1,326 research outputs found
Intranight optical variability of radio-quiet BL Lacertae objects
Aims: Intranight variation (or microvariation) is a common phenomenon of
radio-loud BL Lac objects. However, it is not clear whether the recently found
radio-quiet BL Lac objects have the same properties. The occurrence rate of
intranight variation is helpful in distinguishing the mechanism of the
continuum of radio-quiet BL Lac objects.
Methods: We conducted a photometric monitoring of 8 radio-quiet BL Lac
objects by the Xinglong 2.16m and Lijiang 2.4m telescopes. The differential
light curves are calculated between each target and two comparison stars. To
quantify the variation, the significance of variation is examined by a scaled
-test.
Results: No significant variation is found in the 11 sessions of light curves
of 8 radio-quiet BL Lac objects (one galactic source is excluded). The lack of
microvariation in radio-quiet BL Lac objects is consistent with the detection
rate of microvariation in normal radio-quiet AGNs, but much lower than for
radio-loud AGNs. This result indicates that the continua of the radio-quiet BL
Lac objects are not dominated by jets that will induce frequent
microvariations.Comment: Accepted for publication in Astronomy and Astrophysics; 7 pages, 1
figure, 3 table
Relativistic effects on the observed AGN luminosity distribution
Recently Zhang (2005) has proposed a model to account for the well
established effect that the fraction of type-II AGNs is anti-correlated with
the observed X-ray luminosity; the model consists of an X-ray emitting
accretion disk coaligned to the dusty torus within the standard AGN unification
model. In this paper the model is refined by including relativistic effects of
the observed X-ray radiations from the vicinity of the supermassive black hole
in an AGN. The relativistic corrections improve the combined fitting results of
the observed luminosity distribution and the type-II AGN fraction, though the
improvement is not significant. The type-II AGN fraction prefers non- or mildly
spinning black hole cases and rules out the extremely spinning case.Comment: 9 pages, 4 figures, accepted for publication in PAS
Observe matter falling into a black hole
It has been well known that in the point of view of a distant observer, all
in-falling matter to a black hole (BH) will be eventually stalled and "frozen"
just outside the event horizon of the BH, although an in-falling observer will
see the matter falling straight through the event horizon. Thus in this "frozen
star" scenario, as distant observers, we could never observe matter falling
into a BH, neither could we see any "real" BH other than primordial ones, since
all other BHs are believed to be formed by matter falling towards singularity.
Here we first obtain the exact solution for a pressureless mass shell around a
pre-existing BH. The metrics inside and interior to the shell are all different
from the Schwarzschild metric of the enclosed mass. The metric interior to the
shell can be transformed to the Schwarzschild metric for a slower clock which
is dependent of the location and mass of the shell. Another result is that
there does not exist a singularity nor event horizon in the shell. Therefore
the "frozen star" scenario is incorrect. We also show that for all practical
astrophysical settings the in-falling time recorded by an external observer is
sufficiently short that future astrophysical instruments may be able to follow
the whole process of matter falling into BHs. The distant observer could not
distinguish between a "real" BH and a "frozen star", until two such objects
merge together. It has been proposed that electromagnetic waves will be
produced when two "frozen stars" merge together, but not true when two "real"
bare BHs merge together. However gravitational waves will be produced in both
cases. Thus our solution is testable by future high sensitivity astronomical
observations.Comment: 7 pages, 2 figures. Proceeding of the conference "Astrophysics of
Compact Objects", 1-7 July, Huangshan, China. Abridged abstrac
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