A Fast-Evolving, Luminous Transient Discovered by K2/Kepler
A. RestP. GarnavichDavid KhatamiDaniel KasenB. TuckerE. ShayaRob P. OllingR. F. MushotzkyA. ZentenoS. MargheimG. StrampelliD. J. JamesR. C. SmithF. FörsterV. Ashley Villar
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For decades optical time-domain searches have been tuned to find ordinary supernovae, which rise and fall in brightness over a period of weeks. Recently, supernova searches have improved their cadences and a handful of fast-evolving luminous transients (FELTs) have been identified. FELTs have peak luminosities comparable to Type Ia supernovae, but rise to maximum in $<10$ days and fade from view in $Keywords:
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The presence of strong magnetic fields in neutron stars, such as in magnetars, may significantly affect their crust-core transition properties and the crust size. This knowledge is crucial in the correct interpretation of astrophysical phenomena involving magnetars, such as glitches in observed rotation frequencies, cooling, bursts and possibly tidal polarizabilities. A recently developed meta-modelling technique allows exploring the model dependence of density functional theory equation of state calculations. In this work, we extend this meta-model to investigate the effect of strong magnetic fields on spinodal instabilities of neutron star matter and the associated crust-core properties. Both Tolman-Oppenheimer-Volkov and a full self-consistent numerical calculations are performed for the neutron star structure, the results being quantitatively different for strong magnetic fields.
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Abstract Magnetars are a special kind of neutron stars. There may also be accreting magnetars. From the studies of isolated magnetars, it is known that a neutron star with a strong dipole field only is not a magnetar. Super‐slow X‐ray pulsars may just be accreting high magnetic‐field neutron stars. The ultra‐luminous X‐ray pulsar NuSTAR J095551+6940.8 may be an accreting magnetar. It may be an accreting low magnetic field magnetar with multipole field of 10 14 G and dipole field of 10 12 G. This point of view is consistent with the study of isolated magnetars. An ultra‐luminous X‐ray pulsar phase during binary evolution may result in massive millisecond pulsars. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
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Abstract Magnetars are highly magnetized rotating neutron stars that are predominantly observed as high-energy sources. Six of this class of neutron star are known to also emit radio emission, so magnetars are a favoured model for the origin of at least some of the fast radio bursts (FRBs). If magnetars, or neutron stars in general, are indeed responsible, sharp empirical constraints on the mechanism producing radio emission are required. Here we report on the detection of polarized quasi-periodic substructure in the emission of all well-studied radio-detected magnetars. A correlation previously seen, relating substructure in pulsed emission of radio-emitting neutron stars to their rotational period, is extended and now shown to span more than six orders of magnitude in pulse period. This behaviour is not only seen in magnetars but in members of all classes of radio-emitting rotating neutron stars, regardless of their evolutionary history, their power source or their inferred magnetic field strength. If magnetars are responsible for FRBs, it supports the idea of being able to infer underlying periods from sub-burst timescales in FRBs.
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Situation with highly magnetized neutron stars in binary systems is not yet certain. On the one hand, all best studied magnetars seem to be isolated objects. On the other, there are many claims based on model-dependent analysis of spin properties or/and luminosity of neutron stars in X-ray binaries in favour of large fields. In addition, there are a few results suggesting a magnetar-like activity of neutron stars in close binary systems. Most of theoretical considerations do not favour even existence, not speaking about active decay, of magnetar-scale fields in neutron stars older than $\sim10^6$~yrs. However, alternative scenarios of the field evolution exist. I provide a brief review of theoretical and observational results related to the presence of neutron stars with large magnetic field in binaries and discuss perspectives of future studies.
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iPTF15dtg is a Type Ic supernova (SN) showing a broad light curve around maximum light, consistent with massive ejecta if we assume a radioactive-powering scenario. We study the late-time light curve of iPTF15dtg, which turned out to be extraordinarily luminous for a stripped-envelope (SE) SN. We compare the observed light curves to those of other SE SNe and also with models for the $^{56}$Co decay. We analyze and compare the spectra to nebular spectra of other SE SNe. We build a bolometric light curve and fit it with different models, including powering by radioactivity, magnetar powering, as well as a combination of the two. Between 150 d and 750 d past explosion, iPTF15dtg's luminosity declined by merely two magnitudes instead of the six magnitudes expected from $^{56}$Co decay. This is the first spectroscopically-regular SE SN showing this behavior. The model with both radioactivity and magnetar powering provides the best fit to the light curve and appears to be the more realistic powering mechanism. An alternative mechanism might be CSM interaction. However, the spectra of iPTF15dtg are very similar to those of other SE SNe, and do not show signs of strong CSM interaction. iPTF15dtg is the first spectroscopically-regular SE SN whose light curve displays such clear signs of a magnetar contributing to the powering of the late time light curve. Given this result, the mass of the ejecta needs to be revised to a lower value, and therefore the progenitor mass could be significantly lower than the previously estimated $>$35 $M_{\odot}$.
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I review of the observational properties of Soft Gamma Repeaters (SGRs) and Anomalous X-ray Pulsars (AXPs), two unusual manifestations of neutron stars. I summarize the reasoning for SGRs being "magnetars," neutron stars powered by the decay of a very large magnetic field, and the now compelling evidence that SGRs and AXPs are in fact members of the same source class, as predicted uniquely by the magnetar model. I discuss some open issues in the magnetar model, and the prospects for future work.
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The binary neutron star merger GW170817/GRB170817A confirmed that at least some neutron star mergers are the progenitors of short gamma-ray bursts. Many short gamma-ray bursts have long-term x-ray afterglows that have been interpreted in terms of post-merger millisecond magnetars—rapidly rotating, highly magnetised, massive neutron stars. We review our current understanding of millisecond magnetars born in short gamma-ray bursts, focusing particularly three main topics. First, whether millisecond magnetars really do provide the most plausible explain for the x-ray plateau. Second, determining and observing the gravitational-wave emission from these remnants. Third, determining the equation of state of nuclear matter from current and future x-ray and gravitational-wave measurements.
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I review the observational properties of Soft Gamma Repeaters (SGRs) and Anomalous X-ray Pulsars (AXPs), two unusual manifestations of neutron stars. I summarize the reasoning for SGRs being “magnetars,” neutron stars powered by the decay of a very large magnetic field, and the now compelling evidence that SGRs and AXPs are in fact members of the same source class, as predicted uniquely by the magnetar model. I discuss some open issues in the magnetar model, and the prospects for future work.
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Luminosity evolution of some stripped-envelope supernovae such as Type I superluminous supernovae is difficult to be explained by the canonical 56Ni nuclear decay heating. A popular alternative heating source is rapid spin down of strongly-magnetized rapidly-rotating neutron stars (magnetars). Recent observations have indicated that Type I superluminous supernovae often have bumpy light curves with multiple luminosity peaks. The cause of bumpy light curves is unknown. In this study, we investigate the possibility that the light-curve bumps are caused by variations of the thermal energy injection from magnetar spin down. We find that a temporal increase in the thermal energy injection can lead to multiple luminosity peaks. The multiple luminosity peaks caused by the variable thermal energy injection is found to be accompanied by significant increase in photospheric temperature, and photospheric radii are not significantly changed. We show that the bumpy light curves of SN 2015bn and SN 2019stc can be reproduced by temporarily increasing magnetar spin-down energy input by a factor of 2-3 for 5-20 days. However, not all the light-curve bumps are accompanied by the clear photospheric temperature increase as predicted by our synthetic models. In particular, the secondary light-curve bump of SN 2019stc is accompanied by a temporal increase in photospheric radii rather than temperature, which is not seen in our synthetic models. We, therefore, conclude that not all the light-curve bumps observed in luminous supernovae are caused by the variable thermal energy injection from magnetar spin down and some bumps are likely caused by a different mechanism.
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