56 research outputs found
The evolution of the UV/optical lag spectrum of NGC 7469 seen by the Liverpool Telescope
We present the results regarding the analysis of an intensive monitoring of the Active Galactic Nucleus (AGN) NGC 7469. We observed the source for 4 months with almost daily cadence in the ugriz bands, using the IO:O on the Liverpool Telescope. We measured the lags with respect to the u band and found a clear change in the lag spectrum between the first and the second half of the campaign. Given that the source varies on different timescales during these two segments, it is likely that different components are dominating the variability at different times. This result further confirms that reverberation models require a more complex geometry than a static illuminating point source and that particular attention has to be given in the interpretation of these delays
Comparison of time/phase lags in the hard state and plateau state of GRS 1915+105
We investigate the complex behavior of energy- and frequency-dependent
time/phase lags in the plateau state and the radio-quiet hard state of GRS
1915+105. In our timing analysis, we find that when the source is faint in the
radio, QPOs are observed above 2 Hz and typically exhibit soft lags (soft
photons lag hard photons), whereas QPOs in the radio-bright plateau state are
found below 2.2 Hz and consistently show hard lags. The phase lag at the QPO
frequency is strongly anti-correlated with the QPO frequency, changing sign at
2.2 Hz. However, the phase lag at the frequency of the first harmonic is
positive and nearly independent of frequency at at ~0.172 rad, regardless of
the radio emission. The lag-energy dependence at the first harmonic is also
independent of radio flux. However, the lags at the QPO frequency are negative
at all energies during the radio-quiet state, but lags at the QPO frequency
during the plateau state are positive at all energies and show a
'reflection-type' evolution of the lag-energy spectra with respect to the
radio-quiet state. The lag-energy dependence is roughly logarithmic, but there
is some evidence for a break around 4-6 keV. Finally, the Fourier
frequency-dependent phase lag spectra are fairly flat during the plateau state,
but increase from negative to positive during the radio-quiet state. We discuss
the implications of our results in the light of some generic models.Comment: 9 pages, 7 figures, accepted for publication in Ap
Detection of the high energy cut-off from the Seyfert 1.5 galaxy NGC 5273
We perform the NuSTAR and Swift/XRT joint energy spectral fitting of
simultaneous observations from the broad-line Seyfert 1.5 galaxy NGC 5273. When
fitted with the combination of an exponential cut-off power-law and a
reflection model, a high energy cut-off is detected at 143 keV
with 2-sigma significance. Existence of such cut-off is also consistent with
the observed Comptonizing electron temperature when fitted with a
Comptonization model independently. We observe a moderate hard X-ray
variability of the source over the time-scale of ~12 years using INTEGRAL/ISGRI
observations in the energy range of 20-100 keV. When the hard band count rate
(6-20 keV) is plotted against the soft band count rate (3-6 keV), a hard offset
is observed. Our results indicate that the cut-off energy may not correlate
with the coronal X-ray luminosity in a simple manner. Similarities in
parameters that describe coronal properties indicate that the coronal structure
of NGC 5273 may be similar to that of the broad-line radio galaxy 3C 390.3 and
another galaxy MCG-5-23-16 where the coronal plasma is dominated by electrons,
rather than electron-positron pairs. Therefore, the coronal cooling is equally
efficient to the heating mechanism keeping the cut-off energy at low even at
the low accretion rate.Comment: 11 pages, 6 figures, 1 table, accepted for publication in MNRA
Properties of unique hard X-ray dips observed from GRS 1915+105 and IGR J17091-3624 and their implications
We report a comprehensive study on spectral and timing properties of hard
X-ray dips uniquely observed in some so-called variability classes of the
micro-quasars GRS 1915+105 and IGR J17091-3624. These dips are characterized by
a sudden decline in the 2.0-60.0 keV X-ray intensity by a factor of 4-12
simultaneous with the increase in hardness ratio by a factor of 2-4. Using 31
observations of GRS 1915+105 with RXTE/PCA, we show that different behavior are
observed in different types of variability classes, and we find that a
dichotomy is observed between classes with abrupt transitions vs those with
smoother evolution. For example, both energy-lag spectra and frequency-lag
spectra of hard X-ray dips in classes with abrupt transitions and shorter dip
intervals show hard-lag (hard photons lag soft photons), while both lag spectra
during hard dips in classes with smoother evolution and longer dip intervals
show soft-lag. Both lag time-scales are of the order of 100-600 msec. We also
show that timing and spectral properties of hard X-ray dips observed in light
curves of IGR J17091-3624 during its 2011 outburst are consistent with the
properties of the abrupt transitions in GRS 1915+105 rather than smooth
evolution. A global correlation between the X-ray intensity cycle time and hard
dip time is observed for both abrupt and smooth transition which may be due to
two distinct physical processes whose time-scales are eventually correlated. We
discuss implications of our results in the light of some generic models.Comment: 17 pages, 5 figures, accepted for publication in the Astrophysical
Journa
- …