948 research outputs found
The spectral variability of FSRQs
The optical variability of 29 flat spectrum radio quasars in SDSS Stripe 82
region are investigated by using DR7 released multi-epoch data. All FSRQs show
variations with overall amplitude ranging from 0.24 mag to 3.46 mag in
different sources. About half of FSRQs show a bluer-when-brighter trend, which
is commonly observed for blazars. However, only one source shows a
redder-when-brighter trend, which implies it is rare in FSRQs. In this source,
the thermal emission may likely be responsible for the spectral behavior.Comment: 4 pages, 1 figure, to be published in Journal of Astrophysics and
Astronomy, as a proceeding paper of the conference "Multiwavelength
Variability of Blazars", Guangzhou, China, September 22-24, 201
Variability of the Spectral Energy Distribution of the Blazar S5 0716+714
The emission from blazars is known to be variable at all wavelengths. The
flux variability is often accompanied by spectral changes. Spectral energy
distribution (SED) changes must be associated with changes in the spectra of
emitting electrons and/or the physical parameters of the jet. Meaningful
modeling of blazar broadband spectra is required to understand the extreme
conditions within the emission region. Not only is the broadband SED crucial,
but also information about its variability is needed to understand how the
highest states of emission occur and how they differ from the low states. This
may help in discriminating between models. Here we present the results of our
SED modeling of the blazar S5 0716+714 during various phases of its activity.
The SEDs are classified into different bins depending on the optical brightness
state of the source.Comment: 4 pages, 3 figures, contributed talk presented at the conference
Multifrequency Variability of Blazars, Guangzhou, China, September 22-24,
2010. To appear in Journal of Astrophysics and Astronomy (JAA
The radio structure of ultra-high-energy synchrotron peak BL Lacs
We present the results of EVN and MERLIN 5 GHz observations of nine
ultra-high-energy synchrotron peak BL Lacs (UHBLs) selected as all BL Lacs with
\textbf{log (} from Nieppola et al.. The radio
structure was investigated for these sources, in combination with the available
VLBA archive data. We found that the core-jet structure is detected in five
sources, while four sources only have a compact core on pc scale. The core of
all sources shows high brightness temperature (with mean and median values
\textbf{log (}, which implies that the beaming
effect likely present in all sources. When the multi-epoch VLBI data are
available, we found no significant variations either for core or total flux
density in two sources (2E 0414+0057 and EXO 0706.1+5913), and no evident
proper motion in 2E 0414+0057, while the superluminal motion is likely detected
in EXO 0706.1+5913. Our sources are found to be less compact than the typical
HBLs in Giroletti et al, by comparing the ratio of the VLBI total flux to the
core flux at arcsec scale. Combining all our results, we propose that the
beaming effect might be present in the jets of UHBLs, however, it is likely
weaker than that of typical HBLs. Moreover, we found that UHBLs could be less
Doppler beamed versions of HBLs with similar jet power, by comparing the
distribution of redshift, and radio luminosities. The results are in good
consistence with the expectations from our previous work.Comment: 14 pages, 15 figures and 5 tables. Accepted by MNRA
The nature of the intra-night optical variability in blazars
In this paper we present results of a short-term optical monitoring of 13
blazars. The objects were monitored mostly in the R-band for a total of ~ 160
hours between 2006 and 2011. We study the nature of the short-term variations
and show that most of them could be described as slow, smooth, and (almost)
linear changes of up to ~ 0.1 mag/hour, but many objects show no short-term
variations at all. In fact, we found only ~ 2 per cent chance to observe
variability of more than 0.1 mag/hour for the sample we observed. Hints for
quasi-periodic oscillations at very low amplitude levels are also found for
some objects. We briefly discuss some of the possible mechanisms to generate
the intra-night variability and the quasi-periodic oscillations.Comment: 10 pages, 13 figures, 1 table, Accepted for Publication in MNRA
Optical Intraday Variability Studies of Ten Low Energy Peaked Blazars
We have carried out optical (R band) intraday variability (IDV) monitoring of
a sample of ten bright low energy peaked blazars (LBLs). Forty photometric
observations, of an average of ~ 4 hours each, were made between 2008 September
and 2009 June using two telescopes in India. Measurements with good signal to
noise ratios were typically obtained within 1-3 minutes, allowing the detection
of weak, fast variations using N-star differential photometry. We employed both
structure function and discrete correlation function analysis methods to
estimate any dominant timescales of variability and found that in most of the
cases any such timescales were longer than the duration of the observation. The
calculated duty cycle of IDV in LBLs during our observing run is ~ 52%, which
is low compared to many earlier studies; however, the relatively short periods
for which each source was observed can probably explain this difference. We
briefly discuss possible emission mechanisms for the observed variability.Comment: 20 Pages, 9 Figures, 4 Tables, accepted for publication in MNRA
The optical variability of flat-spectrum radio quasars in the SDSS stripe 82 region
Context. Although a bluer-when-brighter trend is commonly observed in
blazars, the opposite trend of redder-when-brighter has also been found in some
blazars. Aims. We investigate the frequency of the redder-when-brighter trend
in flat-spectrum radio quasars (FSRQs). Methods. We investigate the optical
variability of 29 FSRQs in the SDSS Stripe 82 region using SDSS DR7 released
multi-epoch data covering about nine years. We determined the spectral index by
fitting a powerlaw to SDSS ugriz photometric data, and explored the
relationship between the spectral index and source brightness. Results. For all
FSRQs studied, we detect variations in r band flux of overall amplitude between
0.24 mag and 3.46 mag in different sources. Fourteen of 29 FSRQs display a
bluer-when-brighter trend. However, only one source exhibits a
redder-when-brighter trend, which implies that this behavior is rare in our
FSRQ sample. In this source, the thermal emission from the accretion disk may
be responsible for the redder-when-brighter trend.Comment: 9 pages, 7 figures, Astronomy and Astrophysics accepted, title
revised, the version after language editin
Optical Variability and Colour Behaviour of 3C 345
The colour behaviour of blazars is a subject of much debate. One argument is
that the BL Lac objects show bluer-when-brighter chromatism while the
flat-spectrum radio quasars (FSRQs) display redder-when-brighter trend. Base on
a 3.5-year three-colour monitoring programme, we studied the optical
variability and colour behaviour of one FSRQ, 3C 345. There is at least one
outburst in this period. The overall variation amplitude is 2.640 mags in the
band. Intra-night variability was observed on two nights. The
bluer-when-brighter and redder-when-brighter chromatisms were simultaneously
observed in this object when using different pairs of passbands to compute the
colours. The bluer-when-brighter chromatism is a shared property with the BL
Lacs, while the redder-when-brighter trend is likely due to two less variable
emission features, the Mg\,{\sc ii} line and the blue bump, at short
wavelengths. With numerical simulations, we show that some other strong but
less variable emission lines in the spectrum of FSRQs may also significantly
alter their colour behaviour. Then the colour behaviour of an FSRQ is linked
not only to the emission process in the relativistic jet, but also to the
redshift, the passbands used for computing the colour and the strengths of the
less variable emission features relative to the strength of the non-thermal
continuum.Comment: 9 pages, 9 figures, Accepted by MNRA
Congenital bone marrow failure in DNA-PKcs mutant mice associated with deficiencies in DNA repair
The nonhomologous end-joining (NHEJ) pathway is essential for radioresistance and lymphocyte-specific V(D)J (variable [diversity] joining) recombination. Defects in NHEJ also impair hematopoietic stem cell (HSC) activity with age but do not affect the initial establishment of HSC reserves. In this paper, we report that, in contrast to deoxyribonucleic acid (DNA)–dependent protein kinase catalytic subunit (DNA-PKcs)–null mice, knockin mice with the DNA-PKcs(3A/3A) allele, which codes for three alanine substitutions at the mouse Thr2605 phosphorylation cluster, die prematurely because of congenital bone marrow failure. Impaired proliferation of DNA-PKcs(3A/3A) HSCs is caused by excessive DNA damage and p53-dependent apoptosis. In addition, increased apoptosis in the intestinal crypt and epidermal hyperpigmentation indicate the presence of elevated genotoxic stress and p53 activation. Analysis of embryonic fibroblasts further reveals that DNA-PKcs(3A/3A) cells are hypersensitive to DNA cross-linking agents and are defective in both homologous recombination and the Fanconi anemia DNA damage response pathways. We conclude that phosphorylation of DNA-PKcs is essential for the normal activation of multiple DNA repair pathways, which in turn is critical for the maintenance of diverse populations of tissue stem cells in mice
Tools and techniques for solvent selection: green solvent selection guides
Driven by legislation and evolving attitudes towards environmental issues, establishing green solvents for extractions, separations, formulations and reaction chemistry has become an increasingly important area of research. Several general purpose solvent selection guides have now been published with the aim to reduce use of the most hazardous solvents. This review serves the purpose of explaining the role of these guides, highlighting their similarities and differences. How they can be used most effectively to enhance the greenness of chemical processes, particularly in laboratory organic synthesis and the pharmaceutical industry, is addressed in detail
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