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Register a new userThe Unique Type Ib Supernova 2005bf at Nebular Phases: A Possible Birth Event of A Strongly Magnetized Neutron Star
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Late phase nebular spectra and photometry of Type Ib Supernova (SN) 2005bf taken by the Subaru telescope at ~ 270 and ~ 310 days since the explosion are presented. Emission lines ([OI]6300, 6363, [CaII]7291, 7324, [FeII]7155) show the blueshift of ~ 1,500 - 2,000 km s-1. The [OI] doublet shows a doubly-peaked profile. The line luminosities can be interpreted as coming from a blob or jet containing only ~ 0.1 - 0.4 Msun, in which ~ 0.02 - 0.06 Msun is 56Ni synthesized at the explosion. To explain the blueshift, the blob should either be of unipolar moving at the center-of-mass velocity v ~ 2,000 - 5,000 km s-1, or suffer from self-absorption within the ejecta as seen in SN 1990I. In both interpretations, the low-mass blob component dominates the optical output both at the first peak (~ 20 days) and at the late phase (~ 300 days). The low luminosity at the late phase (the absolute R magnitude M_R ~ -10.2 mag at ~ 270 days) sets the upper limit for the mass of 56Ni < ~ 0.08 Msun, which is in contradiction to the value necessary to explain the second, main peak luminosity (M_R ~ -18.3 mag at ~ 40 days). Encountered by this difficulty in the 56Ni heating model, we suggest an alternative scenario in which the heating source is a newly born, strongly magnetized neutron star (a magnetar) with the surface magnetic field Bmag ~ 10^{14-15} gauss and the initial spin period P0 ~ 10 ms. Then, SN 2005bf could be a link between normal SNe Ib/c and an X-Ray Flash associated SN 2006aj, connected in terms of Bmag and/or P0.
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Observations and modeling for the light curve (LC) and spectra of supernova (SN) 2005bf are reported. This SN showed unique features: the LC had two maxima, and declined rapidly after the second maximum, while the spectra showed strengthening He lines whose velocity increased with time. The double-peaked LC can be reproduced by a double-peaked $^{56}$Ni distribution, with most $^{56}$Ni at low velocity and a small amount at high velocity. The rapid post-maximum decline requires a large fraction of the $gamma$-rays to escape from the $^{56}$Ni-dominated region, possibly because of low-density ``holes. The presence of Balmer lines in the spectrum suggests that the He layer of the progenitor was substantially intact. Increasing $gamma$-ray deposition in the He layer due to enhanced $gamma$-ray escape from the $^{56}$Ni-dominated region may explain both the delayed strengthening and the increasing velocity of the He lines. The SN has massive ejecta ($sim6-7Msun$), normal kinetic energy ($sim 1.0-1.5times 10^{51}$ ergs), high peak bolometric luminosity ($sim 5times 10^{42}$ erg s$^{-1}$) for an epoch as late as $sim$ 40 days, and a large $^{56}$Ni mass ($sim0.32Msun$). These properties, and the presence of a small amount of H suggest that the progenitor was initially massive (M$sim 25-30 Msun$) and had lost most of its H envelope, and was possibly a WN star. The double-peaked $^{56}$Ni distribution suggests that the explosion may have formed jets that did not reach the He layer. The properties of SN 2005bf resemble those of the explosion of Cassiopeia A.
We present BVRI photometry and optical spectroscopy of SN 2005bf near light maximum. The maximum phase is broad and occurred around 2005 May 7, about forty days after the shock breakout. SN 2005bf has a peak bolometric magnitude M_{bol}=-18.0pm 0.2: while this is not particularly bright, it occurred at an epoch significantly later than other SNe Ibc, indicating that the SN possibly ejected ~0.31 M_{sun} of 56Ni, which is more than the typical amount. The spectra of SN 2005bf around maximum are very similar to those of the Type Ib SNe 1999ex and 1984L about 25-35 days after explosion, displaying prominent He I, Fe II, Ca II H & K and the near-IR triplet P Cygni lines. Except for the strongest lines, He I absorptions are blueshifted by <~6500 km/s, and Fe II by ~7500-8000 km/s. No other SNe Ib have been reported to have their Fe II absorptions blueshifted more than their He I absorptions. Relatively weak H-alpha and very weak H-beta may also exist, blueshifted by ~15,000 km/s. We suggest that SN 2005bf was the explosion of a massive He star, possibly with a trace of a hydrogen envelope.
Aims. We present and analyse late-time observations of the type-Ib supernova with possible pre-supernova progenitor detection, iPTF13bvn, taken at $sim$300 days after the explosion, and discuss these in the context of constraints on the supernovas progenitor. Previous studies have proposed two possible natures for the progenitor of the supernova, i.e. a massive Wolf-Rayet star or a lower-mass star in close binary system. Methods. Our observations show that the supernova has entered the nebular phase, with the spectrum dominated by Mg~I]$lambdalambda$4571, [O~I]$lambdalambda$6300, 6364, and [Ca~II]$lambdalambda$7291, 7324 emission lines. We measured the emission line fluxes to estimate the core oxygen mass and compare the [O~I]/[Ca~II] line ratio with other supernovae. Results. The core oxygen mass of the supernova progenitor was estimated to be $lesssim$0.7 M$_odot$, which implies initial progenitor mass not exceeding $sim$15 -- 17 M$_odot$. Since the derived mass is too small for a single star to become a Wolf-Rayet star, this result lends more support to the binary nature of the progenitor star of iPTF13bvn. The comparison of [O~I]/[Ca~II] line ratio with other supernovae also shows that iPTF13bvn appears to be in close association with the lower-mass progenitors of stripped-envelope and type-II supernovae.
We present spectropolarimetric observations of the peculiar Type Ib/c SN 2005bf, in MCG+00-27-005, from 3600-8550AA. The SN was observed on 2005 April 30.9, 18 days after the first B-band light-curve maximum and 6 days before the second B-band light-curve maximum. The degree of the Interstellar Polarization, determined from depolarized emission lines in the spectrum, is found to be large with $p_{max}(ISP)=1.6%$ and $theta(ISP)=149$fdg$7pm4.0$, but this may be an upper limit on the real value of the ISP. After ISP subtraction, significant polarization is observed over large wavelength regions, indicating a significant degree of global asymmetry, $gtrsim 10%$. Polarizations of 3.5% and 4% are observed for absorption components of Ca II H&K and IR triplet, and 1.3% for He I 5876AA and Fe II. On the $Q-U$ plane clear velocity-dependent loop structure is observed for the He I 5876AA line, suggestive of departures from an axial symmetry and possible clumping of the SN ejecta. Weak High Velocity components of $mathrm{Halpha}$, $mathrm{Hbeta}$ and $mathrm{Hgamma}$ are observed, with velocities of -15 000kms. The low degree of polarization observed at H$beta$ suggests that the polarization observed for the other Balmer lines ($sim 0.4%$ above the background polarization) may rather be due to blending of $mathrm{Halpha}$ and $mathrm{Hgamma}$ with polarized Si II and Fe II lines, respectively. We suggest a model in which a jet of material, that is rich in $mathrm{^{56}Ni}$, has penetrated the C-O core, but not the He mantle. The jet axis is tilted with respect to the axis of the photosphere. This accounts for the lack of significant polarization of O I 7774AA, the delayed excitation and, hence, observability of He I and, potentially, the varied geometries of He and Ca.
Magnetars are a special type of neutron stars, considered to have extreme dipole magnetic fields reaching ~1e+11 T. The magnetar 4U 0142+61, one of prototypes of this class, was studied in broadband X-rays (0.5-70 keV) with the Suzaku observatory. In hard X-rays (15-40 keV), its 8.69 sec pulsations suffered slow phase modulations by +/-0.7 sec, with a period of ~15 hours. When this effect is interpreted as free precession of the neutron star, the object is inferred to deviate from spherical symmetry by ~1.6e-4 in its moments of inertia. This deformation, when ascribed to magnetic pressure, suggests a strong toroidal magnetic field, ~1e+12 T, residing inside the object. This provides one of the first observational approaches towards toroidal magnetic fields of magnetars.
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