arXiv subjects

Masahiro Tsujimoto

Publications and source records attributed to Masahiro Tsujimoto.

At least 55 records · Page 3Linked to original sources

arXiv2016

A Systematic X-ray Study of the Dwarf Novae Observed with Suzaku

X-ray behavior of the dwarf novae (DNe) outside the quiescent state has not been fully understood. We thus assembled 21 data sets of the 15 DNe observed by the Suzaku satellite by the end of 2013, which include spectra taken during not only the quiescence, but also the transitional, outburst, and super-outburst states. Starting with the traditional cooling flow model to explain the X-ray emission from the boundary layer, we made several modifications to account for the observed spectra. As a result, we found that the best-fit spectral model depends strongly on the state of the DNe with only a few exceptions. Spectra in the quiescent state are explained by the cooling flow model plus a Fe fluorescent line emission attenuated by an interstellar extinction. Spectra in the transitional state require an additional partial covering extinction. Spectra in the outburst and super-outburst state require additional low-temperature thin-thermal plasma component(s). Spectra in the super-outburst state further require a high value of the minimum temperature of the boundary layer. We present an interpretation on the required modifications to the cooling flow model for each state.

astro-ph.HE↗

arXiv2016

Near-Infrared Spectroscopy of Faint Discrete X-ray Point Sources Constituting the Galactic Ridge X-ray Emission

The Galactic Ridge X-ray Emission (GRXE) is apparently extended X-ray emission along the Galactic Plane. The X-ray spectrum is characterized by hard continuum with a strong Fe K emission feature in the 6-7 keV band. A substantial fraction (~80%) of the GRXE in the Fe band was resolved into point sources by deep Chandra imaging observations, thus GRXE is mostly composed of dim Galactic X-ray point sources at least in this energy band. To investigate the populations of these dim X-ray point sources, we carried out Near-Infrared (NIR) follow-up spectroscopic observations in two deep Chandra fields located in the Galactic plane at (l,b)=(0.1{\arcdeg}, -1.4{\arcdeg}) and (28.5{\arcdeg}, 0.0{\arcdeg}) using NTT/SofI and Subaru/MOIRCS. We obtained well-exposed NIR spectra from 65 objects and found that there are three main classes of Galactic sources based on the X-ray color and NIR spectral features: those having (A) hard X-ray spectra and NIR emission features such as HI(Brγ), HeI, and HeII (2 objects), (B) soft X-ray spectra and NIR absorption features such as HI, NaI, CaI, and CO (46 objects), and (C) hard X-ray spectra and NIR absorption features such as HI, NaI, CaI and CO (17 objects). From these features, we argue that class A sources are Cataclysmic Variables (CVs), and class B sources are late-type stars with enhanced coronal activity, which is in agreement with current knowledge. Class C sources possibly belong to a new group of objects, which has been poorly studied so far. We argue that the candidate sources for class C are the binary systems hosting white dwarfs and late-type companions with very low accretion rates. It is likely that this newly recognized class of the sources contribute to a non-negligible fraction of the GRXE, especially in the Fe K band.

astro-ph.HE↗

arXiv2015

Origin of the X-ray Broad Iron Spectral Feature in GRS 1915+105

The X-ray spectrum of GRS 1915+105 is known to have a ``broad iron spectral feature'' in the spectral hard state. Similar spectral features are often observed in Active Galactic Nuclei (AGNs) and other black-hole binaries (BHBs), and several models have been proposed for explaining it. In order to distinguish spectral models, time variation provides an important key. In AGNs, variation amplitude has been found to drop significantly at the iron K-energy band at timescales of ~10 ks. If spectral variations of black-holes are normalized by their masses, the spectral variations of BHBs at timescales of sub-seconds should exhibit similar characteristics to those of AGNs. In this paper, we investigated spectral variations of GRS 1915+105 at timescales down to ~10 ms. This was made possible for the first time with the Suzaku XIS Parallel-sum clocking (P-sum) mode, which has the CCD energy-resolution as well as a time-resolution of 7.8 ms. Consequently, we found that the variation amplitude of GRS 1915+105 does not drop at the iron K-energy band at any timescales from 0.06 s to 63000 s, and that the entire X-ray flux and the iron feature are independently variable at timescales of hours. These are naturally understood in the framework of the ``partial covering'' model, in which variation timescales of the continuum flux and partial absorbers are independent. The difference of energy dependence of the variation amplitude between AGN and BHB is presumably due to different mechanisms of the outflow winds, i.e., the partial absorbers are due to UV-line driven winds (AGN) or thermally-driven winds (BHB).

astro-ph.HE↗

arXiv2015

Variability of the X-ray Broad Iron Spectral Features in Active Galactic Nuclei and Black-hole Binaries

The "broad iron spectral features" are often seen in X-ray spectra of Active Galactic Nuclei (AGN) and black-hole binaries (BHB). These features may be explained either by the "relativistic disc reflection" scenario or the "partial covering" scenario: It is hardly possible to determine which model is valid from time-averaged spectral analysis. Thus, X-ray spectral variability has been investigated to constrain spectral models. To that end, it is crucial to study iron structure of BHBs in detail at short time-scales, which is, for the first time, made possible with the Parallel-sum clocking (P-sum) mode of XIS detectors on board Suzaku. This observational mode has a time-resolution of 7.8~ms as well as a CCD energy-resolution. We have carried out systematic calibration of the P-sum mode, and investigated spectral variability of the BHB GRS 1915+105. Consequently, we found that the spectral variability of GRS 1915+105 does not show iron features at sub-seconds. This is totally different from variability of AGN such as 1H0707--495, where the variation amplitude significantly drops at the iron K-energy band. This difference can be naturally explained in the framework of the "partial covering" scenario.

astro-ph.HE↗

arXiv2013

Detection of a 522 s Pulsation from the Transient X-ray Source Suzaku J0102.8--7204 (SXP 523) in the Small Magellanic Cloud

During a routine calibration observation of 1E0102.2-7219 in the Small Magellanic Cloud (SMC) carried out in October 2012 for the Suzaku satellite, we detected a transient X-ray source at (RA, Dec) = (01h02m47s, -72d04m54s) in the equinox J2000.0 with a positional uncertainty of ~1"4. We conducted a temporal and spectral analysis of the source and found a coherent pulse signal with a period of 522.3 +/- 0.1 s, and a featureless spectrum described by a single power-law model with a photon index of 1.0 +/- 0.1 and a 0.5-10 keV luminosity of 8.8 x 10^35 erg s^-1 at an assumed distance of 60 kpc. The Suzaku source is likely to be the counterpart of 2XMMJ010247.4-720449, which has been observed several times, including during outburst by Swift. Based on the X-ray characteristics in our data, as well as the transient record and optical and near-infrared features in the literature, we conclude that this source is a high-mass X-ray binary pulsar with a Be star companion in the SMC, which is known to harbor an exceptionally large (80) number of such sources in comparison to our Galaxy.

astro-ph.HE↗

arXiv2013

X-ray Point Source Populations Constituting the Galactic Ridge X-ray Emission

Apparently diffuse X-ray emission has been known to exist along the central quarter of the Galactic Plane since the beginning of the X-ray astronomy, which is referred to as the Galactic Ridge X-ray emission (GRXE). Recent deep X-ray observations have shown that numerous X-ray point sources account for a large fraction of the GRXE in the hard band (2-8 keV). However, the nature of these sources is poorly understood. Using the deepest X-ray observations made in the Chandra Bulge Field (Revnivtsev et al., 2009,2011), we present the result of a coherent photometric and spectroscopic analysis of individual X-ray point sources for the purpose of constraining their nature and deriving their fractional contributions to the hard band continuum and Fe Kαline emission of the GRXE. Based on the X-ray color-color diagram, we divided the point sources into three groups: A (hard), B (soft and broad spectrum), and C (soft and peaked spectrum). The group A sources are further decomposed spectrally into thermal and non-thermal sources with different fractions in different flux ranges. From their X-ray properties, we speculate that the group A non-thermal sources are mostly AGNs and the thermal sources are mostly white dwarf (WD) binaries such as magnetic and non-magnetic cataclysmic variables (CVs), pre-CVs, and symbiotic stars, whereas the group B and C sources are X-ray active stars in flares and quiescence, respectively. In the log N-log S curve of the 2-8 keV band, the group A non-thermal sources is dominant above about 10^{-14} erg cm^{-2} s^{-1}, which is gradually taken over by Galactic sources in the fainter flux ranges. The Fe Kαemission is mostly from the group A thermal (WD binaries) and the group B (X-ray active stars) sources.

astro-ph.HE↗

arXiv2012

Suzaku X-Ray Observation of the Dwarf Nova Z Camelopardalis at the Onset of an Optical Outburst

We present the result of a Suzaku X-ray spectroscopic observation of the dwarf nova Z Camelopardalis, which was conducted by chance at the onset of an optical outburst. We used the X-ray Imaging Spectrometer (a 38 ks exposure) and the Hard X-ray Detector (34 ks) to obtain a 0.35-40 keV spectrum simultaneously. Spectral characteristics suggest that the source was in the X-ray quiescent state despite being in the rising phase of an outburst in the optical band. The spectrum shows a clear signature of circumstellar absorption in excess of interstellar absorption and the reprocessed emission features of Fe fluorescence and Compton scattering. The extra absorption is explained due to partial coverage by either neutral or ionized matter. We found a spectral change during the observation, which is attributable only to the change in the circumstellar absorption. Such an X-ray spectral variation is reported for the first time in dwarf novae. We speculate that the variation in the circumstellar absorption is interpreted as a time-varying disk wind or geometrically flaring disk around the white dwarf during the propagation of a heat wave inward along the accretion disk at the beginning of the outburst, in which optical outburst and X-ray quiescent states co-exist.

astro-ph.HE↗

arXiv2011

Data-oriented Diagnostics of Pileup Effects on the Suzaku XIS

We present the result of a systematic study of pileup phenomena seen in the X-ray Imaging Spectrometer, an X-ray CCD instrument, onboard the Suzaku observatory. Using a data set of observed sources in a wide range of brightness and spectral hardness, we characterized the pileup fraction, spectral hardening, and grade migration as a function of observed count rate in a frame per pixel. Using the pileup fraction as a measure of the degree of pileup, we found that the relative spectral hardening (the hardness ratio normalized to the intrinsic spectral hardness), branching ratio of split events, and that of detached events increase monotonically as the pileup fraction increases, despite the variety of brightness and hardness of the sample sources. We derived the pileup fraction as a function of radius used for event extraction. Upon practical considerations, we found that events outside of the radius with a pileup fraction of 1% or 3% are useful for spectral analysis. We present relevant figures, tables, and software for the convenience of users who wish to apply our method for their data reduction of piled-up sources.

astro-ph.IM↗

arXiv2011

Near-Infrared and X-Ray Observations of XSS J12270-4859

XSS J12270-4859 (J12270) is an enigmatic source of unknown nature. Previous studies revealed that the source has unusual X-ray temporal characteristics, including repetitive short-term flares followed by spectral hardening, non-periodic dips, and dichotomy in activity; i.e. intervals filled with flares and those without. Together with a power-law X-ray spectrum, it is suggested to be a low-mass X-ray binary (LMXB). In order to better understand the object, we present the results of our near-infrared (NIR) photometry and linear polarimetry observations as well as X-ray spectroscopy observations, which overlap with each other partially in time, taken respectively with the InfraRed Survey Facility (IRSF) and the Rossi X-ray Timing Explorer (RXTE). We detected several simultaneous NIR and X-ray flares for the first time. No significant NIR polarization was obtained. We assembled data taken with IRSF, RXTE, Suzaku, Swift, and other missions in the literature and compared the flare profile and the spectral energy distribution (SED) with some representative high-energy sources. Based on some similarities of the repetitive NIR and X-ray flaring characteristics and the broad SED, we argue that J12270 is reminiscent of microquasars with a synchrotron jet, which is at a very low luminosity state of ~1e-4 Eddington luminosity for a stellar mass black hole or neutron star at a reference distance of 1 kpc.

astro-ph.HE↗

arXiv2010

Cross-calibration of the X-ray Instruments onboard the Chandra, INTEGRAL, RXTE, Suzaku, Swift, and XMM-Newton Observatories using G21.5-0.9

Context. The Crab nebula has been used as a celestial calibration source of the X-ray flux and spectral shape for many years by X-ray astronomy missions. However, the object is often too bright for current and future missions equipped with instruments with improved sensitivity. Aims. We use G21.5-0.9 as a viable, fainter substitute to the Crab, which is another pulsar-wind nebula with a time-constant powerlaw spectrum with a flux of a few milli Crab in the X-ray band. Using this source, we conduct a cross-calibration study of the instruments onboard currently active observatories: Chandra ACIS, Suzaku XIS, Swift XRT, XMM-Newton EPIC (MOS and pn) for the soft-band, and INTEGRAL IBIS-ISGRI, RXTE PCA, and Suzaku HXD-PIN for the hard band. Methods. We extract spectra from all the instruments and fit them under the same astrophysical assumptions. We compare the spectral parameters of the G21.5-0.9 model: power-law photon index, H-equivalent column density of the interstellar photoelectric absorption, flux in the soft (2-8 keV) or hard (15-50 keV) energy band. Results. We identify the systematic differences in the best-fit parameter values unattributable to the statistical scatter of the data alone. We interpret these differences as due to residual cross-calibration problems. The differences can be as large as 20% and 9% for the soft-band flux and power-law index, respectively, and 46% for the hard-band flux. The results are plotted and tabulated as a useful reference for future calibration and scientific studies using multiple missions.

astro-ph.HE↗

arXiv2010

Long-Term Spectral Variations of Ultraluminous X-ray Sources in the Interacting Galaxy Systems M51 and NGC4490/85

Variable ultraluminous X-ray sources (ULXs), which are considered to be black hole binaries (BHBs), are known to show state transitions similarly to Galactic BHBs. However, the relation between the ULX states and the Galactic BHB states is still unclear primarily due to less well-understood behaviors of ULXs in contrast to the Galactic BHBs. Here, we report a statistical X-ray spectral study of 34 energy spectra from seven bright ULXs in the interacting galaxy systems M51 and NGC4490/85, using archive data from multiple Chandra and XMM-Newton observations spanning for a few years. In order to compare with Galactic BHB states, we applied representative spectral models of BHBs; a power-law (PL), a multi-color disk black body (MCD), and a slim disk model to all the ULX spectra. We found a hint of a bimodal structure in the luminosity distribution of the samples, suggesting that ULXs have two states with typical luminosities of 3-6*10^{39} and 1.5-3*10^{39} ergs/s. Most spectra in the brighter state are explained by the MCD or the slim disk model, whereas those in the fainter state are explained by the PL model. In particular, the slim disk model successfully explains the observed spectral variations of NGC4490/85 ULX-6 and ULX-8 by changes of the mass accretion rate to a black hole of an estimated mass of <40 Msun. From the best-fit model parameters of each state, we speculate that the brighter state in these two ULXs corresponds to the brightest state of Galactic BHBs, which is often called the ``apparently standard state''. The fainter state of the ULXs has a PL shaped spectrum, but the photon index range is much wider than that seen in any single state of Galactic BHBs. We thus speculate that it is a state unique to ULXs. Some sources show much fainter and steeper spectra than the faint state, which we identified as another state.

astro-ph.HE↗

arXiv2009

Suzaku X-Ray Study of an Anomalous Source XSS J12270-4859

We report the results of the Suzaku X-ray observation of XSS J12270-4859, one of the hard X-ray sources in the INTEGRAL catalogue. The object has been classified as an intermediate polar (IP) by optical spectra and a putative X-ray period of ~860 s. With a 30 ks exposure of Suzaku, we obtained a well-exposed spectrum in the 0.2-70 keV band. We conclude against the previous IP classification based on the lack of Fe Ka emission features in the spectrum and the failure to confirm the previously reported X-ray period. Instead, the X-ray light curve is filled with exotic phenomena, including repetitive flares lasting ~100 s, occasional dips with no apparent periodicities, spectral hardening after some flares, and bimodal changes pivoting between quiet and active phases. The rapid flux changes, the dips, and the power-law spectrum point toward the interpretation that this is a low-mass X-ray binary. Some temporal characteristics are similar to those in the Rapid Burster and GRO J1744-28, making XSS J12270-4859 a very rare object.

astro-ph.HE↗

arXiv2009

Super-hard X-Ray Emission from eta Carinae Observed with Suzaku

We present the Suzaku results of eta Carinae in the 5--50 keV range conducted twice around the apastron in 2005 August for 50 ks and in 2006 February for 20 ks. The X-ray Imaging Spectrometer (XIS) produced hard (5--12 keV) band spectra, resolving K shell lines from highly ionized Fe and Ni. The Hard X-ray Detector yielded a significant detection in the super-hard (15--50 keV) band, which is uncontaminated by near-by sources. We constrained the temperature of the optically-thin thermal plasma emission dominant in the hard band as 3--4 keV using the K-shell line features with the XIS. We found significant excess emission above the thermal emission in the super-hard band with the PIN, confirming the previous INTEGRAL ISGRI report. The entire 5--50 keV spectra were fitted by a combination of a thermal plasma model plus a flat power-law or a very hot thermal bremsstrahlung model for the excess emission. No significant change of the excess emission was found at different epochs within the systematic and statistical uncertainties and no flare-like flux amplification was seen in the hard band, indicating that the excess emission is a steady phenomenon. We argue that the super-hard emission is attributable to the inverse Compton of stellar UV photons by non-thermal electrons or to the thermal bremsstrahlung of very hot plasma, and not to the bremsstrahlung by non-thermal electrons colliding with cold ambient matter.

astro-ph.SR↗

arXiv2008

X-Ray Spectroscopy of the Classical Nova V458 Vulpeculae with Suzaku

We conducted a target of opportunity X-ray observation of the classical nova V458 Vulpeculae 88 days after the explosion using the Suzaku satellite. With a 20 ks exposure, the X-ray Imaging Spectrometer detected X-ray emission significantly harder than typical super-soft source emission. The X-ray spectrum shows K lines from N, Ne, Mg, Si, and S, and L-series emission from Fe in highly ionized states. The spectrum can be described by a single temperature (0.64 keV) thin thermal plasma model in collisional equilibrium with a hydrogen-equivalent extinction column density of ~3e21/cm2, a flux of ~1e-12 erg/s/cm2, and a luminosity of ~6e34 erg/s in the 0.3-3.0 keV band at an assumed distance of 13 kpc. We found a hint of an enhancement of N and deficiencies of O and Fe relative to other metals. The observed X-ray properties can be interpreted as the emission arising from shocks of ejecta from an ONe-type nova.

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arXiv2008

X-Ray Observations of the Sagittarius D HII Region toward the Galactic Center with Suzaku

We present a Suzaku X-ray study of the Sagittarius D (Sgr D) HII region in the Galactic center region. Two 18'x18' images by the X-ray Imaging Spectrometer (XIS) encompass the entire Sgr D complex. Thanks to the low background, XIS discovered two diffuse sources with low surface brightness and obtained their high signal-to-noise ratio spectra. One is associated with the core of the Sgr D HII region, arising from the young stellar cluster. The other is a new object in the vicinity of the region. We also present 3.5 cm and 6.0 cm radio continuum maps of the new source using the 100 m Green Bank Telescope. We conclude that the source is a new supernova remnant (SNR; G1.2--0.0) based on: (1) the 0.9+/-0.2 keV thermal X-ray spectrum with emission lines from highly ionized atoms; (2) the diffuse nature with an apparent extent of ~10 pc at the Galactic center distance inferred from the X-ray absorption (~8.5x10^{22} cm^{-2}); and (3) the nonthermal radio continuum spectral index (~-0.5). Our discovery of an SNR in the Sgr D HII region leads to a revision of the view of this system, which had been considered to be a thermal HII region and its environment.

astro-ph↗

arXiv2007

Suzaku X-Ray Spectroscopy of a Peculiar Hot Star in the Galactic Center Region

We present the results of a Suzaku study of a bright point-like source in the 6.7 keV intensity map of the Galactic center region. We detected an intense FeXXV 6.7 keV line with an equivalent width of ~1 keV as well as emission lines of highly ionized Ar and Ca from a spectrum obtained by the X-ray Imaging Spectrometer. The overall spectrum is described very well by a heavily absorbed (~2x10^{23}cm^{-2}) thin thermal plasma model with a temperature of 3.8+/-0.6 keV and a luminosity of ~3x10^{34} erg s^{-1} (2.0--8.0 keV) at 8 kpc. The absorption, temperature, luminosity, and the 6.7 keV line intensity were confirmed with the archived XMM-Newton data. The source has a very red (J-Ks=8.2 mag) infrared spectral energy distribution (SED), which was fitted by a blackbody emission of ~1000 K attenuated by a visual extinction of ~31 mag. The high plasma temperature and the large X-ray luminosity are consistent with a wind-wind colliding Wolf-Rayet binary. The similarity of the SED to those of the eponymous Quintuplet cluster members suggests that the source is a WC-type source.

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arXiv2007

Spectral Study of the Galactic Ridge X-ray Emission with Suzaku

We have observed a typical Galactic plane field at (l,b) = (28.46d, -0.20d) with Suzaku for 100 ksec to carry out a precise spectral study of the Galactic Ridge X-ray Emission (GRXE). The field is known to be devoid of X-ray point sources brighter than ~2 x 10^{-13} ergs s^{-1} cm^{-2} (2--10 keV), and already deeply observed with Chandra. Thanks to the low and stable background and high spectral resolution of Suzaku, we were able to resolve, for the first time, three narrow iron K-emission lines from low-ionized (6.41 keV), helium-like (6.67 keV), and hydrogenic ions (7.00 keV) in the GRXE spectrum. These line features constrain the GRXE emission mechanisms: The cosmic-ray ion charge exchange model or the non-equilibrium ionization plasma model are unlikely, since they require either broad emission lines or lines at intermediate ionization states. Collisional ionization equilibrium plasma is the likely origin for the 6.67 keV and 7.00 keV lines, while origin of the 6.41 keV line, which is due to fluorescence from cold material, is not elucidated. Low non-X-ray background and little stray-light contamination of Suzaku allowed us to precisely measure the absolute X-ray surface brightness in the direction of the Galactic plane. Excluding the point sources brighter than ~2 x 10^{-13} ergs s^{-1} cm^{-2} (2--10 keV), the total surface brightness on the Galactic plane is ~6.1 x 10^{-11} ergs s^{-1} cm^{-2} deg^{-2} (2--10 keV), including the contribution of the cosmic X-ray background that is estimated to be ~1.3x 10^{-11} ergs s^{-1} cm^{-2} deg^{-2}.

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arXiv2007

Suzaku Detection of an Intense X-Ray Flare from an A-type Star HD161084

We report a serendipitous detection of an intense X-ray flare from the Tycho reference source HD 161084 during a Suzaku observation of the Galactic Center region for 20 ks. The X-ray Imaging Spectrometer (XIS) recorded a flare from this A1-type dwarf or subgiant star with a flux of 1.4x10^{-12} erg s^{-1} cm^{-2} (0.5--10 keV) and a decay time scale of 0.5 hr. The spectrum is hard with a prominent Fe XXV K alpha emission line at 6.7 keV, which is explained by a 5 keV thin-thermal plasma model attenuated by a 1.4x10^{21} cm^{-2} extinction. The low extinction, which is consistent with the optical reddening, indicates that the source is a foreground star toward the Galactic Center region. Based on the spectroscopic parallax distance of 530 pc, the peak X-ray luminosity amounts to 1x10^{32} erg s^{-1} (0.5--10 keV). This is much larger than the X-ray luminosity of ordinary late-type main-sequence stars, and the X-ray emission is unattributable to a hidden late-type companion that comprises a wide binary system with the A-star. We discuss possible natures of HD 161084 and suggest that it is most likely an interacting binary with elevated magnetic activity in the companion such as the Algol-type system. The flux detected by Suzaku during the burst is 100 times larger than the quiescent level measured using the archived XMM-Newton and Chandra data. The large flux amplification makes this star a unique example among sources of this class.

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