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This paper reports on the X-ray emission evolution of the ultra-luminous Galactic X-ray pulsar, Swift J0243.6+6124, during the 2017-2018 giant outburst observed by the MAXI GSC. The 2-30 keV light curve and the energy spectra confirm that the luminosity $L_\mathrm{X}$ reached $2.5\times 10^{39}$ erg s$^{-1}$, 10 times higher than the Eddington limit. When the source was luminous with $L_\mathrm{X}\gtrsim 0.9\times 10^{38}$ erg s$^{-1}$, it exhibited a negative correlation on a hardness-intensity diagram. However, two hardness ratios, a soft color ($=$ 4-10 keV / 2-4 keV) and a hard color ($=$ 10-20 keV / 4-10 keV), showed somewhat different behavior across a characteristic luminosity of $L_\mathrm{c}\simeq 5\times 10^{38}$ erg s$^{-1}$. The soft color changed more than the hard color when $L_\mathrm{X} < L_\mathrm{c}$, whereas the opposite was observed above $L_\mathrm{c}$. The spectral change above $L_\mathrm{c}$ was represented by a broad enhanced feature at $\sim 6$ keV. The pulse profiles made a transition from a single-peak to a double-peak one as the source brightened across $L_\mathrm{c}$. These spectral and pulse-shape properties can be interpreted by a scenario that the accretion columns on the neutron star surface, producing the Comptonized X-ray emission, gradually became taller as $L_\mathrm{X}$ increased. The broad 6 keV enhancement could be a result of cyclotron-resonance absorption at $\sim 10$ keV, corresponding to a surface magnetic field $B_\mathrm{s}\simeq 1.1\times 10^{12}$ G. The spin-frequency derivatives calculated with the Fermi GBM data showed a smooth correlation with $L_\mathrm{X}$ up to the outburst peak, and its linear coefficient is comparable to those of X-ray binary pulsars whose $B_\mathrm{s}$ are $(1-8)\times 10^{12}$ G. These results suggest that $B_\mathrm{s}$ of Swift J0243.6$+$6124 is a few times $10^{12}$ G., 19 pages, 13 figures, accepted for publication in ApJ

The accretion-induced pulse-period changes of the Be/X-ray binary pulsar X Persei were investigated over a period of 1996 January to 2017 September. This study utilized the monitoring data acquired with the RXTE/ASM in 1.5$-$12 keV and MAXI/GSC in 2$-$20 keV. The source intensity changed by a factor of 5$-$6 over this period. The pulsar was spinning down for 1996$-$2003, and has been spinning up since 2003, as already reported. The spin up/down rate and the 3$-$12 keV flux, determined every 250 d, showed a clear negative correlation, which can be successfully explained by the accretion torque model proposed by Ghosh & Lamb (1979). When the mass, radius and distance of the neutron star are allowed to vary over a range of 1.0$-$2.4 solar masses, 9.5$-$15 km, and 0.77$-$0.85 kpc, respectively, the magnetic field strength of $B=(4-25) \times10^{13}\ \rm G$ gave the best fits to the observation. In contrast, the observed results cannot be explained by the values of $B\sim10^{12}\ \rm G$ previously suggested for X Persei, as long as the mass, radius, and distance are required to take reasonable values. Assuming a distance of $0.81\pm0.04$ kpc as indicated by optical astrometry, the mass of the neutron star is estimated as $M=2.03\pm0.17$ solar masses., 11 pages, 9 figures, accepted for publication in PASJ

To observationally study spin-period changes of accreting pulsars caused by the accretion torque, the present work analyzes X-ray light curves of 12 Be binary pulsars obtained by the MAXI/GSC all-sky survey and their pulse periods measured by the Fermi/GBM pulsar project, both covering more than 6 years from 2009 August to 2016 March. The 12 objects were selected because they are accompanied by clear optical identification, and accurate measurements of surface magnetic fields. The luminosity $L$ and the spin-frequency derivatives $\dot{\nu}$, measured during large outbursts with $L\gtrsim 1\times 10^{37}$ erg s$^{-1}$, were found to approximately follow the theoretical relations in the accretion torque models, represented by $\dot{\nu} \propto L^{\alpha}$ ($\alpha\simeq 1$), and the coefficient of proportionality between $\dot{\nu}$ and $L^{\alpha}$, agrees, within a factor of $\sim 3$, with that proposed by Ghosh & Lamb (1979). In the course of the present study, the orbital elements of several sources were refined., Comment: 21 pages, 9 figures, accepted for publication in PASJ

著者人数: 32名, Accepted: 2011-09-20, 資料番号: SA1003145000

著者人数: 37名, Accepted: 2011-07-20, 資料番号: SA1003146000

著者人数: 12名, Accepted: 2007-09-04, 資料番号: SA1000314000

A report is made on the luminosity and pulse-period evolution of the Be binary X-ray pulsar, GX 304$-$1, during a series of outbursts from 2009 to 2013 observed by MAXI/GSC, RXTE/PCA, and Fermi/GBM. In total, twelve outbursts repeated by $\sim$ 132.2 days were observed, which is consistent with the X-ray periodicity of this object observed in the 1970s. These 12 outbursts, together with those in the 1970s, were found to all recur with a well defined period of 132.189$\pm$0.02 d, which can be identified with the orbital period. The pulse period at $\sim 275$ s, obtained from the RXTE/PCA and Fermi/GBM data, apparently exhibited a periodic modulation synchronized with the outburst period, suggesting the pulsar orbital motion, which is superposed on a secular spin-up trend throughout the entire active phase. The observed pulse-period changes were successfully represented by a model composed of the binary orbital modulation and pulsar spin up caused by mass accretion through an accretion disk. The orbital elements obtained from the best-fit model, including the projected orbital semi-major axis $a_{\rm x}\sin i \simeq 500-600$ light-s and an eccentricity $e \simeq 0.5$, are typical of Be binary X-ray pulsars., 10 pages, 6 figures, accepted for publication in PASJ

著者人数: 31名, Accepted: 2006-07-28, 資料番号: SA1000375000

The present paper reports on the RXTE observations of the binary X-ray pulsar 4U0115+63, covering an outburst in 1999 March-April with 44 pointings. The 3-30 keV PCA spectra and the 15-50 keV HEXTE spectra were analyzed jointly for the cyclotron resonance features. When the 3-50 keV luminosity at an assumed distance of 7 kpc was in the range (5-13)x10^{37} erg s^{-1}, harmonic double cyclotron features were observed in absorption at ~11 and ~22 keV, as was measured previously during typical outbursts. As the luminosity decreased below \~5x10^{37} erg s^{-1}, the second resonance disappeared, and the fundamental resonance energy gradually increased, up to $\sim$16 keV at 0.16x10^{37} erg s^{-1}. These results reconfirm the report by Mihara et al. (2004) using Ginga, who observed a single absorption at ~16 keV in a minor (~10^{37} erg s^{-1}) outburst of this object. The luminosity-dependent cyclotron resonance energy may be understood as a result of a decrease in the accretion column height, in response to a decrease in the mass accretion rate.

著者人数: 43名, Accepted: 2014-06-13, 資料番号: SA1140202000

MAXI/GSC observed 21 outbursts from Circinus X-1 between 2009 August and 2013 December. Although 14 outbursts showed ordinary gradual decays, in 7 outbursts we found sudden luminosity decrease in a time scale of a few hours around the periastron, and then the outbursts terminated. These sudden decreases started at the estimated luminosity of a few times $10^{37}$ erg s$^{-1}$ and reached to $\lesssim3\times10^{36}$ erg s$^{-1}$. We propose three interpretations for the sudden luminosity decrease: (1) the end of the outburst during the dip, (2) the propeller effect, and (3) the stripping effect by the stellar wind of the companion star. It is difficult to explain the phenomenon with any of these interpretations alone. The interpretation of (1) is possible for only two outbursts assuming rapid decay. The propeller effect (2) is expected to occur at a constant luminosity, which is incompatible with the observed facts. In wind stripping effect (3), the ram pressure of a typical stellar wind is not sufficient to blow out most of the accretion disk. In this paper, we discuss a possibility of a modified effect of (3) assuming other additional conditions such as wind clumping and disk instability., Comment: 13 pages, 7 figures

We report on the luminosity dependent change of the cyclotron resonance energy obtained from a transient X-ray pulsar, 4U 0115+63. Using RXTE data observed on 1999 March, we found that the fundamental resonance energy stayed constant (∼11 keV) when the source luminosity was above ∼5 × 1037 erg s−1. As the luminosity decreased below ∼5 × 1037 erg s−1, the fundamental resonance energy gradually increased up to ∼16 keV at 0.16 × 1037 erg s−1. The luminosity dependence of the resonance energy can be understood by the change of the accretion column height.

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 $\sim 0.45$ Crab in 4-10 keV, which corresponds to a luminosity of $1.1 \times 10^{38}$ 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 $\sim 76$ keV. This unambiguously reconfirm the previously suggested $\sim$ 80 keV spectral feature in GRO J1008$-$57. The implied surface magnetic field, $6.6\times 10^{12}$ G, is the highest among binary X-ray pulsars from which CRSFs have ever been detected.

The stellar wind around the compact object in luminous wind-accreting high mass X-ray binaries is expected to be strongly ionized with the X-rays coming from the compact object. The stellar wind of hot stars is mostly driven by light absorption in lines of heavier elements, and X-ray photo-ionization significantly reduces the radiative force within the so-called Stroemgren region leading to wind stagnation around the compact object. In close binaries like Vela X-1 this effect might alter the wind structure throughout the system. Using the spectral data from Monitor of All-sky X-ray Image (MAXI), we study the observed dependence of the photoelectric absorption as function of orbital phase in Vela X-1, and find that it is inconsistent with expectations for a spherically-symmetric smooth wind. Taking into account previous investigations we develop a simple model for wind structure with a stream-like photoionization wake region of slower and denser wind trailing the neutron star responsible for the observed absorption curve., 5 pages, 3 figures, accepted in A&A

In the last 10 years, since its last giant outburst in 2006, regular X-ray outbursts (type I) were detected every periastron passage in the Be X-ray binary EXO 2030+375. Recently, however, it was reported that the source started to show a peculiar behavior: its X-ray flux decreased significantly and type I outbursts were missed in several cases. At the same time, the spin frequency of the neutron star, which had been increasing steadily since the end of the 2006 giant outburst, reached a plateau. Very recent observations indicate that the source is now starting to spin down. These observed phenomena have a striking similarity with those which took place 20 years ago, just before the source displayed a sudden orbital phase shift of the outburst peak (1995). This historical event occurred at the time exactly between the two giant outbursts (1985 and 2006). These phenomena suggest the system to have an underlying periodicity of 10.5 years between orbital phase shifts and/or giant outbursts. The suggested periodicity may reflect some long-period dynamics in the circumstellar disk of the Be star, due, e.g., to the Kozai-Lidov effect. A model generating such a periodic change of the Be disk, namely Kozai-Lidov oscillations in the Be disk, is discussed. If this behavior is really periodical, another phase shift of the X-ray outburst peak is predicted to occur around 2016 December., 10 pages, 5 figures, accepted for publication in Astronomy & Astrophysics (A&A), included minor language corrections

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., 14 pages, 8 figures, accepted for publication in PASJ

We present analysis of 4U 1626-67, a 7.7 s pulsar in a low-mass X-ray binary system, observed with the hard X-ray detector of the Japanese X-ray satellite Suzaku in March 2006 for a net exposure of \sim88 ks. The source was detected at an average 10-60 keV flux of \sim4 x10^-10 erg cm^-2 s^-1. The phase-averaged spectrum is reproduced well by combining a negative and positive power-law times exponential cutoff (NPEX) model modified at \sim 37 keV by a cyclotron resonance scattering feature (CRSF). The phase-resolved analysis shows that the spectra at the bright phases are well fit by the NPEX with CRSF model. On the other hand, the spectrum in the dim phase lacks the NPEX high-energy cutoff component, and the CRSF can be reproduced by either an emission or an absorption profile. When fitting the dim phase spectrum with the NPEX plus Gaussian model, we find that the feature is better described in terms of an emission rather than an absorption profile. The statistical significance of this result, evaluated by means of an F-test, is between 2.91 x 10^-3 and 1.53 x 10^-5, taking into account the systematic errors in the background evaluation of HXD-PIN. We find that, the emission profile is more feasible than the absorption one for comparing the physical parameters in other phases. Therefore, we have possibly detected an emission line at the cyclotron resonance energy in the dim phase., Accepted for publication in ApJ on March 16, 2012. 12 pages, 14 figures

著者人数: 31名, Accepted: 2012-01-28, 資料番号: SA1004313000

We report 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 started on August 8, 2009, 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 1.5 degree 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 the similar low-earth orbit if its relation with the geomagnetic cutoff rigidity is extrapolated to the high latitude. We also present the status of the in-orbit operation and the calibration of the effective area and the energy response matrix using Crab-nebula data., 11 pages, 14 figures, accepted for publication in PASJ

著者人数: 37名, Accepted: 2011-09-06, 資料番号: SA1004479000

We report on changes of the cyclotron resonance energies of the recurrent transient pulsar, X0331+53 (V0332+53). The whole RXTE data acquired in the 2004-2005 outburst were utilized. The 3-80 keV source luminosity varied between 1.7x10^36 and 3.5x10^38 ers/s, assuming a distance of 7 kpc. We confirmed that the fundamental cyclotron resonance energy changed from ~22 to ~27 keV in a clear anti-correlation to the source luminosity, and without any hysteresis effects between the rising and declining phases of the outburst. In contrast, the second harmonic energy changed from ~49 to ~54 keV, implying a weaker fractional change as a function of the luminosity. As a result, the observed resonance energy ratio between the second harmonic and the fundamental was ~2.2 when the source was most luminous, whereas the ratio decreased to the nominal value of 2.0 at the least luminous state. Although the significance of this effect is model dependent, these results suggest that the fundamental and second harmonic resonances represent different heights in the accretion column, depending on the mass accretion rate., Comment: 39 pages, 15 figures, 4 tables. Accepted for publication in Astrophysical Journal

Over the 3-year active period from 2008 September to 2011 November, the outburst behavior of the Be/X-ray binary A 0535+26 was continuously monitored with the MAXI/GSC and the Swift/BAT. The source exhibited nine outbursts, every binary revolution of 111.1 days, of which two are categorized into the giant (type-II) outbursts. The recurrence period of these outbursts is found to be $\sim115$ days, significantly longer than the orbital period of 111.1 days. With the MAXI/GSC, a low-level active period, or a "precursor", was detected prior to at least four giant outbursts. The precursor recurrence period agrees with that of the giant outbursts. The period difference of the giant outbursts from the orbital period is possibly related with some structures in the circumstellar disc formed around the Be companion. Two scenarios, one based on a one-armed disc structure and the other a Be-disc precession, are discussed., 15 pages, 7 figures, 1 table. Accepted for publication in PASJ