arXiv subjects

Masaru Matsuoka

Publications and source records attributed to Masaru Matsuoka.

At least 19 recordsLinked to original sources

arXiv2022

Decades' long-term variations in NS-LMXBs observed with MAXI/GSC, RXTE/ASM and Ginga/ASM

We investigated the decades' long-term X-ray variations in bright low-mass X-ray binaries containing a neutron star (NS-LMXB). The light curves of MAXI/GSC and RXTE/ASM covers $\sim$ 26 yr, and high-quality X-ray light curves are obtained from 33 NS-LMXBs. Among them, together with Ginga/ASM, two sources (GX 3$+$1 and GX 9$+$1) showed an apparent sinusoidal variation with the period of $\sim 5$ yr and $\sim 10$ yr in the 34 yr light curve. Their X-ray luminosities were $(1-4)\times10^{37}$ erg s$^{-1}$ in the middle of the luminosity distribution of the NS-LMXB. Other seven sources (Ser X-1, 4U 1735--444, GX 9$+$9, 4U 1746$-$37, 4U 1708$-$40, 4U 1822$-$000, and 1A 1246$-$588) have also similar sinusoidal variation, although the profiles (amplitude, period, and phase) are variable. Compering the 21 sources with known orbital periods, a possible cause of the long-term sinusoidal variation might be the mass transfer cycles induced by the irradiation to the donor star.

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arXiv2018

The 7-year MAXI/GSC X-ray Source Catalog in the High Galactic-Latitude Sky (3MAXI)

We present the third MAXI/GSC catalog in the high Galactic-latitude sky ($|b| > 10^\circ$) based on the 7-year data from 2009 August 13 to 2016 July 31, complementary to that in the low Galactic-latitude sky ($|b| < 10^\circ$; Hori et al. 2018). We compile 682 sources detected at significances of $s_{\rm D,4-10~keV} \geq 6.5$ in the 4--10 keV band. A two-dimensional image fit based on the Poisson likelihood algorithm ($C$-statistics) is adopted for the detections and constraints on their fluxes and positions. The 4--10 keV sensitivity reaches $\approx 0.48$ mCrab, or $\approx 5.9 \times 10^{-12}$ erg cm$^{-2}$ s$^{-1}$, over the half of the survey area. Compared with the 37-month catalog (Hiroi et al. 2013), which adopted a threshold of $s_{\rm D,4-10~keV} \geq 7$, the source number increases by a factor of $\sim$1.4. The fluxes in the 3--4 keV and 10--20 keV bands are further estimated, and hardness ratios (HRs) are calculated using the 3--4 keV, 4--10 keV, 3--10 keV, and 10--20 keV band fluxes. We also make the 4--10 keV lightcurves in one year bins for all the sources and characterize their variabilities with an index based on a likelihood function and the excess variance. Possible counterparts are found from five major X-ray survey catalogs by Swift, Uhuru, RXTE, XMM-Newton, and ROSAT, and an X-ray galaxy-cluster catalog (MCXC). Our catalog provides the fluxes, positions, detection significances, HRs, one-year bin lightcurves, variability indices, and counterpart candidates.

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arXiv2017

Orbital modulations of X-ray light curves of Cyg X-1 in its low/hard and high/soft states

The black hole binary Cygnus X-1 has a 5.6 day orbital period. We first detected a clear intensity modulation with the orbital period in its high/soft state with 6 year MAXI data, as well as in its low/hard state. In the low/hard state, the folded light curves showed an intensity drop at the superior conjunction of the black hole by a modulation factor (MF), which is the amplitude divided by the average, with 8+-1%, 4+-1% and 3+-2% for 2-4 keV, 4-10 and 10-20 keV bands, showing a spectral hardening at the superior conjunction of the black hole. Spectral analysis with a model consisting of a power law and a photoelectric absorption, showed that the hydrogen column density increased from (2.9+-0.4)E+21 to (4.7+-1.1)E+21 cm^-2 around the superior conjunction, although more complex spectral variation, such as a partial absorption, was suggested, and the flux of the power law component decreased with 6+-1%. On the other hand, the MFs of the folded light curves in the high/soft state, were 4+-1% and 4+-2% for 2-4 keV and 4-10 keV bands, respectively. We applied a model consisting of a power law and a diskblackbody with a photoelectric absorption and found a modulation of the flux of the power law component with 7+-5% in MF, while the modulation of the hydrogen column density was less than 1E+21 cm^-2. These results can be interpreted as follows; the modulation of both states can be mainly explained by scattering of the X-rays by an ionized stellar wind, but only at the superior conjunction in the low/hard state, a large photoelectric absorption appears, because of a low ionization state of the wind in the line of sight at the phase. Such a condition can be established by reasonable parameters of an in-homogeneous wind and the observed luminosities.

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arXiv2016

Large X-ray Flares on Stars Detected with MAXI/GSC: A Universal Correlation between the Duration of a Flare and its X-ray Luminosity

23 giant flares from 13 active stars (eight RS CVn systems, one Algol system, three dMe stars and one YSO) were detected during the first two years of our all-sky X-ray monitoring with the gas propotional counters (GSC) of the Monitor of All-sky X-ray Image (MAXI). The observed parameters of all of these MAXI/GSC flares are found to be at the upper ends for stellar flares with the luminosity of 10^(31-34) ergs s-1 in the 2-20 keV band, the emission measure of 10^(54-57) cm-3, the e-folding time of 1 hour to 1.5 days, and the total radiative energy released during the flare of 10^(34-39) ergs. Notably, the peak X-ray luminosity of 5(3-9)*10^33 ergs s-1 in the 2-20 keV band was detected in one of the flares on II Peg, which is one of the, or potentially the, largest ever observed in stellar flares. X-ray flares were detected from GT Mus, V841 Cen, SZ Psc, and TWA-7 for the first time in this survey. Whereas most of our detected sources are multiple-star systems, two of them are single stars (YZ CMi and TWA-7). Among the stellar sources within 100 pc distance, the MAXI/GSC sources have larger rotation velocities than the other sources. This suggests that the rapid rotation velocity may play a key role in generating large flares. Combining the X-ray flare data of nearby stars and the sun, taken from literature and our own data, we discovered a universal correlation of tau~L_X^0.2 for the flare duration tau and the intrinsic X-ray luminosity L_X in the 0.1-100 keV band, which holds for 5 and 12 orders of magnitude in tau and L_X, respectively. The MAXI/GSC sample is located at the highest ends on the correlation.

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arXiv2016

Maxi observations of long X-ray bursts

We report nine long X-ray bursts from neutron stars, detected with Monitor of All-sky X-ray Image (MAXI). Some of these bursts lasted for hours, and hence are qualified as superbursts, which are prolonged thermonuclear flashes on neutron stars and are relatively rare events. MAXI observes roughly 85% of the whole sky every 92 minutes in the 2-20 keV energy band, and has detected nine bursts with a long e-folding decay time, ranging from 0.27 to 5.2 hours, since its launch in 2009 August until 2015 August. The majority of the nine events were found to originate from transient X-ray sources. The persistent luminosities of the sources, when these prolonged bursts were observed, were lower than 1% of the Eddington luminosity for five of them and lower than 20% for the rest. This trend is contrastive to the 18 superbursts observed before MAXI, all but two of which originated from bright persistent sources. The distribution of the total emitted energy, i.e., the product of e-folding time and luminosity, of these bursts clusters around $10^{41}$-$10^{42}$ erg, whereas either of the e-folding time and luminosity ranges for an order of magnitude. Among the nine events, two were from 4U 1850-086 during the phases of relatively low persistent-flux, whereas it usually exhibits standard short X-ray bursts during outbursts.

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arXiv2016

Search for Soft X-ray Flashes at Fireball Phase of Classical/Recurrent Novae using MAXI/GSC data

We searched for precursive soft X-ray flashes (SXFs) associated with optically-discovered classical or recurrent novae in the data of five-years all-sky observations with Gas Slit Camera (GSC) of the Monitor of All-sky X-ray Image (MAXI). We first developed a tool to measure fluxes of point sources by fitting the event distribution with the model that incorporates the point-spread function (PSF-fit) to minimize the potential contamination from nearby sources. Then we applied the PSF-fit tool to 40 classical/recurrent novae that were discovered in optical observations from 2009 August to 2014 August. We found no precursive SXFs with significance above $3 σ$ level in the energy range of 2$-$4 keV between $t_{d}-10$d and $t_{d}$, where $t_{d}$ is the date when each nova was discovered. We obtained the upper limits for the bolometric luminosity of SXFs, and compared them with the theoretical prediction and that observed for MAXI J0158$-$744. This result could constrain the population of massive white dwarfs with a mass of roughly 1.40 solar mass, or larger, in binary systems.

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arXiv2016

MAXI observations of long-term variations of Cygnus X-1 in the low/hard and the high/soft states

Long-term X-ray variability of the black hole binary, Cygnus X-1, was studied with five years of MAXI data from 2009 to 2014, which include substantial periods of the high/soft state, as well as the low/hard state. In each state, Normalized Power Spectrum densities (NPSDs) were calculated in three energy bands of 2-4 keV, 4-10 keV and 10-20 keV. The NPSDs in a frequency from 1e-7 Hz to 1e-4 Hz are all approximated by a power-law function with an index -1.35 ~ -1.29. The fractional RMS variation ($η$), calculated in the above frequency range, was found to show the following three properties; (1) $η$ slightly decreases with energy in the low/hard state; (2) $η$ increases towards higher energies in the high/soft state; and (3) in the 10-20 keV band, $η$ is 3 times higher in the high/soft state than in the low/hard state. These properties were confirmed through studies of intensity-correlated changes of the MAXI spectra. Of these three findings, the first one is consistent with that seen in the short-term variability during the LHS. The latter two can be understood as a result of high variability of the hard-tail component seen in the high/soft state with the above very low frequency range, although the origin of the variability remains inconclusive.

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arXiv2015

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

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." 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 as "purr-type outburst" referring to 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 a repeat of mini-outbursts.

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arXiv2015

Repeated Short-term Spectral Softening in the Low/Hard State of the Galactic Black-Hole Candidate Swift J1753.5-0127

We report MAXI and Swift observations of short-term spectral softenings of the galactic black-hole X-ray binary Swift J1753.5-0127 in the low/hard state. These softening events are characterized by a simultaneous increase of soft X-rays (2-4 keV) and a decrease of hard X-rays (15-50 keV) lasting for a few tens of days. The X-ray energy spectra during the softening periods can be reproduced with a model consisting of a multi-color disk blackbody and its Comptonized component. The fraction of the Comptonized component decreased from 0.30 to 0.15 when the spectrum became softer; meanwhile the inner disk temperature (Tin) increased from 0.2 to 0.45 keV. These results imply that the softening events are triggered by a short-term increase of the mass accretion rate. During the observed spectral softening events, the disk flux (F) and Tin did not obey the relation: F is proportional to Tin^4, suggesting that the inner disk radius does not reach the innermost stable circular orbit.

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arXiv2015

MAXI monitoring of blazars and blackhole binaries

Since August 2009, MAXI experiment on the ISS has been performing all-sky X-ray monitoring. With MAXI, we detected flaring activities of some blazers, including Mrk 421, Mrk 501, and 3C 273. Recently, new X-ray flaring activities were detected from two blazers, MAXI J1930+093 = 2FGL J1931.1+0938 (Atel#5943) and 2MAXI J0243-582 = BZB J0244-5819 (Atel#6012). The MAXI monitoring also covers black hole binaries, including Cyg X-1 and Cyg X-3 which emit GeV gamma-rays. Their gamma-ray emission was found to coincide with their X-ray state transitions. We present light curves and outstanding events of these sources.

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arXiv2015

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 and the Swift XRT 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 $\approx$ 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 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) them, the outburst observed with MAXI may have occurred after the quiescence of 30-40 years.

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arXiv2014

MAXI investigation into the longterm X-ray variability from the very-high-energy gamma-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 $α= 1.60 \pm 0.25$. This index is smaller than that obtained in the higher frequency range ($f > 1 \times 10^{-5}$ Hz), namely, $α= 2.14 \pm 0.06$ 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 ($α= 1$ -- $2$). The physical implications from these results are briefly discussed.

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arXiv2014

MAXI: all-sky observation from the International Space Station

Monitor of All-sky X-ray Image (MAXI) is mounted on the International Space Station (ISS). Since 2009 it has been scanning the whole sky in every 92 minutes with ISS rotation. Due to high particle background at high latitude regions the carbon anodes of three GSC cameras were broken. We limit the GSC operation to low-latitude region around equator. GSC is suffering a double high background from Gamma-ray altimeter of Soyuz spacecraft. MAXI issued the 37-month catalog with 500 sources above ~0.6 mCrab in 4-10 keV. MAXI issued 133 to Astronomers Telegram and 44 to Gammaray burst Coordinated Network so far. One GSC camera had a small gas leak by a micrometeorite. Since 2013 June, the 1.4 atm Xe pressure went down to 0.6 atm in 2014 May 23. By gradually reducing the high voltage we keep using the proportional counter. SSC with X-ray CCD has detected diffuse soft X-rays in the all-sky, such as Cygnus super bubble and north polar spur, as well as it found a fast soft X-ray nova MAXI J0158-744. Although we operate CCD with charge-injection, the energy resolution is degrading. In the 4.5 years of operation MAXI discovered 6 of 12 new black holes. The long-term behaviors of these sources can be classified into two types of the outbursts, 3 Fast Rise Exponential Decay (FRED) and 3 Fast Rise and Flat Top (FRFT). The cause of types is still unknown.

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arXiv2014

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

We report on a 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 MAXI Gas Slit Camera (GSC) and Swift Burst Alert Telescope (BAT) almost simultaneously. We analyzed X-ray and UV data for 270 days since the outburst onset taken by repeated MAXI scans and Swift pointing observations. The obtained X-ray light curve for the inital 90 days is roughly represented by a fast-rise and exponential-decay profile. However, it re-brightened on the ~110 days after the onset and finally went down below both GSC and BAT detec- tion limits on the 240 day. 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 were 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 > 2.9Mo 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 modelled 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-day 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.

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arXiv2014

MAXI observations of GRBs

Monitor of all-sky image (MAXI) Gas Slit Camera (GSC) detects gamma-ray bursts (GRBs) including the 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 is capable of detecting GRBs. Since the beginning of the MAXI operation on August 15, 2009, GSC observed 35 GRBs up to the middle of 2013. One third of them are 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 to that of MAXI/GSC, we find a possibility that many of 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 distribute in 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 a possibility of a confusion of those galactic transients with the MAXI GRB samples.

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arXiv2014

Sudden End of X-Ray Outbursts around Periastron of Circinus X-1 Observed with MAXI

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.

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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

Suzaku Studies of Luminosity-Dependent Changes in the Low-Mass X-ray Binary Aquila X-1

The neutron-star Low-Mass X-ray Binary Aquila X-1 was observed by Suzaku for seven times, from 2007 September 28 to October 30. The observations successfully traced an outburst decay in which the source luminosity decreased almost monotonically from $\sim 10^{37}$ erg s$^{-1}$ to $\sim 10^{34}$ erg s$^{-1}$, by $\sim 3$ orders of magnitude. To investigate luminosity-dependent changes in the accretion geometry, five of the seven data sets with a typical exposure of $\sim 18$ ks each were analyzed; the other two were utilized in a previous work \citep{Sakurai2012}. The source was detected up to 100 keV in the 2nd to the 4th observations, to 40 keV in the 5th, and to 10 keV on the last two occasions. All spectra were reproduced successfully by Comptonized blackbody model with relatively high ($\gtrsim 2.0$) optical depths, plus an additional softer optically-thick component. The faintest three spectra were reproduced alternatively by a single Comptonized blackbody model with a relatively low ($\lesssim 0.8$) optical depth. The estimated radius of the blackbody emission, including seed photons for the Comptonization, was $10 \pm 2$ km at a 0.8--100 keV luminosity of $2.4\times 10^{36}$ erg s$^{-1}$ (the 2nd to the 4th observations). In contrast, it decreased to $7 \pm 1$ km and further to $3 \pm 1$ km, at a luminosity of $(4.8-5.2)\times 10^{35}$ erg s$^{-1}$ (the 5th observation) and $\sim 2\times 10^{34}$ erg s$^{-1}$ (the 6th and 7th), respectively, regardless of the above model ambiguity. This can be taken as evidence for the emergence of a weak magnetosphere of the neutron star.

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