77 research outputs found
Swift/XRT- NuSTAR spectra of type 1 AGN]{Swift/XRT- NuSTAR spectra of type 1 AGN: confirming INTEGRAL results on the high energy cut-off
We present the 0.5 - 78 keV spectral analysis of 18 broad line AGN belonging
to the INTEGRAL complete sample. Using simultaneous Swift-XRT and NuSTAR
observations and employing a simple phenomenological model to fit the data, we
measure with a good constraint the high energy cut-off in 13 sources, while we
place lower limits on 5 objects. We found a mean high-energy cut-off of 111 keV
(standard deviation = 45 keV) for the whole sample, in perfect agreement with
what found in our previous work using non simultaneous observations and with
what recently published using NuSTAR data. This work suggests that simultaneity
of the observations in the soft and hard X-ray band is important but not always
essential, especially if flux and spectral variability are properly accounted
for. A lesser agreement is found when we compare our cut-off measurements with
the ones obtained by Ricci et al. (2017) using Swift-BAT high energy data,
finding that their values are systematically higher than ours. We have
investigated whether a linear correlation exists between photon index and the
cut-off and found a weak one, probably to be ascribed to the non perfect
modelling of the soft part of the spectra, due to the poor statistical quality
of the 2-10 keV X-ray data. No correlation is also found between the Eddington
ratio and the cut-off, suggesting that only using high statistical quality
broad-band spectra is it possible to verify the theoretical predictions and
study the physical characteristics of the hot corona and its geometry.Comment: 12 pages, 11 figures, accepted for publication in Monthly Notices of
the Royal Astronomical Societ
Flat-spectrum radio sources as likely counterparts of unidentified INTEGRAL sources (Research Note)
Many sources in the fourth INTEGRAL/IBIS catalogue are still unidentified,
since they lack an optical counterpart. An important tool that can help in
identifying/classifying these sources is the cross-correlation with radio
catalogues, which are very sensitive and positionally accurate. Moreover, the
radio properties of a source, such as the spectrum or morphology, could provide
further insight into its nature. Flat-spectrum radio sources at high Galactic
latitudes are likely to be AGN, possibly associated to a blazar or to the
compact core of a radio galaxy. Here we present a small sample of 6 sources
extracted from the fourth INTEGRAL/IBIS catalogue that are still
unidentified/unclassified, but which are very likely associated with a bright,
flat-spectrum radio object. To confirm the association and to study the source
X-ray spectral parameters, we performed X-ray follow-up observations with
Swift/XRT. We report the results obtained from this search and discuss the
nature of each source. 5 of the 6 radio associations are also detected in
X-rays; in 3 cases they are the only counterpart found. IGR J06073--0024 is a
flat-spectrum radio quasar at z=1.08, IGR J14488--4008 is a newly discovered
radio galaxy, while IGR J18129--0649 is an AGN of a still unknown type. The
nature of IGR J07225--3810 and IGR J19386--4653 is less well defined, since in
both cases we find another X-ray source in the INTEGRAL error circle;
nevertheless, the flat-spectrum radio source, likely to be a radio loud AGN,
remains a viable and more convincing association in both cases. Only for IGR
J11544--7618 could we not find any convincing counterpart since the radio
association is not an X-ray emitter.Comment: 7 pages, 7 figures, accepted for publication on Astronomy and
Astrophysic
X-ray spectral evolution of V404 Cygni in the initial phase of the 2015 outburst
The black hole binary GS 2023+338 exhibited an unprecedently bright outburst
on June 2015. Since June 17th, the high energy instruments on board INTEGRAL
detected an extremely variable emission during both bright and low luminosity
phases, with dramatic variations of the hardness ratio on time scales of
~seconds. The analysis of the IBIS and SPI data reveals the presence of hard
spectra in the brightest phases, compatible with thermal Comptonization with
temperature kTe ~ 40 keV. The seed photons temperature is best fit by kT0 ~ 7
keV, that is too high to be compatible with blackbody emission from the disk.
This result is consistent with the seed photons being provided by a different
source, that we hypothesize to be a synchrotron driven component in the jet.
During the brightest phase of flares, the hardness shows a complex pattern of
correlation with flux, with a maximum energy released in the range 40-100 keV.
The hard X-ray variability for E > 50 keV is correlated with flux variations in
the softer band, showing that the overall source variability cannot originate
entirely from absorption, but at least part of it is due to the central
accreting source.Comment: 5 pages, 4 figures, accepted for publication in Astrophysical Journal
Letter
Water megamaser emission in hard X-ray selected AGN
Water megamaser emission at 22 GHz has proven to be a powerful tool for
astrophysical studies of AGN allowing an accurate determination of the central
black hole mass and of the accretion disc geometry and dynamics. However, after
searches among thousands of galaxies, only ~ 200 of them have shown such
spectroscopic features, most of them of uncertain classification. In addition,
the physical and geometrical conditions under which maser activates are still
unknown. In this work we aim at characterizing the occurrence of water maser
emission in an unbiased sample of AGN, investigating the relation with the
X-ray properties and the possible favorable geometry needed to detect water
maser. We have searched for 22 GHz maser emission in a hard X-ray selected
sample of AGN, taken from the INTEGRAL/IBIS survey above 20 keV. Of the 380
sources in the sample, only half have water maser data. We have also considered
a sub-sample of 87 sources, volume limited, for which we obtained new Green
Bank Telescope and Effelsberg observations (for 35 sources), detecting one new
maser and increasing its radio coverage to 75%. The detection rate of water
maser emission in the total sample is 15+/-3%, this fraction raises up to
19+/-5% for the complete sub-sample, especially if considering type 2 and
Compton thick AGN. These results demonstrate that the hard X-ray selection may
significantly enhance the maser detection efficiency over comparably large
optical/infrared surveys. A possible decline of the detection fraction with
increasing luminosity might suggest that an extreme luminous nuclear
environment does not favour maser emission. The large fraction of CT AGN with
water maser emission could be explained in terms of geometrical effects, being
the maser medium the very edge-on portion of the obscuring medium.Comment: 21 pages, 3 figures. Accepted for publication in A&A June 202
Disc-Jet coupling in the LMXB 4U1636-53 from INTEGRAL
We report on the spectral analysis results of the neutron star, atoll type,
low mass X-ray Binary 4U1636-53 observed by INTEGRAL and BeppoSAX satellites.
Spectral behavior in three different epochs corresponding to three different
spectral states has been deeply investigated. Two data set spectra show a
continuum well described by one or two soft blackbody plus a Comptonized
components with changes in the Comptonizing electrons and black body
temperature and the accretion rates, which are typical of the spectral
transitions from high to low state. In one occasion INTEGRAL spectrum shows,
for first time in this source, a hard tail dominating the emission above 30
keV. The total spectrum is fitted as the sum of a Comptonized component similar
to soft state and a power-law component (Gamma=2.76), indicating the presence
of a non thermal electron distribution of velocities. In this case, a
comparison with hard tails detected in soft states from neutron stars systems
and some black hole binaries suggests that a similar mechanism could originate
these components in both cases.Comment: 6 pages, 4 figures, 2 tables. accepted Ap
Where are Compton-thick radio galaxies? A hard X-ray view of three candidates
We present a broad-band X-ray spectral analysis of the radio-loud active galactic nuclei NGC 612, 4C 73.08 and 3C 452, exploiting archival data from NuSTAR, XMM-Newton, Swift and INTEGRAL. These Compton-thick candidates are the most absorbed sources among the hard X-ray selected radio galaxies studied in Panessa et al. We find an X-ray absorbing column density in every case below 1.5 × 1024 cm-2, and no evidence for a strong reflection continuum or iron K α line. Therefore, none of these sources is properly Compton-thick. We review other Compton-thick radio galaxies reported in the literature, arguing that we currently lack strong evidences for heavily absorbed radio-loud AGNs
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