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Gravitational-wave emission can lead to the coalescence of close pairs of compact objects orbiting each other.sup.1,2. In the case of neutron stars, such mergers may yield masses above the Tolman-Oppenheimer-Volkoff limit (2 to 2.7 solar masses).sup.3, leading to the formation of black holes.sup.4. For white dwarfs, the mass of the merger product may exceed the Chandrasekhar limit, leading either to a thermonuclear explosion as a type Ia supernova.sup.5,6 or to a collapse forming a neutron star.sup.7,8. The latter case is expected to result in a hydrogen- and helium-free circumstellar nebula and a hot, luminous, rapidly rotating and highly magnetized central star with a lifetime of about 10,000 years.sup.9,10. Here we report observations of a hot star with a spectrum dominated by emission lines, which is located at the centre of a circular mid-infrared nebula. The widths of the emission lines imply that wind material leaves the star with an outflow velocity of 16,000 kilometres per second and that rapid stellar rotation and a strong magnetic field aid the wind acceleration. Given that hydrogen and helium are probably absent from the star and nebula, we conclude that both objects formed recently from the merger of two massive white dwarfs. Our stellar-atmosphere and wind models indicate a stellar surface temperature of about 200,000 kelvin and a luminosity of about 10.sup.4.6 solar luminosities. The properties of the star and nebula agree with models of the post-merger evolution of super-Chandrasekhar-mass white dwarfs.sup.9, which predict a bright optical and high-energy transient upon collapse of the star.sup.11 within the next few thousand years. Our observations indicate that super-Chandrasekhar-mass white-dwarf mergers can avoid thermonuclear explosion as type Ia supernovae, and provide evidence of the generation of magnetic fields in stellar mergers. The merger of two white dwarfs created a massive, hot, luminous, rotating and magnetized star with a lifetime of several thousand years, which will collapse into a type Ic supernova., Author(s): Vasilii V. Gvaramadze [sup.1] [sup.2] [sup.3] , Götz Gräfener [sup.4] , Norbert Langer [sup.4] [sup.5] , Olga V. Maryeva [sup.1] [sup.6] , Alexei Y. Kniazev [sup.1] [sup.7] [sup.8] , [...]

We present full photometric coverage and spectroscopic data for soft gamma-ray burst GRB 201015A with a redshift z = 0.426. Our data span a time range of 85 d following the detection of GRB. These observations revealed an underlying supernova SN 201015A with a maximum at 8.54 ± 1.48 d (rest frame) and an optical peak absolute magnitude |$-19.45_{-0.47}^{+0.85}$| mag. The SN stands out clearly, since the contribution of the afterglow at this time is not dominant, which made it possible to determine SN's parameters. A comparison of these parameters reveals that the SN 201015A is the earliest (the minimum T max) known SN associated with GRBs. Spectroscopic observations during the SN decay stage showed broad lines, indicating a large photospheric velocity, and identified this SN as a Type Ic-BL. Thus, the SN 201015A associated with the GRB 201015A becomes the 27th SN/GRB confirmed by both photometric and spectroscopic observations. Using the results of spectral analysis based on the available data of Fermi -GBM experiment, the parameters E p,i = 20.0 ± 8.5 keV and E iso = (1.1 ± 0.2) × 1050 erg were obtained. According to the position of the burst on the E p,i– E iso correlation, GRB 201015A was classified as a type II (long) GRB, which was also confirmed by the T 90,i– EH diagram. [ABSTRACT FROM AUTHOR]

Introduction. One of the main driving factors of the demand for cable and conductor products is the modern trend of urbanization, which leads to an increase in cities with significantly increased requirements for electrical networks of transportation and distribution of electrical energy. This requires the development of appropriate infrastructure with significant demand for electricity in the commercial, industrial and residential sectors. The construction industry uses a wide range of cables with an appropriate set of electrical and mechanical properties, resistance to the influence of external climatic factors, and, first of all, must meet fire safety requirements. Increasing construction activity is stimulating the market for fire-resistant cables in the construction industry. According to the Construction Products Regulation, power, telecommunication cables, data cables, control and management, fiber optic refer to construction products in the EU member states. These cables are intended for the supply of electrical energy and communication, which are permanently installed in buildings and other engineering structures. Power, telecommunications, data and control cables are considered construction products, and are the only electrical products classified as construction. Over the next 15 years, Eastern Europe is expected to see increased growth in the construction industry, as the end of the war in Ukraine requires nearly $1 trillion in reconstruction. A strategic task in the reconstruction of the country is the use of cable and conductor products with increased operational properties, including in the construction industry. Significant efforts and investments in innovation and certification of cable and conductor products require manufacturers to create construction products of the «Power, control and communication cables» category with a high level of fire safety in accordance with the Euroclassification of cables in terms of reaction to fire depending on the level of safety. Purpose. Analysis of fire safety requirements for cables as a category of construction products and determination of thermal resistance of power cables based on experimental thermal studies of modern electrical insulating compositions. Methodology. The thermal stability of power cables with a voltage of 0,66/1 kV was determined, depending on the design, based on the conducted experimental studies of the thermal stability of electrical insulation materials, between the core filling and the polymer sheath, removed from the cable samples. On the basis of the conducted correlation analysis between thermal resistance and fire load, which are important parameters for confirming the quality and safety of the entire cable, it has been proven that the efficiency of halogen-free compositions to meet fire safety requirements increases with the increase in the cross-section and number of cores in the cable. Practical value. Determining the heat load and fire resistance of cables of various designs and areas of application based on the obtained experimental data on the heat of combustion of polymer cable materials is necessary and justified at the stage of mastering and determining the prospects for the production of cables with modern halogenfree electrical insulation compositions in accordance with fire safety requirements. References 40, figures 6. [ABSTRACT FROM AUTHOR]

Optical, near-infrared (NIR) photometric and spectroscopic studies, along with the optical imaging polarimetric results for SN 2012au, are presented in this article to constrain the nature of the progenitor and other properties. Well-calibrated multiband optical photometric data (from –0.2 to +413 d since B -band maximum) were used to compute the bolometric light curve and to perform semi-analytical light-curve modelling using the minim code. A spin-down millisecond magnetar-powered model explains the observed photometric evolution of SN 2012au reasonably. Early-time imaging polarimetric follow-up observations (–2 to +31 d) and comparison with other similar cases indicate signatures of asphericity in the ejecta. Good spectral coverage of SN 2012au (from –5 to +391 d) allows us to trace the evolution of layers of SN ejecta in detail. SN 2012au exhibits higher line velocities in comparison with other SNe Ib. Late nebular phase spectra of SN 2012au indicate a Wolf–Rayet star as the possible progenitor for SN 2012au, with oxygen, He-core, and main-sequence masses of ∼1.62 ± 0.15 M⊙, ∼4–8 M⊙, and ∼17–25 M⊙, respectively. There is a clear absence of a first overtone of carbon monoxide (CO) features up to +319 d in the K -band region of the NIR spectra. Overall analysis suggests that SN 2012au is one of the most luminous slow-decaying Type Ib SNe, having comparatively higher ejecta mass (∼ 4.7–8.3 M⊙) and kinetic energy (∼ [4.8–5.4] × 1051 erg). Detailed modelling using mesa and the results obtained through stella and snec explosions also strongly support spin-down of a magnetar with mass of around 20 M⊙ and metallicity Z  = 0.04 as a possible powering source of SN 2012au. [ABSTRACT FROM AUTHOR]

In this work we present the photometric and spectroscopic observations of type IIb supernova 2017gpn. This supernova was discovered in the error-box of the LIGO/Virgo G299232 gravitational-wave event. We obtained the light curves in the B and R passbands and modelled them numerically using the one-dimensional radiation hydrocode stella. The best-fitting model has the following parameters: the pre-SN star mass and the radius are M  ≈ 3.5 M⊙ and R  ≈ 50 R⊙, respectively; the explosion energy is |$E_{\rm exp} \approx 1.2\times 10^{51}$|  erg; the mass of radioactive nickel is |$M_{\rm ^{56}Ni} \approx 0.11$|  M⊙, which is completely mixed throughout the ejecta; and the mass of the hydrogen envelope |$M_{\rm H\_{env}}$|  ≈ 0.06 M⊙. Moreover, SN 2017gpn is a confirmed SN IIb that is located at the farthest distance from the centre of its host galaxy NGC 1343 (i.e. the projected distance is ∼21 kpc). This challenges the scenario of the origin of type IIb supernovae from massive stars. [ABSTRACT FROM AUTHOR]

Aims. We study the dark nature of GRB130528A through multi-wavelength observations and conclude that the main reason for the optical darkness is local extinction inside of the host galaxy. Methods. Automatic observations were performed at the Burst Optical Observer and Transient Exploring System (BOOTES)-4/MET robotic telescope.We also triggered target of opportunity (ToO) observations at Observatorio de Sierra Nevada (OSN), IRAM Plateau de Bure Interferometer (PdBI) and Gran Telescopio Canarias (GTC + OSIRIS). The host galaxy photometric observations in optical to near-infrared (nIR) wavelengths were achieved through large ground-based aperture telescopes, such as 10.4 m Gran Telescopio Canarias (GTC), 4.2 m William Herschel Telescope (WHT), 6 m Bolshoi Teleskop Alt-azimutalnyi (BTA) telescope, and 2 m Liverpool Telescope (LT). Based on these observations, spectral energy distributions (SED) for the host galaxy and afterglow were constructed. Results. Thanks to millimetre (mm) observations at PdBI, we confirm the presence of a mm source within the XRT error circle that faded over the course of our observations and identify the host galaxy. However, we do not find any credible optical source within early observations with BOOTES-4/MET and 1.5 m OSN telescopes. Spectroscopic observation of this galaxy by GTC showed a single faint emission line that likely corresponds to [OII] 3727 Å at a redshift of 1.250 ± 0.001, implying a star formation rate (M☉ /yr) > 6.18 M☉ /yr without correcting for dust extinction. The probable line-of-sight extinction towards GRB130528A is revealed through analysis of the afterglow SED, resulting in a value of AGRB ≥ 0.9 at the rest frame; this is comparable to extinction levels found among other dark GRBs. The SED of the host galaxy is explained well (χ2/d.o.f. = 0.564) by a luminous (MB = -21.16), low-extinction (AV = 0, rest frame), and aged (2.6 Gyr) stellar population. We can explain this apparent contradiction in global and line-of-sight extinction if the GRB birth place happened to lie in a local dense environment. In light of having relatively small specific star formation rate ∼5.3 M☉ /yr (L=L*)-1, this also could explain the age of the old stellar population of host galaxy. [ABSTRACT FROM AUTHOR]

We present the low-resolution spectroscopic and UBVRI broad-band photometric investigations of the Type IIb supernova (SN) 2011fu, discovered in UGC 01626. The photometric follow-up of this event was initiated a few days after the explosion and covers a period of about 175 d. The early-phase light curve shows a rise, followed by steep decay in all bands, and shares properties very similar to that seen for SN 1993J, with a possible detection of the adiabatic cooling phase. Modelling of the quasi-bolometric light curve suggests that the progenitor had an extended (∼1 × 1013 cm), low-mass (∼0.1 M⊙) H-rich envelope on top of a dense, compact (∼2 × 1011 cm), more massive (∼1.1 M⊙) He-rich core. The nickel mass synthesized during the explosion was found to be ∼0.21 M⊙, slightly larger than that seen for other Type IIb SNe. The spectral modelling performed with synow suggests that the early-phase line velocities for H and Fe ii features were ∼16 000 and ∼14 000 km s−1, respectively. Then, the velocities declined up to day +40 and became nearly constant at later epochs. [ABSTRACT FROM AUTHOR]

ABSTRACT We present photometry and spectroscopy of the Type IIP supernova (SN IIP) 2009bw in UGC 2890 from a few days after the outburst to 241 d. The light curve of SN 2009bw during the photospheric phase is similar to that of normal SNe IIP but with a brighter peak and plateau ( mag, mag). The luminosity drop from the photospheric to the nebular phase is one of the fastest ever observed, ∼2.2 mag in about 13 d. The radioactive tail of the bolometric light curve indicates that the amount of ejected 56Ni is ≈0.022 M⊙. The photospheric spectra reveal high-velocity lines of Hα and Hβ until about 105 d after the shock breakout, suggesting a possible early interaction between the SN ejecta and pre-existent circumstellar material, and the presence of CNO elements. By modelling the bolometric light curve, ejecta expansion velocity and photospheric temperature, we estimate a total ejected mass of ∼8-12 M⊙, a kinetic energy of ∼0.3 foe and an initial radius of ∼3.6-7 × 1013 cm. [ABSTRACT FROM AUTHOR]

We present optical photometric and low-resolution spectroscopic observations of the Type II plateau supernova (SN) 2008in, which occurred in the outskirts of the nearly face-on spiral galaxy M61. Photometric data in the X-rays, ultraviolet and near-infrared bands have been used to characterize this event. The SN field was imaged with the ROTSE-IIIb optical telescope about seven days before the explosion. This allowed us to constrain the epoch of the shock breakout to JD = 2454825.6. The duration of the plateau phase, as derived from the photometric monitoring, was ~98 days. The spectra of SN 2008in show a striking resemblance to those of the archetypal low-luminosity IIP SNe 1997D and 1999br. A comparison of ejecta kinematics of SN 2008in with the hydrodynamical simulations of Type IIP SNe by Dessart et al. (2010) indicates that it is a less energetic event (~5 x 1050 erg). However, the light curve indicates that the production of radioactive 56Ni is significantly higher than that in the low-luminosity SNe. Adopting an interstellar absorption along the SN direction of Av ~ 0.3 mag and a distance of 13.2 Mpc, we estimated a synthesized 56Ni mass of ~0.015 M⊙. Employing semi- analytical formulae (Litvinova & Nadezhin 1985), we derived a pre-SN radius of ~126 R⊙, an explosion energy of ~5.4x1050 erg and a total ejected mass of ~16.7 M⊙. The latter indicates that the zero age main-sequence mass of the progenitor did not exceed 20 M⊙. Considering the above properties of SN 2008in, and its occurrence in a region of sub-solar metallicity ([O/H] ~ 8.44 dex), it is unlikely that fall- back of the ejecta onto a newly formed black hole occurred in SN 2008in. We therefore favor a low-energy explosion scenario of a relatively compact, moderate-mass progenitor star that generates a neutron star. [ABSTRACT FROM AUTHOR]

We present BVRI photometric and low-resolution spectroscopic investigation of the Type II core-collapse supernova (SN) 2008gz, which occurred in a star-forming arm and within a half-light radius (solar metallicity region) of the nearby spiral galaxy NGC 3672. The SN event was detected late, and a detailed investigation of its light curves and spectra spanning 200 d suggest that it is an event of Type IIP similar to the archetypal SNe 2004et and 1999em. However, in contrast to other events of its class, SN 2008gz exhibits a rarely observed V magnitude drop of 1.5 over the period of a month during the plateau to nebular phase. Using an of 0.21 mag as a lower limit and a distance of 25.5 Mpc, we estimate a synthesized Ni mass of , a mid-plateau of mag and a total radiant energy of . The photospheric velocity is observed to be higher than observed for SN 2004et at similar epochs, indicating that the explosion energy was comparable to or higher than that of SN 2004et. A similar trend was also seen for the expansion velocity of H envelopes. By comparing the properties of SN 2008gz with other well-studied events, as well as by using a recent simulation of pre-SN models by Dessart, Livne & Waldman, we infer an explosion energy range of , and this coupled with the observed width of the forbidden [O ] 6300-6364 Å line at 275 d after the explosion gives an upper limit for the main-sequence (non-rotating, solar metallicity) progenitor mass of 17 M. Our narrow-band Hα observation, taken nearly 560 d after the explosion, and the presence of an emission kink at zero velocity in the Doppler-corrected spectra of SN indicate that the event took place in a low-luminosity star-forming H region. [ABSTRACT FROM AUTHOR]

We present the results of SAO RAS 6-m BTA spectral observations of GRBs and XRFs with the signs of SNe: GRB/XRF 060218 (SN 2006aj) and XRF 080109 (SN 2008D). The properties of these events are similar to those of usual core-collapse SNe without GRBs. Some spectral features in the cases of SN 2006aj and SN 2008D may be interpreted as hydrogen and helium traces. This supports an idea of presence of a relic envelope around the progenitor star and the shock break-out effects observed in X-rays, UV and optics. [ABSTRACT FROM AUTHOR]

ABSTRACT We present photometry and spectroscopy of the Type IIP supernova (SN IIP) 2009bw in UGC 2890 from a few days after the outburst to 241 d. The light curve of SN 2009bw during the photospheric phase is similar to that of normal SNe IIP but with a brighter peak and plateau ( mag, mag). The luminosity drop from the photospheric to the nebular phase is one of the fastest ever observed, ∼2.2 mag in about 13 d. The radioactive tail of the bolometric light curve indicates that the amount of ejected 56Ni is ≈0.022 M⊙. The photospheric spectra reveal high-velocity lines of Hα and Hβ until about 105 d after the shock breakout, suggesting a possible early interaction between the SN ejecta and pre-existent circumstellar material, and the presence of CNO elements. By modelling the bolometric light curve, ejecta expansion velocity and photospheric temperature, we estimate a total ejected mass of ∼8-12 M⊙, a kinetic energy of ∼0.3 foe and an initial radius of ∼3.6-7 × 1013 cm. [ABSTRACT FROM AUTHOR]

Long-duration gamma-ray bursts (GRBs) are thought to result from the explosions of certain massive stars, and some are bright enough that they should be observable out to redshifts of z > 20 using current technology. Hitherto, the highest redshift measured for any object was z = 6.96, for a Lyman-alpha emitting galaxy. Here we report that GRB 090423 lies at a redshift of z approximately 8.2, implying that massive stars were being produced and dying as GRBs approximately 630 Myr after the Big Bang. The burst also pinpoints the location of its host galaxy.