940 research outputs found
Episodic jet power extracted from a spinning black hole surrounded by a neutrino-dominated accretion flow in gamma-ray bursts
It was suggested that the relativistic jets in gamma-ray bursts (GRBs) are
powered via the Blandford-Znajek (BZ) mechanism or the annihilation of
neutrinos and anti-neutrinos from a neutrino cooling-dominated accretion flow
(NDAF). The advection and diffusion of the large-scale magnetic field of a NDAF
is calculated, and the external magnetic field is found to be dragged inward
efficiently by the accretion flow for a typical magnetic Prandtl number P_m=1.
The maximal BZ jet power can be ~10^53-10^54 erg/sec for an extreme Kerr black
hole, if an external magnetic field with 10^14 Gauss is advected by the NDAF.
This is roughly consistent with the field strength of the disk formed after a
tidal disrupted magnetar. The accretion flow near the black hole horizon is
arrested by the magnetic field if the accretion rate is below than a critical
value for a given external field. The arrested accretion flow fails to drag the
field inward and the field strength decays, and then the accretion re-starts,
which leads to oscillating accretion. The typical timescale of such episodic
accretion is in an order of one second. This can qualitatively explain the
observed oscillation in the soft extend emission of short-type GRBs.Comment: 8 pages, to appear in ApJ, references update
The electromagnetic and gravitational-wave radiations of X-ray transient CDF-S XT2
Binary neutron star (NS) mergers may result in remnants of supra-massive or
even stable NS, which have been supported indirectly by observed X-ray plateau
of some gamma-ray bursts (GRBs) afterglow. Recently, Xue et al. (2019)
discovered a X-ray transient CDF-S XT2 that is powered by a magnetar from
merger of double NS via X-ray plateau and following stepper phase. However, the
decay slope after the plateau emission is a little bit larger than the
theoretical value of spin-down in electromagnetic (EM) dominated by losing its
rotation energy. In this paper, we assume that the feature of X-ray emission is
caused by a supra-massive magnetar central engine for surviving thousands of
seconds to collapse black hole. Within this scenario, we present the
comparisons of the X-ray plateau luminosity, break time, and the parameters of
magnetar between CDF-S XT2 and other short GRBs with internal plateau samples.
By adopting the collapse time to constrain the equation of state (EOS), we find
that three EOSs (GM1, DD2, and DDME2) are consistent with the observational
data. On the other hand, if the most released rotation energy of magnetar is
dominated by GW radiation, we also constrain the upper limit of ellipticity of
NS for given EOS, and it is range in . Its GW signal
can not be detected by aLIGO or even for more sensitive Einstein Telescope in
the future.Comment: 13 pages, 5 figures,1 table. Accepted for publication by Research in
Astronomy and Astrophysic
Constraining the Charge of a Black Hole with Electromagnetic Radiation from a Black Hole-Neutron Star System
Black hole-neutron star (BH-NS) mergers are expected to emit
gravitational-wave (GW) and electromagnetic (EM) counterparts when the NS is
tidally disrupted or plunges into the BH. Recently, GW 200105 and GW200115 were
claimed as originating in BH-NS mergers, even GW 200105 remains in debate.
Several optical source candidates are reported to possible associate with the
two GW events, but not confirmed yet. In this work, we assume that the BH is
charged (the NS is naturally charged) and try to constrain the charge of the BH
by using the possible associated EM emission from the charged BH and NS system
working in the inspiral regime. We adopt electric and magnetic dipole
radiations for the binaries which power a Poynting-flux-dominated outflow to
accelerate electrons. Then, it produces the observed EM radiation via
synchrotron radiation. We find that the conversion efficiency in the X-ray band
is much higher than that of the ultraviolet (UV), near-infrared, and radio
bands. The estimated maximum charge-to-mass ratio (the charge for unit mass) of
the BH is and esu for the binary
systems of GW200105 and GW200115, respectively, if magnetic field strength
G and period ms for the NS spin.Comment: 13 pages, 4 Tables, and 2 Figures. Accepted for publication in PR
Optimal sequencing of a set of positive numbers with the variance of the sequence's partial sums maximized
We consider the problem of sequencing a set of positive numbers. We try to
find the optimal sequence to maximize the variance of its partial sums. The
optimal sequence is shown to have a beautiful structure. It is interesting to
note that the symmetric problem which aims at minimizing the variance of the
same partial sums is proved to be NP-complete in the literature.Comment: 12 pages;Accepted for publication in Optimization Lette
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