948 research outputs found
Suppression of backward scattering of Dirac fermions in iron pnictides Ba(FeRuAs)
We report electronic transport of Dirac cones when Fe is replaced by Ru,
which has an isoelectronic electron configuration to Fe, using single crystals
of Ba(FeRuAs). The electronic transport of parabolic bands is
shown to be suppressed by scattering due to the crystal lattice distortion and
the impurity effect of Ru, while that of the Dirac cone is not significantly
reduced due to the intrinsic character of Dirac cones. It is clearly shown from
magnetoresistance and Hall coefficient measurements that the inverse of average
mobility, proportional to cyclotron effective mass, develops as the square root
of the carrier number (n) of the Dirac cones. This is the unique character of
the Dirac cone linear dispersion relationship. Scattering of Ru on the Dirac
cones is discussed in terms of the estimated mean free path using experimental
parameters.Comment: 6 pages, 3 figures, To be published in Phys. Rev.
GRB 070518: A Gamma-ray Burst with Optically Dim Luminosity
We present our optical observations of {\em Swift} GRB 070518 afterglow
obtained at the 0.8-m Tsinghua University-National Astronomical Observatory of
China telescope (TNT) at Xinglong Observatory. Our follow-up observations were
performed from 512 sec after the burst trigger. With the upper limit of
redshift 0.7, GRB 070518 is found to be an optically dim burst. The
spectra indices of optical to X-ray are slightly larger than 0.5,
which implies the burst might be a dark burst. The extinction of the
host galaxy is 3.2 mag inferred from the X-ray hydrogen column density with
Galactic extinction law, and 0.3 mag with SMC extinction law. Also, it is
similar to three other low-redshift optically dim bursts, which belong to XRR
or XRF, and mid-term duration(, except for GRB 070419A,
=116s). Moreover, its band afterglow flux is well fitted by a
single power-law with an index of 0.87. The optical afterglow and the X-ray
afterglow in the normal segment might have the same mechanism, as they are
consistent with the prediction of the classical external shock model. Besides,
GRB 070518 agrees with Amati relation under reasonable assumptions. The
Ghirlanda relation is also tested with the burst.Comment: 8 pages, 4 figures, MNRAS accepte
The GRB 071112C: A Case Study of Different Mechanisms in X-ray and Optical Temporal Evolution
We present the study on GRB 071112C X-ray and optical light curves. In these
two wavelength ranges, we have found different temporal properties. The R-band
light curve showed an initial rise followed by a single power-law decay, while
the X-ray light curve was described by a single power-law decay plus a
flare-like feature. Our analysis shows that the observed temporal evolution
cannot be described by the external shock model in which the X-ray and optical
emission are produced by the same emission mechanism. No significant color
changes in multi-band light curves and a reasonable value of the initial
Lorentz factor ({\Gamma}0 = 275 \pm 20) in a uniform ISM support the afterglow
onset scenario as the correct interpretation for the early R-band rise. The
result suggests the optical flux is dominated by afterglow. Our further
investigations show that the X-ray flux could be created by an additional
feature related to energy injection and X-ray afterglow. Different theoretical
interpretations indicate the additional feature in X-ray can be explained by
either late internal dissipation or local inverse-Compton scattering in the
external shock.Comment: 20 pages, 3 figures, accepted for publication in Ap
Early (0.3 day) R-band light curve of the optical afterglow of GRB030329
We observed the optical afterglow of the bright gamma-ray burst GRB030329 on
the nights of 2003 March 29, using the Kiso observatory (the University of
Tokyo) 1.05 m Schmidt telescope. Data were taken from March 29 13:21:26 UT to
17:43:16 (0.072 to 0.253 days after the burst), using an -band filter. The
obtained -band light curve has been fitted successfully by a single power
law function with decay index of . These results remain
unchanged when incorporating two early photometric data points at 0.065 and
0.073 days, reported by Price et al.(2003) using the SSO 40 inch telescope, and
further including RTT150 data (Burenin et al. 2003) covering at about 0.3 days.
Over the period of 0.065-0.285 days after the burst, any deviation from the
power-law decay is smaller than 0.007 mag. The temporal structure reported
by Uemura et al. (2003) does not show up in our -band light curve.Comment: 9 pages, 2 figures, 1 table, accepted for publication in ApJ
Optical Classification of Gamma-Ray Bursts in the Swift Era
We propose a new method for the classification of optically dark gamma-ray
bursts (GRBs), based on the X-ray and optical-to-X-ray spectral indices of GRB
afterglows, and utilizing the spectral capabilities of Swift. This method
depends less on model assumptions than previous methods, and can be used as a
quick diagnostic tool to identify optically sub-luminous bursts. With this
method we can also find GRBs that are extremely bright at optical wavelengths.
We show that the previously suggested correlation between the optical darkness
and the X-ray/gamma-ray brightness is merely an observational selection effect.Comment: 6 pages, 3 figures; accepted for publication in Ap
Dimerization of Tetherin Is Not Essential for Its Antiviral Activity against Lassa and Marburg Viruses
Tetherin (also known as BST2, CD317 or HM1.24) has recently been reported to inhibit a wide range of viruses. However, the antiviral mechanism of action of tetherin has not been determined. Both ends of the tetherin molecule are associated with the plasma membrane and it forms a homodimer. Therefore, a model in which progeny virions are retained on the cell surface by dimer formation between tetherin molecules on the viral envelope and plasma membrane has been proposed as the antiviral mechanism of action of this molecule. To investigate this possibility, we examined the correlation between dimerization and antiviral activity of tetherin in Lassa and Marburg virus-like particle production systems using tetherin mutants deficient in dimer formation. However, the tetherin mutant with complete loss of dimerization activity still showed apparent antiviral activity, indicating that dimerization of tetherin is not essential for its antiviral activity. This suggests that tetherin retains progeny virions on the cell surface by a mechanism other than dimerization
Very early multi-color observations of the plateau phase of GRB 041006 afterglow
Observations of the optical afterglow of GRB 041006 with the Kiso Observatory
1.05 m Schmidt telescope, the Lulin Observatory 1.0 m telescope and the
Xinglong Observatory 0.6 m telescope. Three-bands (B, V and R) of photometric
data points were obtained on 2004 October 6, 0.025-0.329 days after the burst.
These very early multi band light curves imply the existence of a color
dependent plateau phase. The B-band light curve shows a clear plateau at around
0.03 days after the burst. The R band light curve shows the hint of a plateau,
or a possible slope change, at around 0.1 days after the burst. The overall
behavior of these multi-band light curves may be interpreted in terms of the
sum of two separate components, one showing a monotonic decay the other
exhibiting a rising and a falling phase, as described by the standard afterglow
model.Comment: 11 pages, 2 figures, Accepted to ApJ Letter
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