Single-peaked Light Curves Research Articles

The Origin of the Ha-like Structure in Nebular Spectra of Type IIb Supernovae

Abstract We investigate the origin of the Hα-like structure seen in late-phase nebular spectra of type IIb supernovae (SNe IIb) at ∼200 days after the explosion. We compare the luminosities of the emission lines of the nebular spectra with the light curve peak magnitudes to reveal their power sources. In this work, we analyze 7 SNe IIb, as well as 2 SNe Ib (SN 2007Y and iPTF 13bvn), which show the Hα-like emission in their nebular spectra. The luminosity of the Hα-like emission shows a tight correlation with the light curve peak magnitude, sharing the same behavior as other nebular lines. This result indicates that the Hα-like emission is powered by the radiative decay of 56Co. The line flux is then expected to roughly follow the mass of the emitting layer. The variation expected from the diversity of the H-rich envelope mass among SNe IIb (reaching nearly to an order of magnitude) is however not seen, suggesting that it is most likely contributed predominantly by [N ii], not by Hα. While further analysis is limited by the available sample size, we find an indication that SNe IIb with a double-peak light curve, which is interpreted as an outcome of the more extended and massive hydrogen envelope, tend to show greater luminosity of the Hα-like feature than those with a single-peak light curve. This behavior indicates a possible additional contribution from Hα. Additionally, we also find a correlation between the [Ca ii]/[O i] ratio and the post-maximum decline rate, indicating that the [Ca ii]/[O i] can be used as a diagnostic for the progenitor mass.

Transneptunian objects and Centaurs from light curves

Aims. We compile and analyze an extended database of light curve parameters scattered in the literature to search for correlations and study physical properties, including internal structure constraints. Methods. We analyze a vast light curve database by obtaining mean rotational properties of the entire sample, determining the spin frequency distribution and comparing those data with a simple model based on hydrostatic equilibrium. Results. For the rotation periods, the mean value obtained is 6.95 h for the whole sample, 6.88 h for the Trans-neptunian objects (TNOs) alone and 6.75 h for the Centaurs. From Maxwellian fits to the rotational frequencies distribution the mean rotation rates are 7.35 h for the entire sample, 7.71 h for the TNOs alone and 8.95 h for the Centaurs. These results are obtained by taking into account the criteria of considering a single-peak light curve for objects with amplitudes lower than 0.15 mag and a double-peak light curve for objects with variability >0.15 mag. We investigate the effect of using different values other than 0.15 mag for the transition threshold from albedo-caused light curves to shape-caused light curves. The best Maxwellian fits were obtained with the threshold between 0.10 and 0.15 mag. The mean light-curve amplitude for the entire sample is 0.26 mag, 0.25 mag for TNOs only, and 0.26 mag for the Centaurs. The Period versus B −V color shows a correlation that suggests that objects with shorter rotation periods may have suffered more collisions than objects with larger ones. The amplitude versus Hv correlation clearly indicates that the smaller (and collisionally evolved) objects are more elongated than the bigger ones. Conclusions. From the model results, it appears that hydrostatic equilibrium can explain the statistical results of almost the entire sample, which means hydrostatic equilibrium is probably reached by almost all TNOs in the H range [−1,7]. This implies that for plausible albedos of 0.04 to 0.20, objects with diameters from 300 km to even 100 km would likely be in equilibrium. Thus, the great majority of objects would qualify as being dwarf planets because they would meet the hydrostatic equilibrium condition. The best model density corresponds to 1100 kg/m 3 .

How common are long gamma-ray bursts in the local Universe?

Abstract The two closest gamma-ray bursts so far detected (GRBs 980425 and 060218) were both under-luminous, spectrally soft, long-duration bursts with smooth, single-peaked light curves. Only of the order of 100 GRBs have measured redshifts, and there are, for example, 2704 GRBs in the BATSE (Burst and Transient Source Experiment) catalogue alone. It is therefore plausible that other nearby GRBs have been observed but not identified as being relatively nearby. Here we search for statistical correlations between BATSE long-duration GRBs and galaxy samples with recession velocities v≤ 11 000 km s−1 (z= 0.0367, ≈155 Mpc) selected from two catalogues of nearby galaxies. We also examine the correlations using burst subsamples restricted to those with properties similar to the two known nearby bursts. Our results show correlation of the entire long-GRB sample to remain consistent with zero out to the highest radii considered whereas a subsample selected to be low fluence, spectrally soft, with smooth single-peaked light curves (177 bursts), demonstrates increased correlation with galaxies within ≈155 Mpc. The measured correlation (28 ± 16 per cent of the sample) suggests that BATSE observed between two and nine long-duration GRBs per year similar to, and from within similar distances to, GRBs 980425 and 060218. This implies an observed local rate density (to BATSE limits) of 700 ± 360 Gpc−3 yr−1 within 155 Mpc.

Gamma-ray bursts associated with supernovae: a systematic analysis of BATSE GRB candidates

We examined the properties of a sample of BATSE Gamma--Ray Bursts (GRBs) comprising events which have indications of association with a supernova (SN), some on the basis of indications of re--brightening in the optical afterglow light curve, but in most cases based only on the `loose' temporal and directional coincidence inferred from the cross correlation of catalogs. Despite of the large uncertainties in the latter selection method, the temporal and spectral analysis reveal three interesting statistical results when the sample is compared with that of all the BATSE GRBs: the GRBs tentatively associated with SNe are found to predominantly (in $\sim$ 80% of the cases) have single-peaked light curves, a softer spectrum (i.e. low energy power law index $\alpha \sim$ --1.5) and tend not to follow the Lag-Luminosity and Isotropic Energy--Peak Energy correlations. These three independent statistical properties point toward the existence of a significant number of under-luminous,GRB 980425-like events constituting -- at least from an observational point of view -- a tail or a separate class with respect to the whole of the BATSE GRB events. The unusually high percentage of SN Ibc among those identified by the catalog cross--correlation (factor $\sim 4$ higher than expected from SN catalog statistics) reinforces the non-randomness of (some of) the selected events.

Prompt γ-ray and early afterglow emission in the external shock model

We describe our attempt to determine if γ-ray burst (GRB) and afterglow emissions could both arise in external shocks for simple GRBs – bursts consisting of just a few peaks in their light curves. We calculate peak flux and peak frequency during the γ-ray burst for 10 well-observed bursts using the same set of parameters that are determined from modelling afterglow emissions. We find the γ-ray emission properties for 970508 (which had a single-peak light curve) fit nicely with the extrapolation of its afterglow data, and therefore this burst was probably produced in the external shock. One can explain two other bursts in this sample as forward shock synchrotron emission provided that the magnetic field parameter during the burst is close to equipartition, and larger by a factor ∼102 than the afterglow value at ∼1 d. The remaining seven bursts cannot be explained in the external shock model even if we allow the energy fraction in electrons and magnetic field and the density of the surrounding medium to take on any physically permitted value; the peak of the spectrum is above the cooling frequency, therefore the peak flux is independent of the latter two of these parameters, and is smaller by about an order of magnitude than the observed values. We have also considered inverse-Compton scattering in forward and reverse shock regions and find that it can explain the γ-ray emission for a few bursts, but requires the density to be 1–2 orders of magnitude larger than a typical Wolf–Rayet star wind and much larger than permitted by late afterglow observations. We have also calculated emission from the reverse shock for these ten bursts and find the flux in the optical band for more than half of these bursts to be between 9th and 12th magnitude at the deceleration time if the reverse shock microphysics parameters are the same as those found from afterglow modelling and the deceleration time is of the order of the burst duration. However, the cooling frequency in the reverse shock for most of these bursts is below the optical band, and therefore the observed flux decays rapidly with time (as ∼t−3) and is unobservable after a few deceleration times. It is also possible that the deceleration time is much larger than the burst duration, in which case we expect weak reverse shock emission.

Shading and Smothering of Gamma-Ray Bursts

GRB 980425 is distinctive in that it seems to be associated with SN 1998bw, has no X-ray afterglow, and has a rounded, structureless, single-peak light curve and a relatively soft spectrum. The supernova is itself unusual in that its expansion velocity exceeds c/6. We suggest that many of these features can be accounted for with the hypothesis that we observe the gamma-ray burst along a penumbral line of sight that contains mainly photons that have scattered off ejected baryons. The hypothesis suggests a baryon-poor jet existing within a baryon-rich outflow. The sharp distinction can be attributed to whether or not the magnetic field lines thread an event horizon. Such a configuration suggests that there will be some nonthermal acceleration of pickup ex-neutrons within the baryon-poor jet. This scenario might produce observable spallation products and neutrinos.