212 research outputs found
An XMM-Newton look at the strongly variable radio-weak BL Lac Fermi J1544-0639
Fermi J1544-0639/ASASSN-17gs/AT2017egv was identified as a gamma-ray/optical
transient on May 15, 2017. Subsequent multiwavelength observations suggest that
this source may belong to the new class of radio-weak BL Lacs. We studied the
X-ray spectral properties and short-term variability of Fermi J1544-0639 to
constrain the X-ray continuum emission mechanism of this peculiar source. We
present the analysis of an XMM-Newton observation, 56 ks in length, performed
on February 21, 2018. The source exhibits strong X-ray variability, both in
flux and spectral shape, on timescales of ~10 ks, with a harder-when-brighter
behaviour typical of BL Lacs. The X-ray spectrum is nicely described by a
variable broken power law, with a break energy of around 2.7 keV consistent
with radiative cooling due to Comptonization of broad-line region photons. We
find evidence for a `soft excess', nicely described by a blackbody with a
temperature of ~0.2 keV, consistent with being produced by bulk Comptonization
along the jet.Comment: 11 pages, 12 figures. Accepted for publication in Astronomy &
Astrophysic
Hard X-ray selected giant radio galaxies - I. The X-ray properties and radio connection
We present the first broad-band X-ray study of the nuclei of 14 hard X-ray
selected giant radio galaxies, based both on the literature and on the analysis
of archival X-ray data from NuSTAR, XMM-Newton, Swift and INTEGRAL. The X-ray
properties of the sources are consistent with an accretion-related X-ray
emission, likely originating from an X-ray corona coupled to a radiatively
efficient accretion flow. We find a correlation between the X-ray luminosity
and the radio core luminosity, consistent with that expected for AGNs powered
by efficient accretion. In most sources, the luminosity of the radio lobes and
the estimated jet power are relatively low compared with the nuclear X-ray
emission. This indicates that either the nucleus is more powerful than in the
past, consistent with a restarting of the central engine, or that the giant
lobes are dimmer due to expansion losses.Comment: 11 pages, 3 figures. Accepted for publication in MNRA
FERMI transient J1544-0649: a flaring radio-weak BL Lac
On May 15th, 2017, the \emph{FERMI}/LAT gamma-ray telescope observed a
transient source not present in any previous high-energy catalogue: J1544-0649.
It was visible for two consecutive weeks, with a flux peak on May 21st.
Subsequently observed by a \emph{Swift}/XRT follow-up starting on May 26, the
X-ray counterpart position was coincident with the optical transient
ASASSN-17gs = AT2017egv, detected on May 25, with a potential host galaxy at
=0.171. We conducted a 4-months follow-up in radio (Effelsberg-100m) and
optical (San Pedro M\'artir, 2.1m) bands, in order to build the overall
Spectral Energy Distribution (SED) of this object. The radio data from 5 to 15
GHz confirmed the flat spectrum of the source, favoring a line of sight close
to jet axis, not showing significant variability in the explored post-burst
time-window. The Rx ratio, common indicator of radio loudness, gives a value at
the border between the radio-loud and radio-quiet AGN populations. The
Ca H\&K break value (0.290.05) is compatible with the range
expected for the long-sought intermediate population between BL Lacs and FRI
radio galaxies. An overall SED fitting from Radio to -ray band shows
properties typical of a low-power BL Lac. As a whole, these results suggest
that this transient could well be a new example of the recently discovered
class of radio-weak BL Lac, showing for the first time a flare in the
gamma/X-ray bands.Comment: Accepted for publication in ApJ Letter
The SXI telescope on board EXIST: scientific performances
The SXI telescope is one of the three instruments on board EXIST, a
multiwavelength observatory in charge of performing a global survey of the sky
in hard X-rays searching for Supermassive Black Holes. One of the primary
objectives of EXIST is also to study with unprecedented sensitivity the most
unknown high energy sources in the Universe, like high redshift GRBs, which
will be pointed promptly by the Spacecraft by autonomous trigger based on hard
X-ray localization on board. The recent addition of a soft X-ray telescope to
the EXIST payload complement, with an effective area of ~950 cm2 in the energy
band 0.2-3 keV and extended response up to 10 keV will allow to make broadband
studies from 0.1 to 600 keV. In particular, investigations of the spectra
components and states of AGNs and monitoring of variability of sources, study
of the prompt and afterglow emission of GRBs since the early phases, which will
help to constrain the emission models and finally, help the identification of
sources in the EXIST hard X-ray survey and the characterization of the
transient events detected. SXI will also perform surveys: a scanning survey
with sky coverage of about 2pi and limiting flux of 5x10^{-14}cgs plus other
serendipitous. We give an overview of the SXI scientific performance and also
describe the status of its design emphasizing how it has been derived by the
scientific requirements.Comment: 9 pages, 6 figures, to be published in Proc. of SPIE, vol 7435-11,
200
Physical properties of the nuclear region in Seyfert galaxies derived from observations with the European VLBI Network
We report on sensitive dual-frequency (1.7 and 5 GHz) European VLBI Network
observations of the central region of nine Seyfert galaxies. These sources are
among the faintest and least luminous members of a complete sample of nearby
(d<22 Mpc) low luminosity AGNs. We detect radio emission on milliarcsecond
scale in the nuclei of 4 galaxies, while for the other five sources we set an
upper limit of <~100 microJy. In three sources, namely NGC 3227, NGC 3982, and
NGC 4138, radio emission is detected at both 1.7 and 5 GHz and it is resolved
in two or more components. We describe the structural and spectral properties
of these features; we find that in each of these three nuclei there is one
component with high brightness temperature (typically T_B >10^7.5 K) and
flat/intermediate spectral index (0.3\leq alpha \leq 0.6, S(nu) \sim
nu^(-alpha), accompanied by secondary steep spectrum extended components. In
these cases, non-thermal emission from jets or outflows is thus the most
natural explanation. A faint feature is detected in NGC 4477 at 5 GHz; keeping
in mind the modest significance of this detection (~5sigma), we propose the hot
corona as the origin of non-thermal emission, on the basis of the unrealistic
magnetic field values required by synchrotron self-absorption. Finally, the
five non-detected nuclei remain elusive and further observations on
intermediate scales will be necessary to investigate their nature.Comment: Accepted for publication in MNRA
- …