215 research outputs found
The radio/X-ray correlation in Cyg X-3 and the nature of its hard spectral state
We study the radio/X-ray correlation in Cyg X-3. It has been known that the
soft and hard X-ray fluxes in the hard spectral state are correlated positively
and negatively, respectively, with the radio flux. We show that this implies
that the observed 1--100 keV flux (which is a fair approximation to the
bolometric flux) is completely uncorrelated with the radio flux. We can recover
a positive correlation (seen in other sources and expected theoretically) if
the soft X-rays are strongly absorbed by a local medium. Then, however, the
intrinsic X-ray spectrum of Cyg X-3 in its hard state becomes relatively soft,
similar to that of an intermediate spectral state of black-hole binaries, but
not to their true hard state. We also find the radio spectra in the hard state
of Cyg X-3 are hard on average, and the flux distributions of the radio
emission and soft X-rays can be described by sums of two log-normal functions.
We compare Cyg X-3 with other X-ray binaries using colour-colour,
colour-Eddington ratio and Eddington ratio-radio flux diagrams. We find Cyg X-3
to be spectrally most similar to GRS 1915+105, except that Cyg X-3 is
substantially more radio loud, which appears to be due to its jet emission
enhanced by interaction with the powerful stellar wind from the Wolf-Rayet
donor.Comment: An error in the BAT light curve shown in Fig. A2 corrected. Published
in MNRAS + erratu
Ultrasonic evaluation of induction heat treatment applied to thermoplastic matrix CFRP
Abstract Thermoplastic matrix carbon fibre reinforced polymers (CFRP) are extensively utilized for composites structures in the aerospace and aeronautical industries. Diverse techniques were currently applied to joining composite parts, the most promising method is the induction heat treatment. In this paper, experimental tests were performed on thermoplastic matrix CFRP specimens by varying the induction heat treatment parameters: power, frequency and current. An advanced ultrasonic (UT) non-destructive evaluation based on pulse-echo technique was employed for the investigation of the utilized process parameters through the UT evaluation of the process induced damage and its depth along the thickness of the thermoplastic matrix CFRP laminates
Effective pointing of the ASTRI-Horn telescope using the Cherenkov camera with the Variance method
Cherenkov telescope cameras are not suitable to perform astrometrical
pointing calibration since they are not designed to produce images of the sky,
but rather to detect nanosecond atmospheric flashes due to very high-energy
cosmic radiation. Indeed, these instruments show only a moderate angular
resolution (fractions of degrees) and are almost blind to the steady or
slow-varying optical signal of starlight. For this reason, auxiliary optical
instruments are typically adopted to calibrate the telescope pointing. However,
secondary instruments are possible sources of systematic errors. Furthermore,
the Cherenkov camera is the only one framing exactly the portion of the sky
under study, and hence its exploitation for pointing calibration purposes would
be desirable. In this contribution, we present a procedure to assess the
pointing accuracy of the ASTRI-Horn telescope by means of its innovative
Cherenkov camera. This instrument is endowed with a statistical method, the
so-called Variance method, implemented in the logic board and able to provide
images of the night sky background light as ancillary output. Taking into
account the convolution between the optical point spread function and the pixel
distribution, Variance images can be used to evaluate the position of stars
with sub-pixel precision. In addition, the rotation of the field of view during
observations can be exploited to verify the alignment of the Cherenkov camera
with the optical axis of the telescope, with a precision of a few arcminutes,
as upper limit. This information is essential to evaluate the effective
pointing of the telescope, enhancing the scientific accuracy of the system.Comment: 7 pages, 5 figures, Proceedings of the 37th International Cosmic Ray
Conference (ICRC 2021), Berlin, German
Use of the Peak-Detector mode for gain calibration of SiPM sensors with ASIC CITIROC read-out
The Cherenkov Imaging Telescope Integrated Read Out Chip (CITIROC) is a
32-channel fully analogue front-end ASIC dedicated to the read-out of silicon
photo-multiplier (SiPM) sensors that can be used in a variety of experiments
with different applications: nuclear physics, medical imaging, astrophysics,
etc. It has been adopted as front-end for the focal plane detectors of the
ASTRI-Horn Cherenkov telescope and, in this context, it was modified
implementing the peak detector reading mode to satisfy the instrument
requirements. For each channel, two parallel AC coupled voltage preamplifiers,
one for the high gain and one for the low gain, ensure the read-out of the
charge from 160 fC to 320 pC (i.e. from 1 to 2000 photo-electrons with SiPM
gain = 10, with a photo-electron to noise ratio of 10). The signal in
each of the two preamplifier chains is shaped and the maximum value is captured
by activating the peak detector for an adjustable time interval. In this work,
we illustrate the peak detector operation mode and, in particular, how this can
be used to calibrate the SiPM gain without the need of external light sources.
To demonstrate the validity of the method, we also present and discuss some
laboratory measurements.Comment: 11 pages, 9 figures, 15th Topical Seminar on Innovative Particle and
Radiation Detectors (IPRD19) 14-17 October 2019 Siena, Italy (Submitted to
JINST peer review on 05 January 2020
The puzzling source IGR J17361-4441 in NGC 6388: a possible planetary tidal disruption event
On 2011 August 11, INTEGRAL discovered the hard X-ray source IGR J17361-4441
near the centre of the globular cluster NGC 6388. Follow up observations with
Chandra showed the position of the transient was inconsistent with the cluster
dynamical centre, and thus not related to its possible intermediate mass black
hole. The source showed a peculiar hard spectrum (Gamma \approx 0.8) and no
evidence of QPOs, pulsations, type-I bursts, or radio emission. Based on its
peak luminosity, IGR J17361-4441 was classified as a very faint X-ray
transient, and most likely a low-mass X-ray binary. We re-analysed 200 days of
Swift/XRT observations, covering the whole outburst of IGR J17361-4441 and find
a t^{-5/3} trend evident in the light curve, and a thermal emission component
that does not evolve significantly with time. We investigate whether this
source could be a tidal disruption event, and for certain assumptions find an
accretion efficiency epsilon \approx 3.5E-04 (M_{Ch}/M) consistent with a
massive white dwarf, and a disrupted minor body mass M_{mb}=1.9E+27(M/M_{Ch}) g
in the terrestrial-icy planet regime. These numbers yield an inner disc
temperature of the order kT_{in} \approx 0.04 keV, consistent with the
blackbody temperature of kT_{in} \approx 0.08 keV estimated by spectral
fitting. Although the density of white dwarfs and the number of free-floating
planets are uncertain, we estimate the rate of planetary tidal disruptions in
NGC 6388 to be in the range 3E-06 to 3E-04 yr^{-1}. Averaged over the Milky Way
globular clusters, the upper limit value corresponds to 0.05 yr^{-1},
consistent with the observation of a single event by INTEGRAL and Swift.Comment: Accepted for publication in Monthly Notices of the Royal Astronomical
Society Main Journal on 2014 July 16; 9 pages, 5 figures. Added references;
corrected typo
Cyclotron lines in X-ray pulsars as a probe of relativistic plasmas in superstrong magnetic fields
The systematic search for the presence of cyclotron lines in the spectra of
accreting X-ray pulsars is being carried on with the BeppoSAX satellite since
the beginning of the mission. These highly successful observations allowed the
detection of cyclotron lines in many of the accreting X-ray pulsars observed.
Some correlations between the different measured parameters were found. We
present these correlations and discuss them in the framework of the current
theoretical scenario for the X-ray emission from these sources.Comment: 5 pages, 2 figures, uses aipproc.sty, to appear in Proceeding of
Fifth Compton Symposiu
Compton-thick AGN in the NuSTAR era III: A systematic study of the torus covering factor
We present the analysis of a sample of 35 candidate Compton thick (CT-)
active galactic nuclei (AGNs) selected in the nearby Universe (average redshift
~0.03) with the Swift-BAT 100-month survey. All sources have available
NuSTAR data, thus allowing us to constrain with unprecedented quality important
spectral parameters such as the obscuring torus line-of-sight column density
(N_{H, z}), the average torus column density (N_{H, tor}) and the torus
covering factor (f_c). We compare the best-fit results obtained with the widely
used MyTorus (Murphy et al. 2009) model with those of the recently published
borus02 model (Balokovic et al. 2018) used in the same geometrical
configuration of MyTorus (i.e., with f_c=0.5). We find a remarkable agreement
between the two, although with increasing dispersion in N_{H, z} moving towards
higher column densities. We then use borus02 to measure f_c. High-f_c sources
have, on average, smaller offset between N_{H, z} and N_{H, tor} than low-f_c
ones. Therefore, low f_c values can be linked to a "patchy torus" scenario,
where the AGN is seen through an over-dense region in the torus, while high-f_c
objects are more likely to be obscured by a more uniform gas distribution.
Finally, we find potential evidence of an inverse trend between f_c and the AGN
2-10 keV luminosity, i.e., sources with higher f_c values have on average lower
luminosities.Comment: 35 Pages, 23 Figures. Accepted for publication in Ap
Compton-thick AGNs in the NuSTAR era. V: Joint NuSTAR and XMM-Newton spectral analysis of three "soft-Gamma" candidate CT-AGNs in the Swift-BAT 100-month catalog
We present the joint NuSTAR and XMM-Newton spectral analysis in the 0.6-70
keV band of three candidate Compton thick (CT-) AGN selected in the 100-month
Swift-BAT catalog. These objects were previously classified as CT-AGNs based on
low quality Swift-XRT and Swift-BAT data, and had soft photon indices
(Gamma>2.2) that suggested a potential overestimation of the line of sight
column density. Thanks to the high-quality NuSTAR and XMM-Newton data we were
able to determine that in all three objects the photon index was significantly
overestimated, and two out of three sources are reclassified from CT to Compton
thin, confirming a previously observed trend, i.e., that a significant fraction
of BAT-selected, candidate CT-AGNs with poor soft X-ray data are reclassified
as Compton thin when the NuSTAR data are added to the fit. Finally, thanks to
both the good XMM-Newton spatial resolution and the high NuSTAR and XMM-Newton
spectral quality, we found that the third object in our sample was associated
to the wrong counterpart: the correct one, 2MASX J10331570+5252182, has
redshift z=0.14036, which makes it one of the very few candidate CT-AGNs in the
100-month BAT catalog detected at z>0.1, and a rare CT quasar.Comment: 12 Pages, 4 Figures. Accepted for publication in Ap
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