arXiv subjects

Koji S. Kawabata

Publications and source records attributed to Koji S. Kawabata.

At least 109 records · Page 6Linked to original sources

arXiv2012

Three-Dimensional Explosion Geometry of Stripped-Envelope Core-Collapse Supernovae. I. Spectropolarimetric Observations

We study the multi-dimensional geometry of supernova (SN) explosions by means of spectropolarimetric observations of stripped-envelope SNe, i.e., SNe without a H-rich layer. We perform spectropolarimetric observations of 2 stripped-envelope SNe, the Type Ib SN 2009jf and the Type Ic SN 2009mi. Both objects show non-zero polarization at the wavelength of the strong lines. They also show a loop in the Stokes Q-U diagram, which indicates a non-axisymmetric, three-dimensional ion distribution in the ejecta. We show that five out of six stripped-envelope SNe which have been observed spectropolarimetrically so far show such a loop. This implies that a three-dimensional geometry is common in stripped-envelope SNe. We find that stronger lines tend to show higher polarization. This effect is not related to the geometry, and must be corrected to compare the polarization of different lines or different objects. Even after the correction, however, there remains a dispersion of polarization degree among different objects. Such a dispersion might be caused by three-dimensional clumpy ion distributions viewed from different directions.

astro-ph.SR↗

arXiv2012

The Highly Energetic Expansion of SN2010bh Associated with GRB 100316D

We present the spectroscopic and photometric evolution of the nearby (z = 0.059) spectroscopically confirmed type Ic supernova, SN 2010bh, associated with the soft, long-duration gamma-ray burst (X-ray flash) GRB 100316D. Intensive follow-up observations of SN 2010bh were performed at the ESO Very Large Telescope (VLT) using the X-shooter and FORS2 instruments. Owing to the detailed temporal coverage and the extended wavelength range (3000--24800 A), we obtained an unprecedentedly rich spectral sequence among the hypernovae, making SN 2010bh one of the best studied representatives of this SN class. We find that SN 2010bh has a more rapid rise to maximum brightness (8.0 +/- 1.0 rest-frame days) and a fainter absolute peak luminosity (L_bol~3e42 erg/s) than previously observed SN events associated with GRBs. Our estimate of the ejected (56)Ni mass is 0.12 +/- 0.02 Msun. From the broad spectral features we measure expansion velocities up to 47,000 km/s, higher than those of SNe 1998bw (GRB 980425) and 2006aj (GRB 060218). Helium absorption lines He I lambda5876 and He I 1.083 microm, blueshifted by ~20,000--30,000 km/s and ~28,000--38,000 km/s, respectively, may be present in the optical spectra. However, the lack of coverage of the He I 2.058 microm line prevents us from confirming such identifications. The nebular spectrum, taken at ~186 days after the explosion, shows a broad but faint [O I] emission at 6340 A. The light-curve shape and photospheric expansion velocities of SN 2010bh suggest that we witnessed a highly energetic explosion with a small ejected mass (E_k ~ 1e52 erg and M_ej ~ 3 Msun). The observed properties of SN 2010bh further extend the heterogeneity of the class of GRB supernovae.

astro-ph.HE↗

arXiv2011

Multi-Wavelength Photometric and Polarimetric Observations of the Outburst of 3C 454.3 in Dec. 2009

In December 2009, the bright blazar, 3C 454.3 exhibited a strong outburst in the optical, X-ray and gamma-ray regions. We performed photometric and polarimetric monitoring of this outburst in the optical and near-infrared bands with TRISPEC and HOWPol attached to the Kanata telescope. We also observed this outburst in the infrared band with AKARI, and the radio band with the 32-m radio telescope of Yamaguchi University. The object was in an active state from JD 2455055 to 2455159. It was 1.3 mag brighter than its quiescent state before JD 2455055 in the optical band. After the end of the active state in JD 2455159, a prominent outburst was observed in all wavelengths. The outburst continued for two months. Our optical and nearinfrared polarimetric observations revealed that the position angle of the polarization (PA) apparently rotated clockwise by 240 degrees within 11 d in the active state (JD 2455063-2455074), and after this rotation, PA remained almost constant during our monitoring. In the outburst state, PA smoothly rotated counterclockwise by 350 degrees within 35 d (JD 2455157-2455192). Thus, we detected two distinct rotation events of polarization vector in opposite directions. We discuss these two events compared with the past rotation events observed in 2005, 2007 and 2008.

astro-ph.HE↗

arXiv2011

Correlation between Interstellar Polarization and Dust Temperature: Alignment of Grains by Radiative Torques is Ubiquitous?

We investigate the efficiency of interstellar polarization $p_λ/A_λ$ where $p_λ$ is the fractional linear polarization and $A_λ$ is extinction, in 16 lines of sight as a function of wavelength $λ$. We have used the data obtained with the low-dispersion spectropolarimeter HBS as well as those in literature. It is found that the polarization efficiency $p_λ/A_λ$ is proportional to $\exp(-β/λ)$ in wavelength $λ\approx 0.4-0.8 \micron$, where $β$ is a parameter which varies from 0.5 to 1.2 $\micron$. We find that $β$ is negatively correlated with the dust temperature deduced from infrared data by Schlegel et al., suggesting that the polarization efficiency is higher in short wavelength for higher temperature. According to the alignment theory by radiative torques (RATs), if the radiation is stronger and bluer, RATs will make small grains align better, and the polarization efficiency will increase in short wavelength. Our finding of the correlation between $β$ and the temperature is consistent with what is expected with the alignment mechanism by RATs.

astro-ph.GA↗

arXiv2011

Photopolarimetric Monitoring of Blazars in the Optical and Near-Infrared Bands with the Kanata Telescope. I. Correlations between Flux, Color, and Polarization

We report on the correlation between the flux, color and polarization variations on time scales of days--months in blazars, and discuss their universal aspects. We performed monitoring of 42 blazars in the optical and near-infrared bands from 2008 to 2010 using TRISPEC attached to the "Kanata" 1.5-m telescope. We found that 28 blazars exhibited "bluer-when-brighter" trends in their whole or a part of time-series data sets. This corresponds to 88% of objects that were observed for >10 days. Thus, our observation unambiguously confirmed that the "bluer-when-brighter" trend is common in the emission from blazar jets. This trend was apparently generated by a variation component with a constant and relatively blue color and an underlying red component. Prominent short-term flares on time scales of days--weeks tended to exhibit a spectral hysteresis; their rising phases were bluer than their decay phases around the flare maxima. In contrast to the strong flux--color correlation, the correlation of the flux and polarization degree was relatively weak; only 10 objects showed significant positive correlations. Rotations of polarization were detected only in three objects: PKS 1510-089, 3C 454.3, and PKS 1749+096, and possibly in S5 0716+714. We also investigated the dependence of the degree of variability on the luminosity and the synchrotron peak frequency, ν_peak. As a result, we found that lower luminosity and higher ν_peak objects had smaller variations in their amplitudes both in the flux, color, and polarization degree. Our observation suggests the presence of several distinct emitting sources, which have different variation time-scales, colors, and polarizations. We propose that the energy injection by, for example, internal shocks in relativistic shells is a major factor for blazar variations on time scales of both days and months.

astro-ph.HE↗

arXiv2011

Grisms Developed for FOCAS

Faint Object Camera and Spectrograph (FOCAS) is a versatile common-use optical instrument for the 8.2m Subaru Telescope, offering imaging and spectroscopic observations. FOCAS employs grisms with resolving powers ranging from 280 to 8200 as dispersive optical elements. A grism is a direct-vision grating composed of a transmission grating and prism(s). FOCAS has five grisms with replica surface-relief gratings including an echelle-type grism, and eight grisms with volume-phase holographic (VPH) gratings. The size of these grisms is 110 mm X 106 mm in aperture with a maximum thickness of 110 mm. We employ not only the dichromated gelatin, but also the hologram resin as a recording material for VPH gratings. We discuss the performance of these FOCAS grisms measured in the laboratory, and verify it by test observations, and show examples of astronomical spectroscopic observations.

astro-ph.IM↗

arXiv2011

Prominent Polarized Flares of the Blazars AO 0235+164 and PKS 1510-089

We report on multi-band photometric and polarimetric observations of the blazars AO 0235+164 and PKS 1510-089. These two blazars were active in 2008 and 2009, respectively. In these active states, prominent short flares were observed in both objects, having amplitudes of >1 mag within 10 d. The $V-J$ color became bluer when the objects were brighter in these flares. On the other hand, the color of PKS 1510-089 exhibited a trend that it became redder when it was brighter, except for its prominent flare. This redder-when-brighter trend can be explained by the strong contribution of thermal emission from an accretion disk. The polarization degree increased at the flares, and reached >25 % at the maxima. We compare these flares in AO 0235+164 and PKS 1510-089 with other short flares which were detected by our monitoring of 41 blazars. Those two flares had one of the largest variation amplitudes in both flux and polarization degree. Furthermore, we found a significant positive correlation between the amplitudes of the flux and polarization degree in the short flares. It indicates that the short flares originate from the region where the magnetic field is aligned.

astro-ph.HE↗

arXiv2010

Spectropolarimetry of the superwind filaments of the starburst galaxy M82: kinematics of dust outflow

Spectropolarimetry results for the starburst galaxy M82 are presented. The optical emission lines of the filaments in the energetic outflow ("superwind") from the nuclear starburst region of M82 are substantially polarized. The H-alpha polarization degrees and angles measured by our study are consistent with previous narrowband imaging polarimetry data. The polarized emission lines are redshifted with respect to the emission lines in the total light and systemic motion of the galaxy. The emission line intensity ratios [NII]/H-alpha and [SII]/H-alpha in the polarized light are similar to those of the nuclear star-forming region. In addition, the electron density N_e derived from the[SII]6731/6717 line ratio of the polarized light is 600 - 1000 cm^-3 at a distance of more than 1 kpc from the nucleus, whereas the N_e derived from the total light are less than 300 cm^-3. These facts strongly suggest that the emission from the nuclear starburst of M82 is scattered by dust grains entrained and transported outward by the superwind. A simple hollow biconical outflow model shows that the velocity of the outflowing dust grains, v_d, ranges from 100 to 200 km/s near the nucleus, decreases monotonically with the distance from the nucleus, and reaches about 10 km/s at around 1 kpc. The motion of the dust is substantially slower than that of both ionized gas (~600 km/s) and molecular gas (~200 km/s) at the same distance from the nucleus of M82. This indicates that dust grains in the superwind are kinematically decoupled from both gas components at large radii. Since the dust velocity v_d is much less than the escape velocity of M82 (~170 km/s at 1.5 kpc from the nucleus), most of the dust entrained by the superwind cannot escape to intergalactic space, and may fall back into the galaxy disk without any additional acceleration mechanisms (such as radiation pressure).

astro-ph.CO↗

arXiv2010

Early Spectroscopy of the 2010 Outburst of U Scorpii

We present early spectroscopy of the recurrent nova U~Sco during the outburst in 2010. We successfully obtained time-series spectra at $t_{\rm d}=$0.37--0.44~d, where $t_{\rm d}$ denotes the time from the discovery of the present outburst. This is the first time-resolved spectroscopy on the first night of U Sco outbursts. At $t_{\rm d}\sim 0.4$~d the H$α$ line consists of a blue-shifted ($-5000$ km s$^{-1}$) narrow absorption component and a wide emission component having triple peaks, a blue ($\sim -3000$ km s$^{-1}$), a central ($\sim 0$ km s$^{-1}$) and a red ($\sim +3000$ km s$^{-1}$) ones. The blue and red peaks developed more rapidly than the central one during the first night. This rapid variation would be caused by the growth of aspherical wind produced during the earliest stage of the outburst. At $t_{\rm d}=1.4$~d the H$α$ line has a nearly flat-topped profile with weak blue and red peaks at $\sim \pm 3000$ km s$^{-1}$. This profile can be attributed to a nearly spherical shell, while the asphericity growing on the first night still remains. The wind asphericity is less significant after $t_{\rm d}=9$ d.

astro-ph.SR↗

arXiv2010

Optical and Near-Infrared Photometry of Nova V2362 Cyg : Rebrightening Event and Dust Formation

We present optical and near-infrared (NIR) photometry of a classical nova, V2362 Cyg (= Nova Cygni 2006). V2362 Cyg experienced a peculiar rebrightening with a long duration from 100 to 240 d after the maximum of the nova. Our multicolor observation indicates an emergence of a pseudophotosphere with an effective temperature of 9000 K at the rebrightening maximum. After the rebrightening maximum, the object showed a slow fading homogeneously in all of the used bands for one week. This implies that the fading just after the rebrightening maximum ( less or equal 1 week ) was caused by a slowly shrinking pseudophotosphere. Then, the NIR flux drastically increased, while the optical flux steeply declined. The optical and NIR flux was consistent with blackbody radiation with a temperature of 1500 K during this NIR rising phase. These facts are likely to be explained by dust formation in the nova ejecta. Assuming an optically thin case, we estimate the dust mass of 10^(-8) -- 10^(-10) M_solar, which is less than those in typical dust-forming novae. These results support the senario that a second, long-lasting outflow, which caused the rebrightening, interacted with a fraction of the initial outflow and formed dust grains.

astro-ph.SR↗

arXiv2010

Spectropolarimetry of Extremely Luminous Type Ia Supernova 2009dc: Nearly Spherical Explosion of Super-Chandrasekhar Mass White Dwarf

We present the first spectropolarimetric observations of a candidate of super-Chandrasekhar mass Type Ia supernova (SN): SN 2009dc. The observations were performed at 5.6 and 89.5 days after the B-band maximum. The data taken at the later epoch are used to determine the interstellar polarization. Continuum polarization is found to be small (<0.3 %), indicating that the explosion is nearly spherically symmetric. This fact suggests that a very aspherical explosion is not a likely scenario for SN 2009dc. Polarization at the Si II and Ca II lines clearly shows a loop in the Q-U plane, indicating a non-axisymmetric, clumpy distribution of intermediate-mass elements. The degree of line polarization at the Si and Ca line is moderate (0.5% +- 0.1% and 0.7% +- 0.1%, respectively), but it is higher than expected from the trend of other Type Ia SNe. This may suggest that there are thick enough, clumpy Si-rich layers above the thick 56Ni-rich layers (>~ 1.2 Msun). The observed spectropolarimetric properties, combined with the photometric and spectroscopic properties, suggest that the progenitor of SN 2009dc has a super-Chandrasekhar mass, and that the explosion geometry is globally spherically symmetric, with clumpy distribution of intermediate-mass elements.

astro-ph.CO↗

arXiv2010

Multiband Photopolarimetric Monitoring of the Outburst of the Blazar 3C~454.3 in 2007

We report on optical-near-infrared photopolarimetric observations of a blazar 3C 454.3 over 200 d. The object experienced an optical outburst in July 2007. This outburst was followed by a short state fainter than $V=15.2$ mag lasting $\sim 25$ d. The object, then, entered an active state during which we observed short flares having a timescale of 3-10 d. The object showed two types of features in the color-magnitude relationship. One is a "bluer-when-brighter" trend in the outburst state, and the other is a "redder-when-brighter" trend in the faint state. These two types of features suggest a contribution of a thermal emission to the observed flux, as suspected in previous studies. Our polarimetric observation detected two episodes of the rotation of the polarization vector. The first one was a counterclockwise rotation in the $QU$ plane during the outburst state. After this rotation event of the polarization vector, the object entered a rapidly fading stage. The second one was seen in a series of flares during the active state. Each flare had a specific position angle of polarization, and it apparently rotated clockwise from the first to the last flares. Thus, the object exhibited rotations of the polarization vector in opposite directions. We estimated a decay timescale of the short flares during the active state, and then calculated an upper limit of the strength of the magnetic field, $B$=0.2 G, assuming a typical beaming factor of blazars, $δ=20$. This upper limit of $B$ is smaller than those previously estimated from spectral analysis.

astro-ph.CO↗

arXiv2009

Dwarf Novae in the Shortest Orbital Period Regime: I. A New Short Period Dwarf Nova, OT J055717+683226

We report the observation of a new dwarf nova, OT J055717+683226, during its first-ever recorded superoutburst in December 2006. Our observation shows that this object is an SU UMa-type dwarf nova having a very short superhump period of 76.67+/- 0.03 min (0.05324+/-0.00002 d). The next superoutburst was observed in March 2008. The recurrence time of superoutbursts (supercycle) is, hence, estimated to be ~480 d. The supercycle is much shorter than those of WZ Sge-type dwarf novae having supercycles of >~ 10 yr, which are a major population of dwarf novae in the shortest orbital period regime (<~85 min). Using a hierarchical cluster analysis, we identified seven groups of dwarf novae in the shortest orbital period regime. We identified a small group of objects that have short supercycles, small outburst amplitudes, and large superhump period excesses, compared with those of WZ Sge stars. OT J055717+683226 probably belongs to this group.

astro-ph.SR↗

arXiv2009

Bayesian Approach to Find a Long-Term Trend in Erratic Polarization Variations Observed in Blazars

We developed a method to separate a long-term trend from observed temporal variations of polarization in blazars using a Bayesian approach. The temporal variation of the polarization vector is apparently erratic in most blazars, while several objects occasionally exhibited systematic variations, for example, an increase of the polarization degree associated with a flare of the total flux. We assume that the observed polarization vector is a superposition of distinct two components, a long-term trend and a short-term variation component responsible for short flares. Our Bayesian model estimates the long-term trend which satisfies the condition that the total flux correlates with the polarized flux of the short-term component. We demonstrate that assumed long-term polarization components are successfully separated by the Bayesian model for artificial data. We applied this method to photopolarimetric data of OJ 287, S5 0716+714, and S2 0109+224. Simple and systematic long-term trends were obtained in OJ 287 and S2 0109+224, while no such a trend was identified in S5 0716+714. We propose that the apparently erratic variations of polarization in OJ 287 and S2 0109+224 are due to the presence of the long-term polarization component. The behavior of polarization in S5 0716+714 during our observation period implies the presence of a number of polarization components having a quite short time-scale of variations.

astro-ph.CO↗

arXiv2009

Tohoku-Hiroshima-Nagoya planetary spectra library: A method for characterizing planets in the visible to near infrared

There has not been a comprehensive framework for comparing spectral data from different planets.Such a framework is needed for the study of extrasolar planets and objects within the solar system. We have undertaken observations to compile a library of planet spectra for all planets, some moons, and some dwarf planets in the solar system to study their general spectroscopic and photometric natures. During May and November of 2008, we acquired spectra for the planets using TRISPEC, which is capable of simultaneous three-band spectroscopy in a wide wavelength range of 0.45 - 2.5 microns with low resolving power (lambda-over-Delta-lambda is 140 - 360). Patterns emerge from comparing the spectra. Analyzing their general spectroscopic and photometric natures, we show that it is possible to distinguish between gas planets, soil planets and ice planets. These methods can be applied to extrasolar observations using low resolution spectrography or broad-band filters. The present planet spectral library is the first library to contain observational spectra for all of the solar system planets, based on simultaneous observations in visible and near infrared wavelengths. This library will be a useful reference for analyzing extrasolar planet spectra, and for calibrating planetary data sets.

astro-ph.EP↗

arXiv2009

Spectropolarimetry of the Unique Type Ib Supernova 2005bf: Larger Asymmetry Revealed by Later-Phase Data

We present an optical spectropolarimetric observation of the unique Type Ib supernova (SN) 2005bf at 8 days after the second maximum. The data, combined with the polarization spectrum taken at 6 days before the second maximum (Maund et al. 2007a), enable us to closely examine the intrinsic properties of the SN. The polarization percentage is smaller at the later epoch over a wide wavelength range, while the position angle is similar at the two epochs. We find that an assumption of complete depolarization of strong lines at the emission peak is not necessarily correct. The intrinsic polarization of the SN is larger, and thus, the ejecta of SN 2005bf would be more asymmetric than previously expected. The axis ratio of the photosphere projected on the sky deviates from unity by at least 20 %. If the position angle of interstellar polarization is aligned with the spiral arm of the host galaxy, the deviation is larger than 25 %. The line polarization at the He I, Ca II and Fe II lines is also smaller at the later epoch. The changes in the position angle across these lines, which were observed at the earlier epoch, are still marginally present at the later epoch. The similar polarimetric behavior suggests that the distributions of these ions are correlated. Properties of polarization, as well as the light curve and the spectra both at photospheric and nebular phases, can be explained by an aspherical, possibly unipolar explosion of a WN star in which the blob of 56Ni penetrates C+O core and stops within the He layer.

astro-ph.HE↗

arXiv2009

Nebular Phase Observations of the Type Ib Supernova 2008D/X-ray Transient 080109: Side-Viewed Bipolar Explosion

We present optical spectroscopic and photometric observations of supernova (SN) 2008D, associated with the luminous X-ray transient 080109, at >300 days after the explosion (nebular phases). We also give flux measurements of emission lines from the H II region at the site of the SN, and estimates of the local metallicity. The brightness of the SN at nebular phases is consistent with the prediction of the explosion models with an ejected 56Ni mass of 0.07 Msun, which explains the light curve at early phases. The [O I] line in the nebular spectrum shows a double-peaked profile while the [Ca II] line does not. The double-peaked [O I] profile strongly indicates that SN 2008D is an aspherical explosion. The profile can be explained by a torus-like distribution of oxygen viewed from near the plane of the torus. We suggest that SN 2008D is a side-viewed, bipolar explosion with a viewing angle of > 50^{\circ} from the polar direction.

astro-ph.HE↗

arXiv2009

Early Spectral Evolution of the Rapidly Expanding Type Ia SN 2006X

We present optical spectroscopic and photometric observations of Type Ia supernova (SN) 2006X from --10 to +91 days after the $B$-band maximum. This SN exhibits one of the highest expansion velocity ever published for SNe Ia. At premaximum phases, the spectra show strong and broad features of intermediate-mass elements such as Si, S, Ca, and Mg, while the O{\sc i}$λ$7773 line is weak. The extremely high velocities of Si{\sc ii} and S{\sc ii} lines and the weak O{\sc i} line suggest that an intense nucleosynthesis might take place in the outer layers, favoring a delayed detonation model. Interestingly, Si{\sc ii}$λ$5972 feature is quite shallow, resulting in an unusually low depth ratio of Si{\sc ii}$λ$5972 to $λ$6355, $\cal R$(Si{\sc ii}). The low $\cal R$(Si{\sc ii}) is usually interpreted as a high photospheric temperature. However, the weak Si{\sc iii}$λ$4560 line suggests a low temperature, in contradiction to the low $\cal R$(Si{\sc ii}). This could imply that the Si{\sc ii}$λ$5972 line might be contaminated by underlying emission. We propose that $\cal R$(Si{\sc ii}) may not be a good temperature indicator for rapidly expanding SNe Ia at premaximum phases.

astro-ph.HE↗