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 $\sim$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$^{6}$, 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