously being monitored by MAXI/GSC in 2--20 keV, RXTE/ASM in 2--10 keV, and Swift/BAT in 15--50 keV. We found that the observed ten outbursts are classified into two types by the patterns of the relative intensity evolutions in the two energy bands below/above 15 keV. One type behaves as the 15--50 keV intensity achieves the maximum during the initial hard-state period and drops greatly at the hard-to-soft state transition. On the other hand, the other type does as both the 2--15 keV and the 15--50 keV intensities achieve the maximums after the transition. The former have the longer initial hard-state ($\gtrsim$ 9 d) than the latter's ($\ltsim$5 d). Therefore, we named them as slow-type (S-type) and fast-type (F-type), respectively. These two types also show the differences in the luminosity at the hard-to-soft state transition as well as in the average luminosity before the outburst started, where the S-type are higher than the F-type in the both. These results suggest that the X-ray radiation during the pre-outburst period, which heats up the accretion disk and delays the disk transition (i.e., from a geometrically thick disk to a thin one), would determine whether the following outburst becomes S-type or F-type. The luminosity when the hard-to-soft state transition occurs is higher than $\sim 8 \times10^{36}$ erg s$^{-1}$ in the S-type, which corresponds to 4% of the Eddington luminosity for a 1.4 \Mo NS.

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arXiv2012

Energetic Fermi/LAT GRB100414A: Energetic and Correlations

This study presents multi-wavelength observational results for energetic GRB100414A with GeV photons. The prompt spectral fitting using Suzaku/WAM data yielded spectral peak energies of E^src_peak of 1458.7 (+132.6, -106.6) keV and Eiso of 34.5(+2.0, -1.8) x 10^52 erg with z=1.368. The optical afterglow light curves between 3 and 7 days were effectively fitted according to a simple power law with a temporal index of alpha=-2.6 +/- 0.1. The joint light curve with earlier Swift/UVOT observations yields a temporal break at 2.3 +/- 0.2 days. This was the first \fermi/LAT detected event that demonstrated the clear temporal break in the optical afterglow. The jet opening angle derived from this temporal break was 5.8 degree, consistent with those of other well-observed long gamma-ray bursts (GRBs). The multi-wavelength analyses in this study showed that GRB100414A follows E^src_peak-Eiso and E^src_peak-E_gamma correlations. The late afterglow revealed a flatter evolution with significant excesses at 27.2 days. The most straightforward explanation for the excess is that GRB100414A was accompanied by a contemporaneous supernova. The model light curve based on other GRB-SN events is marginally consistent with that of the observed lightcurve.

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arXiv2012

Outburst of LS V+44 17 Observed by MAXI and RXTE, and Discovery of a Dip Structure in the Pulse Profile

We report on the first observation of an X-ray outburst of a Be/X-ray binary pulsar LS V +44 17/RX J0440.9+4431, and the discovery of an absorption dip structure in the pulse profile. An outburst of this source was discovered by MAXI GSC in 2010 April. It was the first detection of the transient activity of LS V +44 17 since the source was identified as a Be/X-ray binary in 1997. From the data of the follow-up RXTE observation near the peak of the outburst, we found a narrow dip structure in its pulse profile which was clearer in the lower energy bands. The pulse-phase-averaged energy spectra in the 3$-$100 keV band can be fitted with a continuum model containing a power-law function with an exponential cutoff and a blackbody component, which are modified at low energy by an absorption component. A weak iron K$α$ emission line is also detected in the spectra. From the pulse-phase-resolved spectroscopy we found that the absorption column density at the dip phase was much higher than those in the other phases. The dip was not seen in the subsequent RXTE observations at lower flux levels. These results suggest that the dip in the pulse profile originates from the eclipse of the radiation from the neutron star by the accretion column.

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arXiv2011

Combined Spectral and Timing Analysis of the Black Hole Candidate MAXI J1659-152 Discovered by MAXI and Swift

We report on X-ray spectral and timing results of the new black hole candidate (BHC) MAXI J1659-152 with the orbital period of 2.41 hours (shortest among BHCs) in the 2010 outburst from 65 Rossi X-ray Timing Explorer (RXTE) observations and 8 simultaneous Swift and RXTE observations. According to the definitions of the spectral states in Remillard & McClintock (2006), most of the observations have been classified into the intermediate state. All the X-ray broadband spectra can be modeled by a multi-color disk plus a power-law with an exponential cutoff or a multi-color disk plus a Comptonization component. During the initial phase of the outburst, a high energy cutoff was visible at 30-40 keV. The innermost radius of the disk gradually decreased by a factor of more than 3 from the onset of the outburst and reached a constant value of 35 d_10 cos i^-1/2 km, where d_10 is the distance in units of 10 kpc and $i$ is the inclination. The type-C quasi-periodic oscillation (QPO) frequency varied from 1.6 Hz to 7.3 Hz in association with a change of the innermost radius, while the innermost radius remained constant during the type-B QPO detections at 1.6-4.1 Hz. Hence, we suggest that the origin of the type-B QPOs is different from that of type-C QPOs, the latter of which would originate from the disk truncation radius. Assuming the constant innermost radius in the latter phase of the outburst as the innermost stable circular orbit, the black hole mass in MAXI J1659-152 is estimated to be 3.6-8.0 M_solar for a distance of 5.3-8.6 kpc and an inclination angle of 60-75 degrees.

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arXiv2011

Spectral Evolutions in Gamma-Ray Burst Exponential Decays Observed with Suzaku WAM

This paper presents a study on the spectral evolution of gamma-ray burst (GRB) prompt emissions observed with the Suzaku Wide-band All-sky Monitor (WAM). By making use of the WAM data archive, 6 bright GRBs exhibiting 7 well-separated fast-rise-exponential-decay (FRED) shaped light curves are presented and the evaluated exponential decay time constants of the energy-resolved light curves from these FRED peak light curves are shown to indicate significant spectral evolution. The energy dependence of the time constants is well described with a power-law function tau(E) ~ E^gamma, where gamma ~ -(0.34 +/- 0.12) in average, although 5 FRED peaks show consistent value of gamma = -1/2 which is expected in synchrotron or inverse-Compton cooling models. In particular, 2 of the GRBs were located with accuracy sufficient to evaluate the time-resolved spectra with precise energy response matrices. Their behavior in spectral evolution suggests two different origins of emissions. In the case of GRB081224, the derived 1-s time-resolved spectra are well described by a blackbody radiation model with a power-law component. The derived behavior of cooling is consistent with that expected from radiative cooling or expansion of the emission region. On the other hand, the other 1-s time-resolved spectra from GRB100707A is well described by a Band GRB model as well as with the thermal model. Although relative poor statistics prevent us to conclude, the energy dependence in decaying light curve is consistent with that expected in the former emission mechanism model.

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arXiv2011

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 (GSC) in the high/soft state during the outburst in 2010. All the spectra throughout the 8-month period are well reproduced with a model consisting of multi-color disk (MCD) emission and its Comptonization component, whose fraction is <= 25% in the total flux. In spite of the flux variability over a factor of 3, the innermost disk radius is constant at R_in = 61 +/- 2 km for the inclination angle of i = 46 deg and the distance of d=8 kpc. This R_in value is consistent with those of the 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 +/- 0.2 M_sun for i = 46 deg 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 obtain the mass constraint of 4.3 M_sun < M < 13.3 M_sun for the allowed range of d = 6-15 kpc and i < 60 deg. 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.

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arXiv2011

A Spectral Study of the Black Hole Candidate XTE J1752-223 in the High/Soft State with MAXI, Suzaku and Swift

We report on the X-ray spectral analysis of the black hole candidate XTE\ J1752--223 in the 2009--2010 outburst, utilizing data obtained with the MAXI/Gas Slit Camera (GSC), the Swift/XRT, and Suzaku, which work complementarily. As already reported by Nakahira et al. (2010) MAXI monitored the source continuously throughout the entire outburst for about eight months. All the MAXI/GSC energy spectra in the high/soft state lasting for 2 months are well represented by a multi-color disk plus power-law model. The innermost disk temperature changed from $\sim$0.7 keV to $\sim$0.4 keV and the disk flux decreased by an order of magnitude. Nevertheless, the innermost radius is constant at $\sim$41 $D_{3.5}(\cos{\it i})^{-1/2}$ km, where $D_{3.5}$ is the source distance in units of 3.5 kpc and $i$ the inclination. The multi-color disk parameters obtained with the MAXI/GSC are consistent with those with the Swift/XRT and Suzaku. The Suzaku data also suggests a possibility that the disk emission is slightly Comptonized, which could account for broad iron-K features reported previously. Assuming that the obtained innermost radius represents the innermost stable circular orbit for a non-rotating black hole, we estimate the mass of the black hole to be 5.51$\pm$0.28 $M_{\odot}$ $D_{3.5}(\cos{\it i})^{-1/2}$, where the correction for the stress-free inner boundary condition and color hardening factor of 1.7 are taken into account. If the inclination is less than 49$^{\circ}$ as suggested from the radio monitoring of transient jets and the soft-to-hard transition in 2010 April occurred at 1--4% of Eddignton luminosity, the fitting of the Suzaku spectra with a relativistic accretion-disk model derives constraints on the mass and the distance to be 3.1--55 $M_{\odot}$ and 2.3--22 {\rm kpc}, respectively. This confirms that the compact object in XTE J1752--223 is a black hole.

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arXiv2011

Revisit of Local X-ray Luminosity Function of Active Galactic Nuclei with the MAXI Extragalactic Survey

We construct a new X-ray (2--10 keV) luminosity function of Compton-thin active galactic nuclei (AGNs) in the local universe, using the first MAXI/GSC source catalog surveyed in the 4--10 keV band. The sample consists of 37 non-blazar AGNs at $z=0.002-0.2$, whose identification is highly ($>97%$) complete. We confirm the trend that the fraction of absorbed AGNs with $N_{\rm H} > 10^{22}$ cm$^{-2}$ rapidly decreases against luminosity ($L_{\rm X}$), from 0.73$\pm$0.25 at $L_{\rm X} = 10^{42-43.5}$ erg s$^{-1}$ to 0.12$\pm0.09$ at $L_{\rm X} = 10^{43.5-45.5}$ erg s$^{-1}$. The obtained luminosity function is well fitted with a smoothly connected double power-law model whose indices are $γ_1 = 0.84$ (fixed) and $γ_2 = 2.0\pm0.2$ below and above the break luminosity, $L_{*} = 10^{43.3\pm0.4}$ ergs s$^{-1}$, respectively. While the result of the MAXI/GSC agrees well with that of HEAO-1 at $L_{\rm X} \gtsim 10^{43.5}$ erg s$^{-1}$, it gives a larger number density at the lower luminosity range. Comparison between our luminosity function in the 2--10 keV band and that in the 14--195 keV band obtained from the Swift/BAT survey indicates that the averaged broad band spectra in the 2--200 keV band should depend on luminosity, approximated by $Γ\sim1.7$ for $L_{\rm X} \ltsim 10^{44}$ erg s$^{-1}$ while $Γ\sim 2.0$ for $L_{\rm X} \gtsim 10^{44}$ erg s$^{-1}$. This trend is confirmed by the correlation between the luminosities in the 2--10 keV and 14--195 keV bands in our sample. We argue that there is no contradiction in the luminosity functions between above and below 10 keV once this effect is taken into account.

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arXiv2011

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

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 $3\times10^{-9}$ ergs cm$^{-2}$ s$^{-1}$ in 2--20 keV band during the scan transit starting at UT 2010-09-07 18:24:30.The estimated X-ray luminosity at the scan in the energy band is 3$\times10^{32}$ ergs s$^{-1}$,indicating that the event is among the largest X-ray flares fromT 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.

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arXiv2011

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

We present the first unbiased source catalog of the Monitor of All-sky X-ray Image (MAXI) mission at high Galactic latitudes ($|b| > 10^{\circ}$), 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 develop 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 7 sigma significance level with a limiting sensitivity of $\sim1.5\times10^{-11}$ ergs cm$^{-2}$ s$^{-1}$ (1.2 mCrab) in the 4--10 keV band. Among them, we identify 38 Galactic/LMC/SMC objects, 48 galaxy clusters, 39 Seyfert galaxies, 12 blazars, and 1 galaxy. Other 4 sources are confused with multiple objects, and one remains unidentified. The log $N$ - log $S$ relation of extragalactic objects is in a 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 in 30 years ago.

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arXiv2011

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

The monitor of 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 has peculiarly narrow spectral energy distribution and is represented by Comptonized blackbody model. This spectrum can be interpreted as photospheric emission from the low baryon-load GRB fireball. Calculating the parameter of fireball, we found the size of the base of the flow $r_0 = (4.3 \pm 0.9) \times 10^{9} \, Y^{\prime \, -3/2}$ cm and Lorentz factor of the plasma $Γ= (110 \pm 10) \, Y^{\prime \, 1/4}$, where $Y^{\prime}$ is a ratio between the total fireball energy and the energy in the blackbody component of the gamma-ray emission. This $r_0$ is factor of a few 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.

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