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We propose and experimentally demonstrate a silicon based ultra-compact and low loss higher order mode bandpass filter (HMF), which was inversely designed by a direct-binary search topology optimization algorithm. The proposed HMF is a directional coupler with optimally positioned etched circular air holes in the coupling region which footprint is only 1.8 × 3 μm2. The experimental results show that the HMF has low insertion loss of <1 dB and low crosstalk of <−15 dB in the wavelength range of 1520 ∼ 1580 nm. It can be extended to selectively pass other higher order modes. [ABSTRACT FROM AUTHOR]

The radiation mechanism of the prompt emission of gamma-ray bursts (GRBs) remains an open question. Although their spectra are usually well fitted with the empirical Band function, which is widely believed to be fully nonthermal and interpreted as an optically thin synchrotron emission, accumulating evidence shows that a thermal component actually exists. In this paper, a multicolor blackbody (mBB) model is proposed for the time-integrated spectrum of GRB 081221 by assuming a power-law distribution of the thermal luminosities with temperature, which manifests photospheric emissions from a different radius and/or angle. The effects of the minimum temperature kTmin, the maximum temperature kTmax, and the power-law index m of the luminosity distribution of an mBB are discussed. The fitting to the time-integrated spectrum during the bright phase (from 20 to 30 s since the trigger) of GRB 081221 by the mBB model yields kTmin = 4.4 ± 0.3 keV, , and . When the time bin is small enough, the time-resolved spectra of GRB 081221 are well fitted with a series of single-temperature blackbodies. Our results imply the prompt emission of GRB 081221 is dominated by the photosphere emission and its time-integrated spectrum is a superposition of pure blackbody components at different times, indicating that some empirical Band spectra may be interpreted as mBB if the temperature is widely distributed. [ABSTRACT FROM AUTHOR]

Short gamma-ray bursts may originate from the merger of a double neutron star (NS) or the merger of a black hole (BH) and an NS. We propose that the bright X-ray flare related to the central engine reactivity may indicate a BH–NS merger, since such a merger can provide more fallback materials and therefore a more massive accretion disk than the NS–NS merger. Based on the 49 observed short bursts with the Swift/X-ray Telescope follow-up observations, we find that three bursts have bright X-ray flares, among which three flares from two bursts are probably related to the central engine reactivity. We argue that these two bursts may originate from the BH–NS merger rather than the NS–NS merger. Our suggested link between the central-engine-powered bright X-ray flare and the BH–NS merger event can be checked by future gravitational wave detections from advanced LIGO and Virgo. [ABSTRACT FROM AUTHOR]

Two plateaus and a following bump in the X-ray light curve of GRB 170714A have been detected by the Swift/X-ray Telescope, which could be very significant for the central engine of gamma-ray bursts (GRBs), implying that the origin of this burst might be different from those of other ultra-long GRBs. We propose that merging two neutron stars into a hyper-massive quark star (QS) and then collapsing into a black hole (BH), with a delay time around 104 s, could be responsible for these X-ray components. The hyper-massive QS is initially in a fluid state, being turbulent and differentially rotating, but would solidify and release its latent heat, injecting it into the GRB fireball (lasting about 103 s during the liquid–solid phase transition). A magnetic field as high as ∼1015 G can be created by dynamo action of the newborn liquid QS, and a magnetar-like central engine (after solidification) supplies significant energy for the second plateau. More energy could be released during a fall-back accretion after the post-merger QS collapses to a BH, and the X-ray bump forms. This post-merger QS model could be tested by future observations, with either advanced gravitational wave detectors (e.g., advanced LIGO and VIRGO) or X-ray/optical telescopes. [ABSTRACT FROM AUTHOR]

GRB 160821B is a short gamma-ray burst (SGRB) at redshift z = 0.16, with a duration less than 1 s and without any “extended emission” detected up to more than 100 s in both Swift/BAT and Fermi/GBM bands. An X-ray plateau with a sharp drop 180 s after the BAT trigger was observed with Swift/XRT. No supernova or kilo-nova signature was detected. Assuming the central engine of this SGRB is a recently born supra-massive magnetar, we can explain the SGRB as jet radiation and its X-ray plateau as the internal energy dissipation of the pulsar wind as it spins down. We constrain its surface magnetic field to Bp < 3.12 × 1016 G and initial spin period to P0 < 8.5 × 10−3 s. Its equation of state is consistent with the GM1 model with MTOV ∼ 2.37 M⊙ and ellipticity ϵ < 0.07. Its gravitational wave (GW) radiation may be detectable with the future Einstein Telescope, but is much weaker than the current detectability limit of Advanced LIGO. The GW radiation of such an event would be detectable by Advanced LIGO if it occurred at a distance of 100 Mpc (z = 0.023). [ABSTRACT FROM AUTHOR]

This work focuses on a sample of seven extremely late-time X-ray flares with peak time , among which two flares can be confirmed as the late-time activity of central engine. The main purpose is to investigate the mechanism of such late-time flares based on the internal origin assumption. In the hyper-accreting black hole (BH) scenario, we study the possibility of two well-known mechanisms acting as the central engine to power such X-ray flares, i.e., the neutrino–antineutrino annihilation and the Blandford–Znajek (BZ) process. Our results show that the annihilation luminosity is far below the observational data. Thus, the annihilation mechanism cannot account for such late-time flares. For the BZ process, if the role of outflows is taken into consideration, the inflow mass rate near the horizon will be quite low such that the magnetic field will probably be too weak to power the observed X-ray flares. We therefore argue that, for the late-time flares with internal origin, the central engine is unlikely to be associated with BHs. On the contrary, a fast rotating neutron star with strong bipolar magnetic fields may be responsible for such flares. [ABSTRACT FROM AUTHOR]

A neutrino-dominated accretion disk around a stellar-mass black hole (BH) can power a gamma-ray burst (GRB) via annihilation of neutrinos launched from the disk. For the BH hyperaccretion system, high accretion rate should trigger the violent evolution of the BH’s characteristics, which further leads to the evolution of the neutrino annihilation luminosity. In this paper, we consider the evolution of the accretion system to analyze the mean time-dependent neutrino annihilation luminosity with the different mean accretion rates and initial BH parameters. By time-integrating the luminosity, the total neutrino annihilation energy with the reasonable initial disk mass can satisfy most short-duration GRBs and about half of long-duration GRBs. Moreover, the extreme Kerr BH should exist in the cental engines of some high-luminosity GRBs. GRBs with higher energy have to request the alternative magnetohydrodynamics processes in the centers, such as the Blandford–Znajek jet from the accretion system or the millisecond magnetar. [ABSTRACT FROM AUTHOR]

Stellar-mass black holes (BHs) surrounded by neutrino-dominated accretion flows (NDAFs) are plausible sources of power for gamma-ray bursts (GRBs) via neutrino emission and their annihilation. The progenitors of short-duration GRBs (SGRBs) are generally considered to be compact binary mergers. According to the simulation results, the disk mass of the NDAF is limited after merger events. We can estimate such disk masses using the current SGRB observational data and fireball model. The results show that the disk mass of a certain SGRB mainly depends on its output energy, jet opening angle, and central BH characteristics. Even for the extreme BH parameters, some SGRBs require massive disks, which approach or exceed the limits in simulations. We suggest that there may exist alternative MHD processes or mechanisms that increase the neutrino emission to produce SGRBs with reasonable BH parameters and disk masses. [ABSTRACT FROM AUTHOR]

A multilayered configuration broad bandwidth polarization insensitive reflector realized by a multi-subpart profile grating structure is presented. The properties of the reflector are investigated by rigorous coupled-wave analysis. It is shown that over a broadband spectrum of 1.62-1.76mm, the reflector demonstrates high reflectivity (R > 99%), low polarization-dependent loss (PDL < 0.02 dB) and good angular insensitivity of about 29.6deg for both transverse electric and transverse magnetic polarized waves. [ABSTRACT FROM AUTHOR]