291 research outputs found
A significant hardening and rising shape detected in the MeV/GeV nuFnu spectrum from the recently-discovered very-high-energy blazar S4 0954+65 during the bright optical flare in 2015 February
We report on Fermi Large Area Telescope (LAT) and multi-wavelength results on
the recently-discovered very-high-energy (VHE, 100 GeV) blazar S4 0954+65
() during an exceptionally bright optical flare in 2015 February.
During the time period (2015 February, 13/14, or MJD 57067) when the MAGIC
telescope detected VHE -ray emission from the source, the Fermi-LAT
data indicated a significant spectral hardening at GeV energies, with a
power-law photon index of ---compared with the 3FGL value
(averaged over four years of observation) of . In contrast,
Swift/XRT data showed a softening of the X-ray spectrum, with a photon index of
(compared with averaged during the flare from
MJD 57066 to 57077), possibly indicating a modest contribution of synchrotron
photons by the highest-energy electrons superposed on the inverse Compton
component. Fitting of the quasi-simultaneous ( day) broadband spectrum with
a one-zone synchrotron plus inverse-Compton model revealed that GeV/TeV
emission could be produced by inverse-Compton scattering of external photons
from the dust torus. We emphasize that a flaring blazar showing high flux of
photons cm s ( 100 MeV) and a
hard spectral index of detected by Fermi-LAT on daily
time scales is a promising target for TeV follow-up by ground-based Cherenkov
telescopes to discover high-redshift blazars, investigate their temporal
variability and spectral features in the VHE band, and also constrain the
intensity of the extragalactic background light.Comment: 15 pages, 3 figures, 2 tables. Accepted by PAS
Precursor flares in OJ 287
We have studied three most recent precursor flares in the light curve of the
blazar OJ 287 while invoking the presence of a precessing binary black hole in
the system to explain the nature of these flares. Precursor flare timings from
the historical light curves are compared with theoretical predictions from our
model that incorporate effects of an accretion disk and post-Newtonian
description for the binary black hole orbit. We find that the precursor flares
coincide with the secondary black hole descending towards the accretion disk of
the primary black hole from the observed side, with a mean z-component of
approximately z_c = 4000 AU. We use this model of precursor flares to predict
that precursor flare of similar nature should happen around 2020.96 before the
next major outburst in 2022.Comment: to appear in the Astrophysical Journa
Locating the γ-ray emission site in Fermi/LAT blazars – II. Multifrequency correlations
In an attempt to constrain and understand the emission mechanism of γ-rays, we perform a cross-correlation analysis of 15 blazars using light curves in millimetre, optical and γ-rays. We use discrete correlation function and consider only correlations significant at the 99 per cent level. A strong correlation was found between 37 and 95 GHz with a near-zero time delay in most of the sources, and ∼1 month or longer in the rest. A similar result was obtained between the optical and γ-ray bands. Of the 15 sources, less than 50 per cent showed a strong correlation between the millimetre and γ-ray or millimetre and optical bands. The primary reason for the lack of statistically significant correlation is the absence of a major outburst in the millimetre bands of most of the sources during the 2.5 yr time period investigated in our study. This may indicate that only the long-term variations or large flares are correlated between these bands. The variability of the sources at every waveband was also inspected using fractional rms variability (F_(var)). The F_(var) displays an increase with frequency reaching its maximum in the γ-rays
Ecological consequences of early Late Pleistocene megadroughts in tropical Africa
Extremely arid conditions in tropical Africa occurred in several discrete episodes between 135 and 90 ka, as demonstrated by lake core and seismic records from multiple basins [Scholz CA, Johnson TC, Cohen AS, King JW, Peck J, Overpeck JT, Talbot MR, Brown ET, Kalindekafe L, Amoako PYO, et al. (2007) Proc Natl Acad Sci USA 104:16416–16421]. This resulted in extraordinarily low lake levels, even in Africa\u27s deepest lakes. On the basis of well dated paleoecological records from Lake Malawi, which reflect both local and regional conditions, we show that this aridity had severe consequences for terrestrial and aquatic ecosystems. During the most arid phase, there was extremely low pollen production and limited charred-particle deposition, indicating insufficient vegetation to maintain substantial fires, and the Lake Malawi watershed experienced cool, semidesert conditions (\u3c400 mm/yr precipitation). Fossil and sedimentological data show that Lake Malawi itself, currently 706 m deep, was reduced to an ≈125 m deep saline, alkaline, well mixed lake. This episode of aridity was far more extreme than any experienced in the Afrotropics during the Last Glacial Maximum (≈35–15 ka). Aridity diminished after 95 ka, lake levels rose erratically, and salinity/alkalinity declined, reaching near-modern conditions after 60 ka. This record of lake levels and changing limnological conditions provides a framework for interpreting the evolution of the Lake Malawi fish and invertebrate species flocks. Moreover, this record, coupled with other regional records of early Late Pleistocene aridity, places new constraints on models of Afrotropical biogeographic refugia and early modern human population expansion into and out of tropical Africa
The 72-Hour WEBT Microvariability Observation of Blazar S5 0716+714 in 2009
Context. The international whole earth blazar telescope (WEBT) consortium
planned and carried out three days of intensive micro-variability observations
of S5 0716+714 from February 22, 2009 to February 25, 2009. This object was
chosen due to its bright apparent magnitude range, its high declination, and
its very large duty cycle for micro-variations. Aims. We report here on the
long continuous optical micro-variability light curve of 0716+714 obtained
during the multi-site observing campaign during which the Blazar showed almost
constant variability over a 0.5 magnitude range. The resulting light curve is
presented here for the first time. Observations from participating
observatories were corrected for instrumental differences and combined to
construct the overall smoothed light curve. Methods. Thirty-six observatories
in sixteen countries participated in this continuous monitoring program and
twenty of them submitted data for compilation into a continuous light curve.
The light curve was analyzed using several techniques including Fourier
transform, Wavelet and noise analysis techniques. Those results led us to model
the light curve by attributing the variations to a series of synchrotron
pulses. Results. We have interpreted the observed microvariations in this
extended light curve in terms of a new model consisting of individual
stochastic pulses due to cells in a turbulent jet which are energized by a
passing shock and cool by means of synchrotron emission. We obtained an
excellent fit to the 72-hour light curve with the synchrotron pulse model
Locating the γ-ray emission site in Fermi/LAT blazars – II. Multifrequency correlations
In an attempt to constrain and understand the emission mechanism of γ-rays, we perform a cross-correlation analysis of 15 blazars using light curves in millimetre, optical and γ-rays. We use discrete correlation function and consider only correlations significant at the 99 per cent level. A strong correlation was found between 37 and 95 GHz with a near-zero time delay in most of the sources, and ∼1 month or longer in the rest. A similar result was obtained between the optical and γ-ray bands. Of the 15 sources, less than 50 per cent showed a strong correlation between the millimetre and γ-ray or millimetre and optical bands. The primary reason for the lack of statistically significant correlation is the absence of a major outburst in the millimetre bands of most of the sources during the 2.5 yr time period investigated in our study. This may indicate that only the long-term variations or large flares are correlated between these bands. The variability of the sources at every waveband was also inspected using fractional rms variability (F_(var)). The F_(var) displays an increase with frequency reaching its maximum in the γ-rays
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