Kenzo Kinugasa

THE TYPE IC HYPERNOVA SN 2002AP

Photometric and spectroscopic data of the energetic Type Ic supernova (SN) 2002ap are presented, and the properties of the SN are investigated through models of its spectral evolution and its light curve. The SN is spectroscopically similar to the hypernova SN 1997ef. However, its kinetic energy [~(4-10) ? 1051 ergs] and the mass ejected (2.5-5 M?) are smaller, resulting in a faster evolving light curve. The SN synthesized ~0.07 M? of 56Ni, and its peak luminosity was similar to that of normal SNe. Brightness alone should not be used to define a hypernova, whose defining character, namely very broad spectral features, is the result of high kinetic energy. The likely main-sequence mass of the progenitor star was 20-25 M?, which is also lower than that of both hypernovae SN 1997ef and SN 1998bw. SN 2002ap appears to lie at the low-energy and low-mass end of the hypernova sequence as it is known so far. Observations of the nebular spectrum, which is expected to dominate by the summer of 2002, are necessary to confirm these values.

Discovery of Non-Thermal X-Rays from the Northwest Shell of the New SNR RX J1713.7–3946: The Second SN 1006?

We report ASCA results of a featureless X-ray spectrum from RX J1713.7−3946, a new shell-like SNR discovered with the ROSAT all sky survey. The northwest part of RX J1713.7−3946 was in the field of the ASCA Galactic Plane Survey Project and was found to exhibit a shell-like structure. The spectrum, however shows neither line emission nor any signature of a thermal origin. Instead, a power-law model with a photon index of 2.4-2.5 gives reasonable fit to the spectrum, suggesting a non-thermal origin. Together with the similarity to SN1006, we propose that RX J1713.7−3946 is the second example, after SN1006, of a synchrotron X-ray radiation from a shell of SNRs. Since the synchrotron X-rays suggest existence of extremely high energy charged particles in the SNR shell, our discovery should have strong impact on the origin of the cosmic X-rays.

Early Phase Obserbations of Extermely Luminous Type Ia Supernova 2009dc

We present early phase observations in optical and near-infrared wavelengths for the extremely luminous Type Ia supernova (SN Ia) 2009dc. The decline rate of the light curve is ?m 15(B) = 0.65 ? 0.03, which is one of the slowest among SNe Ia. The peak V-band absolute magnitude is estimated to be MV = ?19.90 ? 0.15?mag if no host extinction is assumed. It reaches MV = ?20.19 ? 0.19?mag if we assume the host extinction of AV = 0.29?mag. SN 2009dc belongs to the most luminous class of SNe Ia, like SNe 2003fg and 2006gz. Our JHKs -band photometry shows that this SN is also one of the most luminous SNe Ia in near-infrared wavelengths. We estimate the ejected 56Ni mass of 1.2 ? 0.3 M ? for the no host extinction case (and of 1.6 ? 0.4 M ? for the host extinction of AV = 0.29?mag). The C II ?6580 absorption line remains visible until a week after the maximum brightness, in contrast to its early disappearance in SN 2006gz. The line velocity of Si II ?6355 is about 8000?km?s?1 around the maximum, being considerably slower than that of SN 2006gz. The velocity of the C II line is similar to or slightly less than that of the Si II line around the maximum. The presence of the carbon line suggests that the thick unburned C+O layer remains after the explosion. Spectropolarimetric observations by Tanaka et?al. indicate that the explosion is nearly spherical. These observational facts suggest that SN 2009dc is a super-Chandrasekhar mass SN Ia.

Discovery of a 7 Second X-Ray Pulsar, AX J1845.0?0300

We report here a serendipitous discovery of a faint X-ray pulsar, AX J1845.0-0300, with ASCA. Its period of ~7 s, together with its low luminosity and the soft spectrum, suggest that it may be the eighth candidate member of braking X-ray pulsars. Two observations, 3.5 yr apart, showed a significant flux decrease of more than 1 order of magnitude. The source is in the Scutum arm region (l ~ 30°), which was claimed as a colony of X-ray pulsars by Koyama et al. This discovery strengthens the hypothesis by Koyama et al. that there may be a number of low-luminosity pulsars on the Galactic plane, particularly in the Scutum arm region.

Spin-Down of the 65 Millisecond X-Ray Pulsar in the Supernova Remnant G11.2–0.3

We report here the first measurement of the spin-down rate of the 65 ms X-ray pulsar within the supernova remnant G11.2-0.3. The period derivative is measured to be = (4.40) × 10-14 s s-1 using the Advanced Satellite for Cosmology and Astrophysics and BeppoSAX. From the pulsar period and its derivative, the corresponding surface magnetic field (B = 1.7 × 1012 G) and the characteristic age [P/(2) = 2.4 × 104 yr] are derived. The physical association of the pulsar and supernova remnant with the historical record of A.D. 386 is discussed in the context of the measured . If the pulsar was formed during the historical event, the initial pulse period should have been relatively slow (P0 62 ms). This initial period is more than a factor of 3 larger than that of a small number of prototypical objects, suggesting a diversity of initial periods for newly formed neutron stars.

Suzaku Observations of Tycho's Supernova Remnant

Tychos supernova remnant was observed by the XIS and HXD instruments onboard the Suzaku satellite on 2006 June 26-29 for 92 ks. The spectrum up to 30 keV was well fitted with a two-component model, consisting of a power-law with photon index of 2.7 and a thermal bremsstrahlung model with temperature of 4.7 keV. The former component can alternatively be modeled as synchrotron emission from a population of relativistic electrons with an estimated roll-off energy of around 1 keV. In the XIS spectra, in addition to the prominent Fe K_alpha line (6.445 keV), we observe for the first time significant K_alpha line emission from the trace species Cr and Mn at energies of 5.48 keV and 5.95 keV, respectively. Faint K_beta lines from Ca (4.56 keV) and Fe (7.11 keV) are also seen. The ionization states of Cr and Mn, based on their line centroids, are estimated to be similar to that of Fe K_alpha (Fe XV or XVI).

Survey of period variations of superhumps in SU UMa-type dwarf novae. V. The fifth year (2012–2013)

Continuing the project described by Kato et al. (2009a, arXiv:0905.1757), we collected times of superhump maxima for SU UMa-type dwarf novae mainly observed during the 2012-2013 season. We found three objects (V444 Peg, CSS J203937 and MASTER J212624) having strongly positive period derivatives despite the long orbital period (Porb). By using the period of growing stage (stage A) superhumps, we obtained mass ratios for six objects. We characterized nine new WZ Sge-type dwarf novae. We made a pilot survey of the decline rate of slowly fading part of SU UMa-type and WZ Sge-type outbursts. The decline time scale was found to generally follow the expected Porb^(1/4) dependence and WZ Sge-type outbursts also generally follow this trend. There are some objects which show slower decline rates, and we consider these objects good candidates for period bouncers. We also studied unusual behavior in some objects, including BK Lyn which made a transition from an ER UMa-type state to the novalike (standstill) state in 2013 and unusually frequent occurrence of superoutbursts in NY Ser and CR Boo. We applied least absolute shrinkage and selection operator (Lasso) power spectral analysis, which has been proven to be very effective in analyzing the Kepler data, to ground-based photometry of BK Lyn and detected the dramatic disappearance of the signal of negative superhumps in 2013. We suggested that the mass-transfer rates did not vary strongly between the ER UMa-type state and novalike state in BK Lyn, and this transition was less likely caused by a systematic variation of the mass-transfer rate.

ASCA Observations of the Crab-Like Supernova Remnant 3C 58

We present here X-ray observation of a Crab-like supernova remnant (SNR) 3C 58 with ASCA. We find that the integrated energy spectrum over the nebula is consistent with previous results, showing a power-law spectrum with the photon index of 7 = 2.2-2.4, modified by interstellar absorption of about (3-4) x 10 21 cm -2 . The inclusion of a blackbody component, which is attributable to the central compact source, significantly improves the spectral fit. Stringent upper limits for any line emitting thin hot plasma are established. We find for the first time that the nebular spectrum is harder in the central part of the SNR, becoming softer toward the periphery, while the absorption column is uniform across the nebula. Correspondingly, the nebular size decreases with increasing photon energy, which is a steeper function of the radius than that of the Crab nebula. The results are compared with synchrotron energy-loss models> and the nature of the putative pulsar is discussed. A timing analysis was performed to search for pulsed X-ray emission from the central compact source. No significant pulsations were observed; and we present the upper limit for the pulsed fraction.

Period Change of Superhumps in a WZ Sge-Type Dwarf Nova, HV Virginis

After 10 years of quiescence, HV Vir underwent a superoutburst in 2002 January. We report on time-series observations that clearly reveal the period change of ordinary superhumps during the superoutburst. We derived a mean superhump period of 0.058260d and a positive period derivative of 7 × 10 −5 . These results are in good agreement with a value obtained from the 1992 superoutburst. We also detected early superhumps, which were not clearly recognized in the past outburst, and a possible rebrightening. Both of them are common characteristics of WZ Sge-type stars.

Discovery of a 220 Second X-Ray Pulsar, AX J1749.2–2725

We report here the discovery of a faint 220 s X-ray pulsar, AX J1749.2-2725, with ASCA. In the analysis, we used a method that can detect a pulsating source in a field of view contaminated by stray light. The source was observed on four occasions between 1995 March and 1997 September. The observed source flux was ~3.0 × 10-11 ergs s-1 cm-2 (2-10 keV) in 1995 March and became an order of magnitude fainter during the 2.5 yr interval. We cannot rule out the possibility that AX J1749.2-2725 is a nearby (d 1.3 kpc) DQ Her-type white dwarf binary, while the hard X-ray spectrum and the transient behavior suggest that it is a distant (d ~ 8.5 kpc) binary system containing a neutron star.