arXiv subjects

Masaomi Tanaka

Publications and source records attributed to Masaomi Tanaka.

At least 145 records · Page 8Linked to original sources

arXiv2016

J-GEM Follow-Up Observations to Search for an Optical Counterpart of The First Gravitational Wave Source GW150914

We present our optical follow-up observations to search for an electromagnetic counterpart of the first gravitational wave source GW150914 in the framework of the Japanese collaboration for Gravitational wave ElectroMagnetic follow-up (J-GEM), which is an observing group utilizing optical and radio telescopes in Japan, as well as those in New Zealand, China, South Africa, Chile, and Hawaii. We carried out a wide-field imaging survey with Kiso Wide Field Camera (KWFC) on the 1.05-m Kiso Schmidt telescope in Japan and a galaxy-targeted survey with Tripole5 on the B&C 61-cm telescope in New Zealand. Approximately 24 deg2 regions in total were surveyed in i-band with KWFC and 18 nearby galaxies were observed with Tripole5 in g-, r-, and i-bands 4-12 days after the gravitational wave detection. Median 5-sigma depths are i~18.9 mag for the KWFC data and g~18.9 mag, r~18.7 mag, and i~18.3 mag for the Tripole5 data. Probability for a counterpart to be in the observed area is 1.2% in the initial skymap and 0.1% in the final skymap. We do not find any transient source associated to an external galaxy with spatial offset from its center, which is consistent with the local supernova rate. We summarize future prospects and ongoing efforts to pin down electromagnetic counterparts of binary black hole mergers as well as neutron star mergers.

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arXiv2016

OISTER Optical and Near-Infrared Observations of the Super-Chandrasekhar Supernova Candidate SN 2012dn: Dust Emission from the Circumstellar Shell

We present extensively dense observations of the super-Chandrasekhar supernova (SC SN) candidate SN 2012dn from $-11$ to $+140$ days after the date of its $B$-band maximum in the optical and near-infrared (NIR) wavelengths conducted through the OISTER ToO program. The NIR light curves and color evolutions up to 35 days after the $B$-band maximum provided an excellent match with those of another SC SN 2009dc, providing a further support to the nature of SN 2012dn as a SC SN. We found that SN 2012dn exhibited strong excesses in the NIR wavelengths from $30$ days after the $B$-band maximum. The $H$ and $K_{s}$-band light curves exhibited much later maximum dates at $40$ and $70$ days after the $B$-band maximum, respectively, compared with those of normal SNe Ia. The $H$ and $K_{s}$-band light curves subtracted by those of SN 2009dc displayed plateaued evolutions, indicating a NIR echo from the surrounding dust. The distance to the inner boundary of the dust shell is limited to be $4.8 - 6.4\times10^{-2}$ pc. No emission lines were found in its early phase spectrum, suggesting that the ejecta-CSM interaction could not occur. On the other hand, we found no signature that strongly supports the scenario of dust formation. The mass loss rate of the pre-explosion system is estimated to be $10^{-6}-10^{-5}$ M$_{\odot}$ yr$^{-1}$, assuming that the wind velocity of the system is $10-100$ km~s$^{-1}$, which suggests that the progenitor of SN 2012dn could be a recurrent nova system. We conclude that the progenitor of this SC SN could be explained by the single-degenerate scenario.

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arXiv2016

An Effective Selection Method for Low-Mass Active Black Holes and First Spectroscopic Identification

We present a new method to effectively select objects which may be low-mass active black holes (BHs) at galaxy centers using high-cadence optical imaging data, and our first spectroscopic identification of an active 2.7x10^6 Msun BH at z=0.164. This active BH was originally selected due to its rapid optical variability, from a few hours to a day, based on Subaru Hyper Suprime-Cam~(HSC) g-band imaging data taken with 1-hour cadence. Broad and narrow H-alpha and many other emission lines are detected in our optical spectra taken with Subaru FOCAS, and the BH mass is measured via the broad H-alpha emission line width (1,880 km s^{-1}) and luminosity (4.2x10^{40} erg s^{-1}) after careful correction for the atmospheric absorption around 7,580-7,720A. We measure the Eddington ratio to be as low as 0.05, considerably smaller than those in a previous SDSS sample with similar BH mass and redshift, which indicates one of the strong potentials of our Subaru survey. The g-r color and morphology of the extended component indicate that the host galaxy is a star-forming galaxy. We also show effectiveness of our variability selection for low-mass active BHs.

astro-ph.GA↗

arXiv2016

Rapidly Rising Transients from Subaru Hyper Suprime-Cam Transient Survey

We present rapidly rising transients discovered by a high-cadence transient survey with Subaru telescope and Hyper Suprime-Cam. We discovered five transients at z=0.384-0.821 showing the rising rate faster than 1 mag per 1 day in the restframe near-ultraviolet wavelengths. The fast rising rate and brightness are the most similar to SN 2010aq and PS1-13arp, for which the ultraviolet emission within a few days after the shock breakout was detected. The lower limit of the event rate of rapidly rising transients is ~9 % of core-collapse supernova rates, assuming a duration of rapid rise to be 1 day. We show that the light curves of the three faint objects agree with the cooling envelope emission from the explosion of red supergiants. The other two luminous objects are, however, brighter and faster than the cooling envelope emission. We interpret these two objects to be the shock breakout from dense wind with the mass loss rate of ~10^{-3} Msun yr^{-1}, as also proposed for PS1-13arp. This mass loss rate is higher than that typically observed for red supergiants. The event rate of these luminous objects is >~1 % of core-collapse supernova rate, and thus, our study implies that more than ~1 % of massive stars can experience an intensive mass loss at a few years before the explosion.

astro-ph.HE↗

arXiv2015

Radioactive decay products in neutron star merger ejecta: heating efficiency and $γ$-ray emission

The radioactive decay of the freshly synthesized $r$-process nuclei ejected in compact binary mergers power optical/infrared macronovae (kilonovae) that follow these events. The light curves depend critically on the energy partition among the different products of the radioactive decay and this plays an important role in estimates of the amount of ejected $r$-process elements from a given observed signal. We study the energy partition and $γ$-ray emission of the radioactive decay. We show that $20$-$50\%$ of the total radioactive energy is released in $γ$-rays on timescales from hours to a month. The number of emitted $γ$-rays per unit energy interval has roughly a flat spectrum between a few dozen keV and $1$ MeV so that most of this energy is carried by $\sim 1$ MeV $γ$-rays. However at the peak of macronova emission the optical depth of the $γ$-rays is $\sim 0.02$ and most of the $γ$-rays escape. The loss of these $γ$-rays reduces the heat deposition into the ejecta and hence reduces the expected macronova signals if those are lanthanides dominated. This implies that the ejected mass is larger by a factor of $2$-$3$ than what was previously estimated. Spontaneous fission heats up the ejecta and the heating rate can increase if a sufficient amount of transuranic nuclei are synthesized. Direct measurements of these escaping $γ$-rays may provide the ultimate proof for the macronova mechanisms and an identification of the $r$-process nucleosynthesis sites. However, the chances to detect these signals are slim with current X-ray and $γ$-ray missions. New detectors, more sensitive by at least a factor of ten, are needed for a realistic detection rate.

astro-ph.HE↗

arXiv2015

Kepler's Supernova: An Overluminous Type Ia Event Interacting with a Massive Circumstellar Medium at a Very Late Phase

We have analyzed XMM-Newton, Chandra, and Suzaku observations of Kepler's supernova remnant (SNR) to investigate the properties of both the SN ejecta and the circumstellar medium (CSM). For comparison, we have also analyzed two similarly-aged, ejecta-dominated SNRs: Tycho's SNR, thought to be the remnant of a typical Type Ia SN, and SNR 0509-67.5 in the Large Magellanic Cloud, thought to be the remnant of an overluminous Type Ia SN. By simply comparing the X-ray spectra, we find that line intensity ratios of iron-group elements (IGE) to intermediate-mass elements (IME) for Kepler's SNR and SNR 0509-67.5 are much higher than those for Tycho's SNR. We therefore argue that Kepler is the product of an overluminous Type Ia SN. This inference is supported by our spectral modeling, which reveals the IGE and IME masses respectively to be ~0.95 M_sun and ~0.12 M_sun (Kepler's SNR), ~0.75 M_sun and ~0.34 M_sun (SNR 0509-67.5), and ~0.35 M_sun and ~0.70 M_sun (Tycho's SNR). We find that the CSM component in Kepler's SNR consists of tenuous diffuse gas (~0.3 M_sun) present throughout the entire remnant, plus dense knots (~0.035 M_sun). Both of these components have an elevated N abundance (N/H ~ 4 times the solar value), suggesting that they originate from CNO-processed material from the progenitor system. The mass of the diffuse CSM allows us to infer the pre-SN mass-loss rate to be ~1.5e-5 (V_w/10 km/s) M_sun/yr, in general agreement with results from recent hydrodynamical simulations. Since the dense knots have slow proper motions and relatively small ionization timescales, they were likely located a few pc away from the progenitor system. Therefore, we argue that Kepler's SN was an overluminous event that started to interact with massive CSM a few hundred years after the explosion. This supports the possible link between overluminous SNe and the so-called "Ia-CSM" SNe.

astro-ph.HE↗

arXiv2015

OISTER Optical and Near-Infrared Observations of Type Iax Supernova 2012Z

We report observations of the Type Iax supernova (SN Iax) 2012Z at optical and near-infrared wavelengths from immediately after the explosion until $\sim$ $260$ days after the maximum luminosity using the Optical and Infrared Synergetic Telescopes for Education and Research (OISTER) Target-of-Opportunity (ToO) program and the Subaru telescope. We found that the near-infrared (NIR) light curve evolutions and color evolutions are similar to those of SNe Iax 2005hk and 2008ha. The NIR absolute magnitudes ($M_{J}\sim-18.1$ mag and $M_{H}\sim-18.3$ mag) and the rate of decline of the light curve ($Δ$ $m_{15}$($B$)$=1.6 \pm 0.1$ mag) are very similar to those of SN 2005hk ($M_{J}\sim-17.7$ mag, $M_{H}\sim$$-18.0$ mag, and $Δ$ $m_{15}$($B$)$\sim1.6$ mag), yet differ significantly from SNe 2008ha and 2010ae ($M_{J}\sim-14 - -15$ mag and $Δ$ $m_{15}$($B$)$\sim2.4-2.7$ mag). The estimated rise time is $12.0 \pm 3.0$ days, which is significantly shorter than that of SN 2005hk or any other Ia SNe. The rapid rise indicates that the $^{56}$Ni distribution may extend into the outer layer or that the effective opacity may be lower than that in normal SNe Ia. The late-phase spectrum exhibits broader emission lines than those of SN 2005hk by a factor of 6--8. Such high velocities of the emission lines indicate that the density profile of the inner ejecta extends more than that of SN 2005hk. We argue that the most favored explosion scenario is a `failed deflagration' model, although the pulsational delayed detonations is not excluded.

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arXiv2014

Radiation hydrodynamics simulations of wide-angle outflows from super-critical accretion disks around black holes

By performing two-dimensional radiation hydrodynamics simulations with large computational domain of 5000 Schwarzschild radius, we revealed that wide-angle outflow is launched via the radiation force from the super-critical accretion flows around black holes. The angular size of the outflow, of which the radial velocity (v_r) is over the escape velocity (v_esc), increases with an increase of the distance from the black hole. As a result, the mass is blown away with speed of v_r > v_esc in all direction except for the very vicinity of the equatorial plane, theta=0-85^circ, where theta is the polar angle. The mass ejected from the outer boundary per unit time by the outflow is larger than the mass accretion rate onto the black hole, ~150L_Edd/c^2, where L_Edd and c are the Eddington luminosity and the speed of light. Kinetic power of such wide-angle high-velocity outflow is comparable to the photon luminosity and is a few times larger than the Eddington luminosity. This corresponds to ~10^39-10^40 erg/s for the stellar mass black holes. Our model consistent with the observations of shock excited bubbles observed in some ultra-luminous X-ray sources (ULXs), supporting a hypothesis that ULXs are powered by the super-critical accretion onto stellar mass black holes.

astro-ph.HE↗

arXiv2014

The red supergiant and supernova rate problems: implications for core-collapse supernova physics

Mapping supernovae to their progenitors is fundamental to understanding the collapse of massive stars. We investigate the red supergiant problem, which concerns why red supergiants with masses $\sim16$-$30 M_\odot$ have not been identified as progenitors of Type IIP supernovae, and the supernova rate problem, which concerns why the observed cosmic supernova rate is smaller than the observed cosmic star formation rate. We find key physics to solving these in the compactness parameter, which characterizes the density structure of the progenitor. If massive stars with compactness above $ξ_{2.5} \sim 0.2$ fail to produce canonical supernovae, (i) stars in the mass range $16$-$30 M_\odot$ populate an island of stars that have high $ξ_{2.5}$ and do not produce canonical supernovae, and (ii) the fraction of such stars is consistent with the missing fraction of supernovae relative to star formation. We support this scenario with a series of two- and three-dimensional radiation hydrodynamics core-collapse simulations. Using more than 300 progenitors covering initial masses $10.8$-$75 M_\odot$ and three initial metallicities, we show that high compactness is conducive to failed explosions. We then argue that a critical compactness of $\sim 0.2$ as the divide between successful and failed explosions is consistent with state-of-the-art three-dimensional core-collapse simulations. Our study implies that numerical simulations of core collapse need not produce robust explosions in a significant fraction of compact massive star initial conditions.

astro-ph.HE↗

arXiv2014

Discovery of Dramatic Optical Variability in SDSS J1100+4421: A Peculiar Radio-Loud Narrow-Line Seyfert 1 Galaxy?

We present our discovery of dramatic variability in SDSS J1100+4421 by the high-cadence transient survey Kiso Supernova Survey (KISS). The source brightened in the optical by at least a factor of three within about half a day. Spectroscopic observations suggest that this object is likely a narrow-line Seyfert 1 galaxy (NLS1) at z=0.840, however with unusually strong narrow emission lines. The estimated black hole mass of ~ 10^7 Msun implies bolometric nuclear luminosity close to the Eddington limit. SDSS J1100+4421 is also extremely radio-loud, with a radio loudness parameter of R ~ 4 x 10^2 - 3 x 10^3, which implies the presence of relativistic jets. Rapid and large-amplitude optical variability of the target, reminiscent of that found in a few radio- and gamma-ray loud NLS1s, is therefore produced most likely in a blazar-like core. The 1.4 GHz radio image of the source shows an extended structure with a linear size of about 100 kpc. If SDSS J1100+4421 is a genuine NLS1, as suggested here, this radio structure would then be the largest ever discovered in this type of active galaxies.

astro-ph.GA↗

arXiv2014

Kiso Supernova Survey (KISS): Survey Strategy

The Kiso Supernova Survey (KISS) is a high-cadence optical wide-field supernova (SN) survey. The primary goal of the survey is to catch the very early light of a SN, during the shock breakout phase. Detection of SN shock breakouts combined with multi-band photometry obtained with other facilities would provide detailed physical information on the progenitor stars of SNe. The survey is performed using a 2.2x2.2 deg field-of-view instrument on the 1.05-m Kiso Schmidt telescope, the Kiso Wide Field Camera (KWFC). We take a three-minute exposure in g-band once every hour in our survey, reaching magnitude g~20-21. About 100 nights of telescope time per year have been spent on the survey since April 2012. The number of the shock breakout detections is estimated to be of order of 1 during our 3-year project. This paper summarizes the KISS project including the KWFC observing setup, the survey strategy, the data reduction system, and CBET-reported SNe discovered so far by KISS.

astro-ph.SR↗

arXiv2014

Random walks and effective optical depth in relativistic flow

We investigate the random walk process in relativistic flow. In the relativistic flow, photon propagation is concentrated in the directions of the flow velocity due to relativistic beaming effect. We show that, in the pure scattering case, the number of scatterings is proportional to the size parameter $ξ\equiv L/l_0$ if the flow velocity $β\equiv v/c$ satisfies $β/Γ\gg ξ^{-1}$, while it is proportional to $ξ^2$ if $β/Γ\ll ξ^{-1}$ where $L$ and $l_0$ are the size of the system in the observer frame and the mean free path in the comoving frame, respectively. We also examine the photon propagation in the scattering and absorptive medium. We find that, if the optical depth for absorption $τ_{\rm a}$ is considerably smaller than the optical depth for scattering $τ_{\rm s}$ ($τ_{\rm a}/τ_{\rm s} \ll 1$) and the flow velocity satisfies $β\gg \sqrt{2τ_{\rm a}/τ_{\rm s}}$, the effective optical depth is approximated by $τ_*\simeqτ_{\rm a}(1+β)/β$. Furthermore, we perform Monte Carlo simulations of radiative transfer and compare the results with the analytic expression for the number of scattering. The analytic expression is consistent with the results of the numerical simulations. The expression derived in this Letter can be used to estimate the photon production site in relativistic phenomena, e.g., gamma-ray burst and active galactic nuclei.

astro-ph.HE↗

arXiv2014

Early-phase photometry and spectroscopy of transitional Type Ia SN 2012ht: Direct constraint on the rise time

We report photometric and spectroscopic observations of the nearby Type Ia Supernova (SN Ia) 2012ht from $-15.8$ days to $+49.1$ days after $B$-band maximum. The decline rate of the light curve is $Δm_{15}$($B$)$=1.39~\pm~0.05$ mag, which is intermediate between normal and subluminous SNe Ia, and similar to that of the `transitional' Type Ia SN 2004eo. The spectral line profiles also closely resemble those of SN 2004eo. We were able to observe SN 2012ht at very early phase, when it was still rising and was about three magnitudes fainter than at the peak. The rise time to the $B$-band maximum is estimated to be $17.6 \pm 0.5$ days and the time of the explosion is MJD $56277.98 \pm 0.13$. SN 2012ht is the first transitional SN Ia whose rise time is directly measured without using light curve templates, and the fifth SN Ia overall. This rise time is consistent with those of the other four SNe within the measurement error, even including the extremely early detection of SN 2013dy. The rising part of the light curve can be fitted by a quadratic function, and shows no sign of a shock-heating component due to the interaction of the ejecta with a companion star. The rise time is significantly longer than that inferred for subluminous SNe such as SN 1991bg, which suggests that a progenitor and/or explosion mechanism of transitional SNe Ia are more similar to normal SNe Ia rather than subluminous SNe Ia.

astro-ph.SR↗

arXiv2013

Radioactively Powered Emission from Black Hole-Neutron Star Mergers

Detection of electromagnetic counterparts of gravitational wave (GW) sources is important to unveil the nature of compact binary coalescences. We perform three-dimensional, time-dependent, multi-frequency radiative transfer simulations for radioactively powered emission from the ejecta of black hole (BH) - neutron star (NS) mergers. Depending on the BH to NS mass ratio, spin of the BH, and equations of state of dense matter, BH-NS mergers can eject more material than NS-NS mergers. In such cases, radioactively powered emission from the BH-NS merger ejecta can be more luminous than that from NS-NS mergers. We show that, in spite of the expected larger distances to BH-NS merger events, observed brightness of BH-NS mergers can be comparable to or even higher than that of NS-NS mergers. We find that, when the tidally disrupted BH-NS merger ejecta are confined to a small solid angle, the emission from BH-NS merger ejecta tends to be bluer than that from NS-NS merger ejecta for a given total luminosity. Thanks to this property, we might be able to distinguish BH-NS merger events from NS-NS merger events by multi-band observations of the radioactively powered emission. In addition to the GW observations, such electromagnetic observations can potentially provide independent information on the nature of compact binary coalescences.

astro-ph.HE↗

arXiv2013

Progenitor models of the electromagnetic transient associated with the short GRB 130603B

An electromagnetic transient powered by the radioactive decay of r-process elements, a so-called kilonova/macronova, is one of the possible observable consequences of compact binary mergers including at east one neutron star. Recent observations strongly suggest the first discovery of the electromagnetic transient, which is associated with the short GRB 130603B. We explore a possible progenitor of this event combining the numerical-relativity simulations and radiative transfer simulations of the dynamical ejecta of binary neutron star and black hole - neutron star mergers. We show that the ejecta models within a realistic parameter range consistently reproduce the observed near-infrared excess. We also show that the soft equation of state models for binary neutron star mergers and the stiff equation of state models for black hole - neutron star mergers are favored to reproduce the observed luminosity.

astro-ph.HE↗

arXiv2013

Detectability of High-Redshift Superluminous Supernovae with Upcoming Optical and Near-Infrared Surveys - II. Beyond z=6

Observational identification of the first stars is one of the great challenges in the modern astronomy. Although a single first star is too faint to be detected, supernova explosions of the first stars can be bright enough. An important question is whether such supernovae can be detected in the limited observational area with realistic observational resources. We perform detailed simulations to study the detectability of superluminous supernovae (SLSNe) at high redshifts, using the observationally-calibrated star formation rate density and supernova occurrence rate. We show that a 100 deg^2 survey with the limiting magnitude of 26 mag in near-infrared wavelengths will be able to discover about 10 SLSNe at z > 10. If the survey is extended to 200 deg^2 with 27 mag depth, about 10 SLSNe can be discovered at z > 15. We emphasize that the observations at > 3 um are crucial to detect and select SLSNe at z > 10. Our simulations are also applied to the planned survey with Euclid, WFIRST, and WISH. These surveys will be able to detect about 1000, 400, and 3000 SLSNe up to z ~ 5, 7, and 12, respectively. We conclude that detection of SLSNe at z > 10 is in fact achievable in the near future.

astro-ph.CO↗

arXiv2013

Radiative Transfer Simulations for Neutron Star Merger Ejecta

The merger of binary neutron stars (NSs) is among the most promising gravitational wave (GW) sources. Next-generation GW detectors are expected to detect signals from the NS merger within 200 Mpc. Detection of electromagnetic wave (EM) counterpart is crucial to understand the nature of GW sources. Among possible EM emission from the NS merger, emission powered by radioactive r-process nuclei is one of the best targets for follow-up observations. However, prediction so far does not take into account detailed r-process element abundances in the ejecta. We perform radiative transfer simulations for the NS merger ejecta including all the r-process elements from Ga to U for the first time. We show that the opacity in the NS merger ejecta is about kappa = 10 cm^2 g^{-1}, which is higher than that of Fe-rich Type Ia supernova ejecta by a factor of ~ 100. As a result, the emission is fainter and longer than previously expected. The spectra are almost featureless due to the high expansion velocity and bound-bound transitions of many different r-process elements. We demonstrate that the emission is brighter for a higher mass ratio of two NSs and a softer equation of states adopted in the merger simulations. Because of the red color of the emission, follow-up observations in red optical and near-infrared (NIR) wavelengths will be the most efficient. At 200 Mpc, expected brightness of the emission is i = 22 - 25 AB mag, z = 21 - 23 AB mag, and 21 - 24 AB mag in NIR JHK bands. Thus, observations with wide-field 4m- and 8m-class optical telescopes and wide-field NIR space telescopes are necessary. We also argue that the emission powered by radioactive energy can be detected in the afterglow of nearby short gamma-ray bursts.

astro-ph.HE↗

arXiv2013

The very energetic, broad-lined type Ic Supernova 2010ah (PTF10bzf) in the context of GRB/SNe

SN2010ah, a very broad-lined type Ic SN discovered by the Palomar Transient Factory, was interesting because of its relatively high luminosity and the high velocity of the absorption lines, which was comparable to that of GRB/SNe, suggesting a high explosion kinetic energy. However, no GRB was detected in association with the SN. Here, the properties of SN2010ah are determined with higher accuracy than previous studies through modelling. New Subaru telescope photometry is presented. A bolometric light curve is constructed taking advantage of the spectral similarity with SN1998bw. Radiation transport tools are used to reproduce the spectra and the light curve. The results thus obtained regarding ejecta mass, composition and kinetic energy are then used to compute a synthetic light curve. This is in reasonable agreement with the early bolometric light curve of SN2010ah, but a high abundance of 56Ni at high velocity is required to reproduce the early rise, while a dense inner core must be used to reproduce the slow decline at late phases. The high-velocity 56Ni cannot have been located on our line of sight, which may be indirect evidence for an off-axis, aspherical explosion. The main properties of SN2010ah are: ejected mass ~ 3 Mo; kinetic energy ~10^52 erg, M(56Ni) ~ 0.25 Mo. The mass located at v >~ 0.1c is ~0.2 Mo. Although these values, in particular the kinetic energy, are quite large for a SN Ic, they are all smaller (especially the ejecta mass) than those typical of GRB/SNe. This confirms the tendency for these quantities to correlate, and suggests that there are minimum requirements for a GRB/SN, which SN2010ah may not meet although it comes quite close. Depending on whether a neutron star or a black hole was formed following core collapse, SN2010ah was the explosion of a CO core of ~ 5-6 Mo, pointing to a progenitor mass of ~24 - 28 Mo.

astro-ph.CO↗