20 research outputs found
Microlensing pulsars
We investigate the possibilities that pulsars act as the lens in
gravitational microlensing events towards the galactic bulge or a spiral arm.
Our estimation is based on expectant survey and observations of FAST (Five
hundred meter Aperture Spherical Telescope) and SKA (Square Kilometer Array),
and two different models of pulsar distribution are used. We find that the
lensing rate is > 1 event/decade, being high enough to search the real events.
Therefore, the microlensing observations focusing on pulsars identified by FAST
or SKA in the future are meaningful. As an independent determination of pulsar
mass, a future detection of microlensing pulsars should be significant in the
history of studying pulsars, especially in constraining the state of matter
(either hadronic or quark matter) at supra-nuclear densities. The observations
of such events by using advanced optical facilities (e.g., the James Webb Space
Telescope and the Thirty Meter Telescope) in future are highly suggested.Comment: 5pages, 2figure
On the orbital and physical parameters of the HDE 226868/Cygnus X-1 binary system
In this paper we explore the consequences of the recent determination of the
mass m=(8.7 +/- 0.8)M_Sun of Cygnus X-1, obtained from the Quasi-Periodic
Oscillation (QPO)-photon index correlation scaling, on the orbital and physical
properties of the binary system HDE 226868/Cygnus X-1. By using such a result
and the latest spectroscopic optical data of the HDE 226868 supergiant star we
get M=(24 +/- 5)M_Sun for its mass. It turns out that deviations from the third
Kepler law significant at more than 1-sigma level would occur if the
inclination i of the system's orbital plane to the plane of the sky falls
outside the range 41-56 deg: such deviations cannot be due to the first
post-Newtonian (1PN) correction to the orbital period because of its smallness;
interpreted in the framework of the Newtonian theory of gravitation as due to
the stellar quadrupole mass moment Q, they are unphysical because Q would take
unreasonably large values. By conservatively assuming that the third Kepler law
is an adequate model for the orbital period we obtain i=(48 +/- 7) deg which
yields for the relative semimajor axis a=(42 +/- 9)R_Sun. Our estimate for the
Roche's lobe of HDE 226868 is r_M = (21 +/- 6)R_Sun.Comment: Latex2e, 7 pages, 1 table, 4 figures. To appear in ApSS (Astrophysics
and Space Science
Open Questions in GRB Physics
Open questions in GRB physics are summarized as of 2011, including
classification, progenitor, central engine, ejecta composition, energy
dissipation and particle acceleration mechanism, radiation mechanism, long term
engine activity, external shock afterglow physics, origin of high energy
emission, and cosmological setting. Prospects of addressing some of these
problems with the upcoming Chinese-French GRB mission, SVOM, are outlined.Comment: 27 pages. To appear in a special issue of Comptes Rendus Physique
"GRB studies in the SVOM era", Eds. F. Daigne, G. Dubu
Photon mixing in universes with large extra-dimensions
In presence of a magnetic field, photons can mix with any particle having a
two-photon vertex. In theories with large compact extra-dimensions, there
exists a hierachy of massive Kaluza-Klein gravitons that couple to any photon
entering a magnetic field. We study this mixing and show that, in comparison
with the four dimensional situation where the photon couples only to the
massless graviton, the oscillation effect may be enhanced due to the existence
of a large number of Kaluza-Klein modes. We give the conditions for such an
enhancement and then investigate the cosmological and astrophysical
consequences of this phenomenon; we also discuss some laboratory experiments.
Axions also couple to photons in the same way; we discuss the effect of the
existence of bulk axions in universes with large extra-dimensions. The results
can also be applied to neutrino physics with extra-dimensions.Comment: 41 pages, LaTex, 6 figure
Capturing the electromagnetic counterparts of binary neutron star mergers through low-latency gravitational wave triggers
© 2016 The Authors. We investigate the prospects for joint low-latency gravitational wave (GW) detection and prompt electromagnetic (EM) follow-up observations of coalescing binary neutron stars (BNSs). For BNS mergers associated with short duration gamma-ray bursts (SGRBs), we for the first time evaluate the feasibility of rapid EM follow-ups to capture the prompt emission, early engine activity, or reveal any potential by-products such as magnetars or fast radio bursts. To achieve our goal, we first simulate a population of coalescing BNSs using realistic distributions of source parameters and estimate the detectability and localization efficiency at different times before merger. We then use a selection of facilities with GW follow-up agreements in place, from low-frequency radio to high-energy y-ray to assess the prospects of prompt follow-up. We quantify our assessment using observational SGRB flux data extrapolated to be within the horizon distances of the advanced GW interferometric detectors LIGO and Virgo and to the prompt phase immediately following the binary merger. Our results illustrate that while challenging, breakthrough multimessenger science is possible with EM follow-up facilities with fast responses and wide fields-of-view. We demonstrate that the opportunity to catch the prompt stage (<5s) of SGRBs can be enhanced by speeding up the detection pipelines of both GW observatories and EM follow-up facilities. We further show that the addition of an Australian instrument to the optimal detector network could possibly improve the angular resolution by a factor of 2 and thereby contribute significantly to GW-EM multimessenger astronomy
Turbulence induced additional deceleration in relativistic shock wave propagation: implications for gamma-ray burst
The late afterglow of gamma-ray burst is believed to be due to progressive
deceleration of the forward shock wave driven by the gamma-ray burst ejecta
propagating in the interstellar medium. We study the dynamic effect of
interstellar turbulence on shock wave propagation. It is shown that the shock
wave decelerates more quickly than previously assumed without the turbulence.
As an observational consequence, an earlier jet break will appear in the light
curve of the forward shock wave. The scatter of the jet-corrected energy
release for gamma-ray burst, inferred from the jet-break, may be partly due to
the physical uncertainties in the turbulence/shock wave interaction. This
uncertainties also exist in two shell collisions in the well-known internal
shock model proposed for gamma-ray burst prompt emission. The large scatters of
known luminosity relations of gamma-ray burst may be intrinsic and thus
gamma-ray burst is not a good standard candle. We also discuss the other
implications.Comment: accepted for publication in Astrophysics and Space Scienc
An X-ray Pulsar with a Superstrong Magnetic Field in the Soft Gamma-Ray Repeater SGR1806-20
Soft gamma-ray repeaters (SGRs) emit multiple, brief (approximately O.1 s) intense outbursts of low-energy gamma-rays. They are extremely rare; three are known in our galaxy and one in the Large Magellanic Cloud. Two SGRs are associated with young supernova remnants (SNRs), and therefore most probably with neutron stars, but it remains a puzzle why SGRs are so different from 'normal' radio pulsars. Here we report the discovery of pulsations in the persistent X-ray flux of SGR1806-20, with a period of 7.47 s and a spindown rate of 2.6 x 10(exp -3) s/yr. We argue that the spindown is due to magnetic dipole emission and find that the pulsar age and (dipolar) magnetic field strength are approximately 1500 years and 8 x 10(exp 14) gauss, respectively. Our observations demonstrate the existence of 'magnetars', neutron stars with magnetic fields about 100 times stronger than those of radio pulsars, and support earlier suggestions that SGR bursts are caused by neutron-star 'crust-quakes' produced by magnetic stresses. The 'magnetar' birth rate is about one per millenium, a substantial fraction of that of radio pulsars. Thus our results may explain why some SNRs have no radio pulsars
Unusual Central Engine Activity in the Double Burst GRB 110709B
The double burst, GRB 110709B, triggered Swift/BAT twice at 21:32:39 UT and
21:43:45 UT, respectively, on 9 July 2011. This is the first time we observed a
GRB with two BAT triggers. In this paper, we present simultaneous Swift and
Konus-WIND observations of this unusual GRB and its afterglow. If the two
events originated from the same physical progenitor, their different
time-dependent spectral evolution suggests they must belong to different
episodes of the central engine, which may be a magnetar-to-BH accretion system.Comment: 10 pages, 14 figures, 2 tables. ApJ accepte
Discerning the physical origins of cosmological Gamma-ray bursts based on multiple observational criteria: the cases of z=6.7 GRB 080913, z=8.3 GRB 090423, and some short/hard GRBs
(Abridged) The two high-redshift gamma-ray bursts, GRB 080913 at z=6.7 and
GRB 090423 at z=8.3, recently detected by Swift appear as intrinsically short,
hard GRBs. They could have been recognized by BATSE as short/hard GRBs should
they have occurred at z <= 1. We perform a more thorough investigation on two
physically distinct types (Type I/II) of cosmological GRBs and their
observational characteristics. We reiterate the definitions of Type I/II GRBs
and review the observational criteria and their physical motivations. Contrary
to the traditional approach of assigning the physical category based on the
gamma-ray properties (duration, hardness, and spectral lag), we take an
alternative approach to define the Type I and Type II Gold Samples using
several criteria that are more directly related to the GRB progenitors, and
study the properties of the two Gold Samples and compare them with the
traditional long/soft and short/hard samples. We find that the Type II Gold
Sample reasonably tracks the long/soft population, although it includes several
intrinsically short (shorter than 1s in the rest frame) GRBs. The Type I Gold
Sample only has 5 GRBs, 4 of which are not strictly short but have extended
emission. Other short/hard GRBs detected in the Swift era represent the BATSE
short/hard sample well, but it is unclear whether all of them belong to Type I.
We suggest that some (probably even most) high-luminosity short/hard GRBs
instead belong to Type II. We suggest that GRB 080913 and GRB 090423 are more
likely Type II events. We re-emphasize the importance of invoking multiple
observational criteria, and cautiously propose an operational procedure to
infer the physical origin of a given GRB with available multiple observational
criteria, with various caveats laid out.Comment: 32 pages, ApJ, in press. The strengths and weaknesses of physical
classification and its relation to phenomenological classification are fully
discussed in a newly added section 3. Discussions on GRBs 090423, 090426, and
090510 are include
A surge of light at the birth of a supernova.
It is difficult to establish the properties of massive stars that explode as supernovae. The electromagnetic emission during the first minutes to hours after the emergence of the shock from the stellar surface conveys important information about the final evolution and structure of the exploding star. However, the unpredictable nature of supernova events hinders the detection of this brief initial phase. Here we report the serendipitous discovery of a newly born, normal type IIb supernova (SN 2016gkg), which reveals a rapid brightening at optical wavelengths of about 40 magnitudes per day. The very frequent sampling of the observations allowed us to study in detail the outermost structure of the progenitor of the supernova and the physics of the emergence of the shock. We develop hydrodynamical models of the explosion that naturally account for the complete evolution of the supernova over distinct phases regulated by different physical processes. This result suggests that it is appropriate to decouple the treatment of the shock propagation from the unknown mechanism that triggers the explosion