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1. State transitions of GRS 1739−278 in the 2014 outburst

Author

Taketoshi Yoshii, Sili Wang, Aya Kubota, Yutaro Tachibana, Nobuyuki Kawai, Masayuki Sudo, and Megumi Shidatsu

Subjects
High Energy Astrophysical Phenomena (astro-ph.HE), Physics, 010504 meteorology & atmospheric sciences, Astrophysics::High Energy Astrophysical Phenomena, Time evolution, FOS: Physical sciences, Astronomy and Astrophysics, Astrophysics, 01 natural sciences, Spectral line, Accretion (astrophysics), Relativistic disk, symbols.namesake, Rotating black hole, Accretion disc, Space and Planetary Science, 0103 physical sciences, Eddington luminosity, symbols, Spectral analysis, Astrophysics::Earth and Planetary Astrophysics, Astrophysics - High Energy Astrophysical Phenomena, 010303 astronomy & astrophysics, Astrophysics::Galaxy Astrophysics, 0105 earth and related environmental sciences
Abstract

We report on the X-ray spectral analysis and time evolution of GRS 1739$-$278 during its 2014 outburst based on MAXI/GSC and Swift/XRT observations. Over the course of the outburst, a transition from the low/hard state to the high/soft state and then back to the low/hard state was seen. During the high/soft state, the innermost disk temperature mildly decreased, while the innermost radius estimated with the multi-color disk model remained constant at $\sim18\ (\frac{D}{8.5\ \mathrm{kpc}}) \ {(\frac{\cos i}{\cos 30^{\circ}})}^{-1/2}$ km, where $D$ is the source distance and $i$ is the inclination of observation. This small innermost radius of the accretion disk suggests that the central object is more likely to be a Kerr black hole rather than a Schwardzschild black hole. Applying a relativistic disk emission model to the high/soft state spectra, a mass upper limit of $18.3\ \mathrm{M_{\odot}}$ was obtained based on the inclination limit $i, This paper was accepted by PASJ and published on 2018 May 25

Published
2018
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2. Late Engine Activity of GRB 161017A Revealed by Early Optical Observations

Author

Katsuaki Asano, Nobuyuki Kawai, Makoto Arimoto, Kotaro Morita, S. Harita, Ryosuke Itoh, Yutaro Tachibana, Yoichi Yatsu, T. Fujiwara, Taketoshi Yoshii, and Katsuhiro L. Murata

Subjects
Physics, High Energy Astrophysical Phenomena (astro-ph.HE), Jet (fluid), 010308 nuclear & particles physics, Astrophysics::High Energy Astrophysical Phenomena, Flux, FOS: Physical sciences, Astronomy and Astrophysics, Context (language use), Astrophysics, Kinetic energy, Plateau (mathematics), 01 natural sciences, Afterglow, Space and Planetary Science, 0103 physical sciences, Gamma-ray burst, Astrophysics - High Energy Astrophysical Phenomena, 010303 astronomy & astrophysics, Fermi Gamma-ray Space Telescope
Abstract

The long gamma-ray burst GRB 161017A was detected by Fermi and Swift, and its afterglow was observed by the MITSuME 50-cm optical telescope promptly about 50 s after the burst. Early optical observations revealed that the optical lightcurve exhibits a plateau and rebrightening in the early afterglow phase about 500 and 5000 s after the trigger, respectively. By investigating the behavior of the spectral and temporal flux variation, it was found that the plateau and re-brightening cannot be explained in the context of the simple standard afterglow model. These observational features can be explained with two independent refreshed shocks, which indicate the long-acting central engine. We evaluated the physical parameters of the subsequent shells, and we then determined the kinetic energy ratio of the two colliding shells to the leading shell to be roughly 1 and 8, respectively. In addition, two prominent X-ray flares about 200 s after the trigger may be signatures of delayed ejections of the energetic jets responsible for the refreshed shocks. Such late activity of the central engine and X-ray flares play a crucial role in understanding the mechanisms for jet formation and photon emission.

Published
2018
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3. MITSuME observation of V404 Cygni in the 2015 outburst: Two optical variable components with different variability

Author

Hidekazu Hanayama, Yutaro Tachibana, Taketoshi Yoshii, and Nobuyuki Kawai

Subjects
High Energy Astrophysical Phenomena (astro-ph.HE), Physics, Spectral index, 010308 nuclear & particles physics, Component (thermodynamics), Astrophysics::High Energy Astrophysical Phenomena, FOS: Physical sciences, Astronomy and Astrophysics, Astrophysics, medicine.disease_cause, Light curve, 01 natural sciences, law.invention, Telescope, Space and Planetary Science, Observatory, law, 0103 physical sciences, medicine, Spectral energy distribution, Black-body radiation, Astrophysics - High Energy Astrophysical Phenomena, 010303 astronomy & astrophysics, Astrophysics::Galaxy Astrophysics, Ultraviolet
Abstract

The black hole binary V404 Cygni (= GS 2023+338) had an outburst on June 15, 2015 after 26 years of quiescence. We report the multi-color optical observation (g', Rc, and Ic) of this object at the beginning of its outburst performed by the MITSuME 50cm telescope in Akeno, Yamanashi, and the MURIKABUSHI 105 cm Telescope at Ishigakijima Astronomical Observatory. With time domain analysis of the multi-color light curves, we successfully decomposed optical variations into two components: a highly-variable, and a little-variable component. The loci of the little-variable component in the color-color diagram are consistent with that of the multi-temperature blackbody radiation, while those of the highly-variable component trace out a power-law spectrum with a spectral index {\alpha}~0.6-1.0. For the little-variable component, we argue that an irradiated disk with the innermost temperature higher than ~2.0$\times10^4$ K and the outermost temperature lower than ~6.5$\times10^3$ K is the most plausible source. The gradual rise trend of the light curve during our observation is probably due to the growing of the little-variable component. The observed spectral energy distribution (SED) from the optical to ultraviolet can be expressed by a model consisting of a power-law component and an irradiated disk component., Comment: 11 pages, 7 figures, 3 tables

Published
2017
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4. Discovery of the new X-ray transient MAXI J1807+132: a Candidate of a Neutron Star Low-mass X-ray binary

Author

Taiki Kawamuro, Satoshi Nakahira, Taketoshi Yoshii, Kazuo Makishima, Yoshihiro Ueda, Motoko Serino, Hitoshi Negoro, Nobuyuki Kawai, J. A. Kennea, Megumi Shidatsu, Yutaro Tachibana, and Wataru Iwakiri

Subjects
Physics, High Energy Astrophysical Phenomena (astro-ph.HE), Photon, X-ray transient, 010308 nuclear & particles physics, Astrophysics::High Energy Astrophysical Phenomena, X-ray binary, FOS: Physical sciences, Astronomy and Astrophysics, Astrophysics, Radius, 01 natural sciences, Spectral line, Neutron star, Space and Planetary Science, 0103 physical sciences, Black-body radiation, Astrophysics - High Energy Astrophysical Phenomena, Low Mass, 010303 astronomy & astrophysics
Abstract

We report on the detection and follow-up multi-wavelength observations of the new X-ray transient MAXI J1807+132 with the MAXI/GSC, Swift, and ground-based optical telescopes. The source was first recognized with the MAXI/GSC on 2017 March 13. About a week later, it reached the maximum intensity ($\sim$10 mCrab in 2-10 keV), and then gradually faded in $\sim$10 days by more than one order of magnitude. Time-averaged Swift/XRT spectra in the decaying phase can be described by a blackbody with a relatively low temperature (0.1-0.5 keV), plus a hard power-law component with a photon index of $\sim$2. These spectral properties are similar to those of neutron star low-mass X-ray binaries (LMXBs) in their dim periods. The blackbody temperature and the radius of the emission region varied in a complex manner as the source became dimmer. The source was detected in the optical wavelength on March 27-31 as well. The optical flux decreased monotonically as the X-ray flux decayed. The correlation between the X-ray and optical fluxes is found to be consistent with those of known neutron star LMXBs, supporting the idea that the source is likely to be a transient neutron star LMXB., Comment: 13 pages, 11 figures, accepted for publication in ApJ

Published
2017
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5. No Evidence of Intrinsic Optical/Near-Infrared Linear Polarization for V404 Cygni During its Bright Outburst in 2015: Broadband Modeling and Constraint on Jet Parameters

Author

Makoto Watanabe, A. Ikebe, Daisuke Kuroda, S. Gouda, Yoshiyuki Inoue, K. Sekiguchi, Kumiko Morihana, Kensuke Hosoya, Masataka Imai, Yumiko Oasa, Tatsuya Nakaoka, Tomoki Morokuma, Makoto Uemura, Tomohito Ohshima, Nobuyuki Kawai, Katsutoshi Takaki, Satoshi Honda, C. C. Cheung, Keisuke Isogai, Michitoshi Yoshida, Jun Takahashi, Taichi Kato, T. Fujiwara, Yoshihiko Saito, Yasuyuki T. Tanaka, Daisaku Nogami, Yasushi Fukazawa, Yuya Gouda, Hiroshi Akitaya, Hinako Akimoto, Takumi Nagayama, Masashi Kimura, Taketoshi Yoshii, Yuhei Takagi, Katsuhiro L. Murata, Yutaro Tachibana, Kyoko Watanabe, Yoichi Yatsu, Kensei Shiki, Ryosuke Itoh, Koji S. Kawabata, and Miho Kawabata

Subjects
Astrophysics::High Energy Astrophysical Phenomena, FOS: Physical sciences, Astrophysics, 01 natural sciences, law.invention, stars: jets, symbols.namesake, law, 0103 physical sciences, Astrophysics::Solar and Stellar Astrophysics, Blazar, infrared: stars, 010303 astronomy & astrophysics, Astrophysics::Galaxy Astrophysics, stars: individual: V404 Cyg, Physics, High Energy Astrophysical Phenomena (astro-ph.HE), polarization, 010308 nuclear & particles physics, Linear polarization, stars, Astronomy and Astrophysics, radiation mechanisms: non-thermal, Light curve, Position angle, Synchrotron, Particle acceleration, Lorentz factor, binaries: general, Space and Planetary Science, symbols, Astrophysics - High Energy Astrophysical Phenomena, Flare
Abstract

著者人数: 42名, Accepted: 2016-03-14, 資料番号: SA1160038000

Published
2016
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6. Two optical emission components with different variability in V404 Cygni

Author

Taketoshi Yoshii, Nobuyuki Kawai, and Yutaro Tachibana

Subjects
Physics, Spectral index, Accretion disc, Space and Planetary Science, Astronomy and Astrophysics, Black-body radiation, Astrophysics, Optical emission spectroscopy, Time domain, Light curve, Optical observation, Spectral line
Abstract

V404 Cygni went into an outburst again on June 15, 2015 after 26 years of quietness. Soon after the notifications, we started intense optical observation campaign of this source. The spectral index between RC and IC-band was stable over the outburst, whereas that between g′ and RC-band varied violently. With the time domain analysis of the multi-color optical light curves, we successfully decomposed optical variations into three components: highly-variable component (HVC), little-variable component (LVC). The loci of the LVC in the color-color diagram is consistent with that of a single temperature blackbody radiation or a multi-color blackbody radiation from a standard accretion disk, while those of the HVC trace that of power-law spectra.

Published
2016
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7. Kiso Supernova Survey (KISS): Survey strategy

Author

Mitsuru Kokubo, Tomoki Morokuma, Greg Aldering, Yutaro Tachibana, Ryosuke Itoh, Fumiaki Nakata, Takao Soyano, Akihiko Tomita, Toru Misawa, Yuji Urata, Yuki Kikuchi, Taketoshi Yoshii, Yuki Sarugaku, Mark M. Phillips, Hiroyuki Mito, N. N. Pavlyuk, Takashi Miyata, Katsutoshi Takaki, Kei Sano, Takahiro Ui, Koji S. Kawabata, Michael Richmond, Tsutomu Aoki, D. Y. Tsvetkov, Yoshikazu Nakada, Naoto Kobayashi, Hiroyuki Maehara, Ikuru Iwata, Norio Okada, Nidia Morrell, Nobuyuki Kawai, Nobuharu Ukita, Noriyuki Matsunaga, Hiroshi Akitaya, Nozomu Tominaga, Maximilian Stritzinger, Emma S. Walker, Shigeyuki Sako, Ko Arimatsu, Takumi Shibata, Mamoru Doi, Paolo A. Mazzali, Rina Okamoto, Ken'ichi Tarusawa, Takashi Horiuchi, Francesco Taddia, Masaomi Tanaka, Ken Tateuchi, Daisuke Kuroda, Carlos Contreras, Emiko Matsumoto, Yuki Moritani, Yutaro Kitagawa, Kensho Mori, Yoshihiko Saito, D. K. Sahu, Elena Pian, Satoshi Miyazaki, and Eric Hsiao

Subjects
general [Supernovae], FOS: Physical sciences, RR Lyrae variable, Surveys, 01 natural sciences, supernovae: general, surveys, Observatory, 0103 physical sciences, observations [Cosmology], 010303 astronomy & astrophysics, Solar and Stellar Astrophysics (astro-ph.SR), Physics, High Energy Astrophysical Phenomena (astro-ph.HE), 010308 nuclear & particles physics, Astronomy, Astronomy and Astrophysics, Schmidt camera, Galactic plane, Redshift, Supernova, Stars, Astrophysics - Solar and Stellar Astrophysics, Space and Planetary Science, cosmology: observations, Variable star, Astrophysics - High Energy Astrophysical Phenomena
Abstract

著者人数: 59名, Accepted: 2014-08-26, 資料番号: SA1005030000

Published
2014
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8. MAXI Observations of Gamma-ray Bursts

Author

M. Sasaki, Takahiro Toizumi, Fumitoshi Suwa, Hisaki Yamada, Hiroshi Tomida, Ryuichi Usui, Juri Sugimoto, T. Sakamoto, Hitoshi Negoro, Yujin E. Nakagawa, T. Onodera, Makoto Yamauchi, Masaki Ishikawa, K. Ishikawa, Y. Nishimura, K. Fukushima, Y. Nakano, Y. Ogawa, K. Yoshidome, Takayuki Yamamoto, Tatehiro Mihara, Taketoshi Yoshii, Motoki Nakajima, Nobuyuki Kawai, Masanori Ohno, Toshihiro Takagi, Megumi Shidatsu, Kazuhiko Suzuki, Taiki Kawamuro, Satoshi Nakahira, Masashi Kimura, Yoshihiro Ueda, M. Higa, Motoko Serino, Hiroshi Tsunemi, Mutsumi Sugizaki, Yohko Tsuboi, Yutaro Tachibana, Masaru Matsuoka, Atsumasa Yoshida, Shiro Ueno, Mikio Morii, and Kazutaka Yamaoka

Subjects
Physics, High Energy Transient Explorer, Astrophysics::High Energy Astrophysical Phenomena, gamma-ray burst: general, Astrophysics::Instrumentation and Methods for Astrophysics, Astronomy and Astrophysics, S distribution, Astrophysics, methods: data analysis, Space and Planetary Science, medicine, medicine.symptom, Gamma-ray burst, X-rays: bursts, Confusion
Abstract

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

Published
2014

9. GRB 130427A: A nearby ordinary monster

Author

D. Bersier, T. A. Pritchard, B. Sbarufatti, P. D'Avanzo, Valerio D'Elia, A. Maselli, G. Calderone, Drejc Kopač, Paul Kuin, G. Tagliaferri, Phil Evans, J. A. Nousek, T. Sakamoto, H. Hanayama, Giacomo Vianello, Nobuyuki Kawai, J. P. Osborne, A. P. Beardmore, B. Wiegand, P. Romano, David N. Burrows, Neil Gehrels, A. Gomboc, M. Perri, Iain A. Steele, L. Nava, Taketoshi Yoshii, S. R. Oates, Nial R. Tanvir, S. D. Barthelmy, Chryssa Kouveliotou, C. Pagani, C. Guidorzi, Andrew J. Levan, Giancarlo Ghirlanda, J. A. Kennea, M. Capalbi, Shiho Kobayashi, M. de Pasquale, J. R. Cummings, Y. Yatsu, A. Melandri, M. G. Bernardini, R. Harrison, D. Malesani, V. La Parola, Giancarlo Cusumano, Klaas Wiersema, M. J. Page, P. T. O'Brien, Sergio Campana, B.-B. Zhang, V. Mangano, Y. Saito, James Chiang, F. E. Marshall, J. Japelj, K. L. Page, Stefano Covino, Carole Mundell, Fabio Virgili, Motoko Serino, Gabriele Ghisellini, Daisuke Kuroda, D. Fugazza, Ruben Salvaterra, INAF - Osservatorio Astronomico di Brera (OAB), Istituto Nazionale di Astrofisica (INAF), Racah Institute of Physics (Racah Institute of Physics), The Hebrew University of Jerusalem (HUJ)-Racah Institute of Physics, AstroParticule et Cosmologie (APC (UMR_7164)), Observatoire de Paris, Université Paris sciences et lettres (PSL)-Université Paris sciences et lettres (PSL)-Commissariat à l'énergie atomique et aux énergies alternatives (CEA)-Université Paris Diderot - Paris 7 (UPD7)-Centre National de la Recherche Scientifique (CNRS)-Institut National de Physique Nucléaire et de Physique des Particules du CNRS (IN2P3), RIKEN - Institute of Physical and Chemical Research [Japon] (RIKEN), Istituto di Astrofisica e Planetologia Spaziali - INAF (IAPS), Kyoto University [Kyoto], Institut de médecine moléculaire de Rangueil (I2MR), Université Toulouse III - Paul Sabatier (UT3), Université Fédérale Toulouse Midi-Pyrénées-Université Fédérale Toulouse Midi-Pyrénées-IFR150-Institut National de la Santé et de la Recherche Médicale (INSERM), Astrophysics Research Institute [Liverpool] (ARI), Liverpool John Moores University (LJMU), Chimie Et Interdisciplinarité : Synthèse, Analyse, Modélisation (CEISAM), Université de Nantes - UFR des Sciences et des Techniques (UN UFR ST), Université de Nantes (UN)-Université de Nantes (UN)-Centre National de la Recherche Scientifique (CNRS)-Institut de Chimie du CNRS (INC), ASI-Science Data Center, Rome, NASA Goddard Space Flight Center (GSFC), Faculty of Mathematics and Physics [Ljubljana] (FMF), University of Ljubljana, Dark Cosmology Centre (DARK), Niels Bohr Institute [Copenhagen] (NBI), Faculty of Science [Copenhagen], University of Copenhagen = Københavns Universitet (KU)-University of Copenhagen = Københavns Universitet (KU)-Faculty of Science [Copenhagen], University of Copenhagen = Københavns Universitet (KU)-University of Copenhagen = Københavns Universitet (KU), Mullard Space Science Laboratory (MSSL), University College of London [London] (UCL), Osservatorio Astrofisico di Catania, Institute of Space and Astronautical Science (ISAS), Japan Aerospace Exploration Agency [Sagamihara] (JAXA), INAF-IASF Milano, Department of Physics and Astronomy [Leicester], University of Leicester, Institute of Zoology, Universität Regensburg (UR), Commissariat à l'énergie atomique et aux énergies alternatives (CEA)-Institut National de Physique Nucléaire et de Physique des Particules du CNRS (IN2P3)-Observatoire de Paris, Université Paris sciences et lettres (PSL)-Université Paris sciences et lettres (PSL)-Université Paris Diderot - Paris 7 (UPD7)-Centre National de la Recherche Scientifique (CNRS), Kyoto University, Université de Toulouse (UT)-Université de Toulouse (UT)- Institut Fédératif de Recherche Bio-médicale Institution (IFR150)-Institut National de la Santé et de la Recherche Médicale (INSERM), Université de Nantes (UN)-Université de Nantes (UN)-Institut de Chimie du CNRS (INC)-Centre National de la Recherche Scientifique (CNRS), University of Copenhagen = Københavns Universitet (UCPH)-University of Copenhagen = Københavns Universitet (UCPH)-Faculty of Science [Copenhagen], and University of Copenhagen = Københavns Universitet (UCPH)-University of Copenhagen = Københavns Universitet (UCPH)

Subjects
media_common.quotation_subject, Astrophysics::High Energy Astrophysical Phenomena, FOS: Physical sciences, Astrophysics, Astrophysics::Cosmology and Extragalactic Astrophysics, 01 natural sciences, Luminosity, 0103 physical sciences, 010306 general physics, 010303 astronomy & astrophysics, Astrophysics::Galaxy Astrophysics, ComputingMilieux_MISCELLANEOUS, media_common, QB, High Energy Astrophysical Phenomena (astro-ph.HE), Physics, [PHYS]Physics [physics], Multidisciplinary, Gamma ray, GRB 130427A, Light curve, Universe, Afterglow, Stars, Astrophysics - High Energy Astrophysical Phenomena, Gamma-ray burst, [PHYS.ASTR]Physics [physics]/Astrophysics [astro-ph]
Abstract

Long-duration Gamma-Ray Bursts (GRBs) are an extremely rare outcome of the collapse of massive stars, and are typically found in the distant Universe. Because of its intrinsic luminosity ($L\sim 3 \times 10^{53}$ erg s$^{-1}$) and its relative proximity ($z=0.34$), GRB 130427A was a unique event that reached the highest fluence observed in the gamma-ray band. Here we present a comprehensive multiwavelength view of GRB 130427A with Swift, the 2-m Liverpool and Faulkes telescopes and by other ground-based facilities, highlighting the evolution of the burst emission from the prompt to the afterglow phase. The properties of GRB 130427A are similar to those of the most luminous, high-redshift GRBs, suggesting that a common central engine is responsible for producing GRBs in both the contemporary and the early Universe and over the full range of GRB isotropic energies., 64 pages, 11 figures; published in Science. Update for a few references and correction of a typo in the name of one of the authors

Published
2014
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10. MITSuME observation of V404 Cygni in the 2015 outburst: Two optical variable components with different variability.

Author

Yutaro TACHIBANA, Taketoshi YOSHII, Hidekazu HANAYAMA, and Nobuyuki KAWAI

Subjects
*V404 cygni, *BLACK holes, *ASTRONOMICAL observatories
Abstract

The black hole binary V404 Cygni (= GS 2023+338) had an outburst on 2015 June 15 after 26 years of its quiescence. We report on the multi-color optical observation (g', RC, and IC) of this object at the beginning of its outburst performed by the MITSuME 50 cm telescope in Akeno, Yamanashi, and the MURIKABUSHI 105cm telescope at Ishigakijima Astronomical Observatory. Through time domain analysis of the multi-color light curves, we successfully decomposed the optical variations into two components: a highly variable and a less-variable component. The loci of the less-variable component in the color-color diagram are consistent with that of multi-temperature blackbody radiation, while those of the highly variable component trace out a power-law spectrum with a spectral index α ~ 0.6-1.0. For the less-variable component, we argue that an irradiated disk with the innermost temperature higher than ~2.0 × 104 K and the outermost temperature lower than ~6.5 × 103 K is the most plausible source. The gradual rising trend of the light curve during our observation is probably due to the growing of the less-variable component. The observed spectral energy distribution from optical to ultraviolet can be expressed by a model consisting of a power-law component and an irradiated disk component. [ABSTRACT FROM AUTHOR]

Published
2017
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11. A soft X-ray lag detected in Centaurus A.

Author

Yutaro TACHIBANA, Taiki KAWAMURO, Yoshihiro UEDA, Megumi SHIDATSU, Makoto ARIMOTO, Taketoshi YOSHII, Yoichi YATSU, Yoshihiko SAITO, PIKE, Sean, and Nobuyuki KAWAI

Subjects
LIGHT curves, CENTAURUS (Constellation), ENERGY bands, MAGNETIC fields, ELECTRONS
Abstract

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

Published
2016
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12. Discovery of the New X-Ray Transient MAXI J1807+132: A Candidate of a Neutron Star Low-mass X-Ray Binary.

Author

Megumi Shidatsu, Wataru Iwakiri, Satoshi Nakahira, Kazuo Makishima, Motoko Serino, Yutaro Tachibana, Taketoshi Yoshii, Nobuyuki Kawai, Hitoshi Negoro, Taiki Kawamuro, Yoshihiro Ueda, and Jamie Kennea

Subjects
X-ray binaries, X-ray astronomy, NEUTRON stars, TRANSIENTS (Dynamics), ACCRETION (Astrophysics)
Abstract

We report on the detection and follow-up multi-wavelength observations of the new X-ray transient MAXI J1807+132 with the MAXI/GSC, Swift, and ground-based optical telescopes. The source was first recognized with the MAXI/GSC on 2017 March 13. About a week later, it reached maximum intensity (∼10 mCrab in 2–10 keV), and then gradually faded in ∼10 days by more than one order of magnitude. Time-averaged Swift/X-ray Telescope spectra in the decaying phase can be described by a blackbody with a relatively low temperature (0.1–0.5 keV), plus a hard power-law component with a photon index of ∼2. These spectral properties are similar to those of neutron star low-mass X-ray binaries (LMXBs) in their dim periods. The blackbody temperature and the radius of the emission region varied in a complex manner as the source became dimmer. The source was detected in the optical wavelength on March 27–31 as well. The optical flux decreased monotonically as the X-ray flux decayed. The correlation between the X-ray and optical fluxes is found to be consistent with those of known neutron star LMXBs, supporting the idea that the source is likely to be a transient neutron star LMXB. [ABSTRACT FROM AUTHOR]

Published
2017
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13. MULTI-WAVELENGTH OBSERVATIONS OF THE BLACK WIDOW PULSAR 2FGL J2339.6-0532 WITH OISTER AND SUZAKU.

Author

Yoichi Yatsu, Jun Kataoka, Yosuke Takahashi, Yutaro Tachibana, Nobuyuki Kawai, Shimpei Shibata, Sean Pike, Taketoshi Yoshii, Makoto Arimoto, Yoshihiko Saito, Takeshi Nakamori, Kazuhiro Sekiguchi, Daisuke Kuroda, Kenshi Yanagisawa, Hidekazu Hanayama, Makoto Watanabe, Ko Hamamoto, Hikaru Nakao, Akihito Ozaki, and Kentaro Motohara

Subjects
WAVELENGTHS, PULSARS, STELLAR orbits, STAR observations, SOLAR wind
Abstract

Multi-wavelength observations of the black widow binary system 2FGL J2339.6-0532 are reported. The Fermi gamma-ray source 2FGL J2339.6-0532 was recently categorized as a black widow in which a recycled millisecond pulsar (MSP) is evaporating the companion star with its powerful pulsar wind. Our optical observations show clear sinusoidal light curves due to the asymmetric temperature distribution of the companion star. Assuming a simple geometry, we constrained the range of the inclination angle of the binary system to , which enables us to discuss the interaction between the pulsar wind and the companion in detail. The X-ray spectrum consists of two components: a soft, steady component that seems to originate from the surface of the MSP, and a hard, variable component from the wind-termination shock near the companion star. The measured X-ray luminosity is comparable to the bolometric luminosity of the companion, meaning that the heating efficiency is less than 0.5. In the companion orbit, 1011 cm from the pulsar, the pulsar wind is already in the particle-dominant stage with a magnetization parameter of . In addition, we precisely investigated the time variations of the X-ray periodograms and detected a weakening of the orbital modulation. The observed phenomenon may be related to unstable pulsar wind activity or weak mass accretion, both of which can result in the temporal extinction of the radio pulse. [ABSTRACT FROM AUTHOR]

Published
2015
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