Electromagnetic counterparts of binary neutron star mergers leading to a strongly magnetized long-lived remnant neutron star

We explore the electromagnetic counterparts that will associate with binary neutron star mergers for the case that remnant massive neutron stars survive for $\gtrsim 0.5$ s after the merger. For this study, we employ the outflow profiles obtained by long-term general-relativistic neutrino-radiation magneto-hydrodynamics simulations with a mean field dynamo effect. We show that a synchrotron afterglow with high luminosity can be associated with the merger event if the magnetic fields of the remnant neutron stars are significantly amplified by the dynamo effect. We also perform a radiative transfer calculation for kilonovae and find that for the highly amplified magnetic field cases, the kilonovae can be bright in the early epoch ($t\leq 0.5\,{\rm d}$), while it shows rapid declining ($\lesssim 1\,{\rm d}$) emission and long-lasting ($\sim 10\,{\rm d}$) emission in the optical and near-infrared wavelength, respectively. All these features have not been found in GW170817, indicating that the merger remnant neutron star formed in GW170817 might have collapsed to a black hole within several hundreds ms or magnetic-field amplification might be a minor effect

Lanthanide Features in Near-infrared Spectra of Kilonovae

The observations of GW170817/AT2017gfo have provided us with evidence thatbinary neutron star mergers are sites of $r$-process nucleosynthesis. However,the observed signatures in the spectra of GW170817/AT2017gfo have not beenfully decoded especially in the near-infrared (NIR) wavelengths. In this paper,we investigate the kilonova spectra over the entire wavelength range with theaim of elemental identification. We systematically calculate the strength ofbound-bound transitions by constructing a hybrid line list that is accurate forimportant strong transitions and complete for weak transitions. We find thatthe elements on the left side of the periodic table, such as Ca, Sr, Y, Zr, Ba,La, and Ce, tend to produce prominent absorption lines in the spectra. This isbecause such elements have a small number of valence electrons and low-lyingenergy levels, resulting in strong transitions. By performing self-consistentradiative transfer simulations for the entire ejecta, we find that La III andCe III appear in the NIR spectra, which can explain the absorption features at$\lambda\sim 12000$-14000 A in the spectra of GW170817/AT2017gfo. The massfractions of La and Ce are estimated to be $>2\times 10^{-6}$ and $\sim$(1-100)$\times 10^{-5}$, respectively. An actinide element Th can also be asource of absorption as the atomic structure is analogous to that of Ce.However, we show that Th III features are less prominent in the spectra becauseof the denser energy levels of actinides compared to those of lanthanides.<br

A Case Study of On-the-Fly Wide-Field Radio Imaging Applied to the Gravitational-wave Event GW 151226

We apply a newly-developed On-the-Fly mosaicing technique on the NSF's Karl G. Jansky Very Large Array (VLA) at 3 GHz in order to carry out a sensitive search for an afterglow from the Advanced LIGO binary black hole merger event GW 151226. In three epochs between 1.5 and 6 months post-merger we observed a 100 sq. deg region, with more than 80% of the survey region having a RMS sensitivity of better than 150 uJy/beam, in the northern hemisphere having a merger containment probability of 10%. The data were processed in near-real-time, and analyzed to search for transients and variables. No transients were found but we have demonstrated the ability to conduct blind searches in a time-frequency phase space where the predicted afterglow signals are strongest. If the gravitational wave event is contained within our survey region, the upper limit on any late-time radio afterglow from the merger event at an assumed mean distance of 440 Mpc is about 1e29 erg/s/Hz. Approximately 1.5% of the radio sources in the field showed variability at a level of 30%, and can be attributed to normal activity from active galactic nuclei. The low rate of false positives in the radio sky suggests that wide-field imaging searches at a few Gigahertz can be an efficient and competitive search strategy. We discuss our search method in the context of the recent afterglow detection from GW 170817 and radio follow-up in future gravitational wave observing runs.Comment: 11 pages. 6 figures. 1 table. Accepted for publication in ApJ Letter

GSK-3β-dependent downregulation of γ-taxilin and αNAC merge to regulate ER stress responses

The signaling pathway leading to the endoplasmic reticulum (ER) stress responses has not been fully elucidated. Here we showed that glycogen synthase kinase-3β (GSK-3β)-dependent downregulation of γ-taxilin and nascent polypeptide-associated complex α-subunit (αNAC) mediates hypoxia-induced unfolded protein responses (UPRs) and the subsequent apoptotic and autophagic pathways. The degradation of γ-taxilin or αNAC was sufficient to initiate UPRs in normoxic cells. However, the ER stress signaling pathways initiated by γ-taxilin or αNAC were distinct, triggering different ER stress sensors and activating different downstream pathways. Hypoxia caused GSK-3β-dependent tau hyperphosphorylation and cleavage in neuronal cells, but γ-taxilin ablation induced tau hyperphosphorylation alone and αNAC ablation induced neither changes. Notably, downregulation of γ-taxilin and αNAC occurs in the brain of patients with Alzheimer's disease. These results suggest that GSK-3β-dependent downregulation of γ-taxilin and αNAC, which differently activate the UPRs, merge to regulate hypoxia-induced ER stress responses and provide a new insight into the pathogenesis of neurodegenerative diseases

Prospects of gravitational-wave follow-up through a wide-field ultraviolet satellite: A Dorado case study

The detection of gravitational waves from the binary neuron star merger GW170817 and electromagnetic counterparts GRB170817A and AT2017gfo kick-started the field of gravitational-wave multimessenger astronomy. The optically red to near-infrared emission ("red" component) of AT2017gfo was readily explained as produced by the decay of newly created nuclei produced by rapid neutron capture (a kilonova). However, the ultraviolet to optically blue emission ("blue" component) that was dominant at early times (up to 1.5 days) received no consensus regarding its driving physics. Among many explanations, two leading contenders are kilonova radiation from a lanthanide-poor ejecta component and shock interaction (cocoon emission). In this work, we simulate AT2017gfo-like light curves and perform a Bayesian analysis to study whether an ultraviolet satellite capable of rapid gravitational-wave follow-up, could distinguish between physical processes driving the early "blue" component. We find that ultraviolet data starting at 1.2 hr distinguishes the two early radiation models up to 160 Mpc, implying that an ultraviolet mission like Dorado would significantly contribute to insights into the driving emission physics of the postmerger system. While the same ultraviolet data and optical data starting at 12 hr have limited ability to constrain model parameters separately, the combination of the two unlocks tight constraints for all but one parameter of the kilonova model up to 160 Mpc. We further find that a Dorado-like ultraviolet satellite can distinguish the early radiation models up to at least 130 (60) Mpc if data collection starts within 3.2 (5.2) hr for AT2017gfo-like light curves