Thermal synchrotron radiation and its Comptonization in compact X-ray sources

We investigate thermal synchrotron radiation at semi-relativistic and relativistic temperatures. We find an analytic expression for the angle-averaged emission coefficient, and show that it is significantly more accurate that those derived previously. We also present analytic approximations to the synchrotron turnover frequency. Then, we treat Comptonization of the synchrotron radiation, and give simple expressions for the spectral shape and the emitted power. We also consider modifications of the above results by bremsstrahlung. The importance of Comptonization of synchrotron radiation in compact X-ray sources is then studied. We consider emission from hot accretion flows and from active coronae above optically-thick accretion discs in black-hole binaries and AGNs. Synchrotron Comptonization is found to be, in general, negligible in luminous sources, except for those with hardest X-ray spectra and stellar masses. Increasing the black-hole mass results in a strong reduction of the maximum Eddington ratio possible from to this process. X-ray spectra of intermediate-luminosity sources, e.g., low-luminosity AGNs, can be explained by synchrotron Comptonization only in the case of hot accretion flows. Then, bremsstrahlung emission always dominates X-ray spectra of very weak sources. Finally, we consider weakly-magnetized neutron stars and find that synchrotron Comptonization can account for the power-law X-ray spectra observed in their low states.Comment: 17 pages, 17 postscript figures. Accepted to MNRA

Effects of non-thermal tails in Maxwellian electron distributions on synchrotron and Compton processes

We investigate how the presence of a non-thermal tail beyond a Maxwellian electron distribution affects the synchrotron process as well as Comptonization in plasmas with parameters typical for accretion flows onto black holes. We find that the presence of the tail can significantly increase the net (after accounting for self-absorption) cyclo-synchrotron emission of the plasma, which emission then provides seed photons for Compton upscattering. Thus, the luminosity in the thermally-Comptonized spectrum is enhanced as well. The importance of these effects increases with both increasing Eddington ratio and the black hole mass. The enhancement of the Comptonized synchrotron luminosity can be as large as by factors of $\sim 10^3$ and $\sim 10^5$ for stellar and supermassive black holes, respectively, when the energy content in the non-thermal tail is 1 per cent. The presence of the tail only weakly hardens the thermal Comptonization spectrum but it leads to formation of a high-energy tail beyond the thermal cut-off, which two effects are independent of the nature of the seed photons. Since observations of high-energy tails in Comptonization spectra can constrain the non-thermal tails in the electron distribution and thus the Comptonized synchrotron luminosity, they provide upper limits on the strength of magnetic fields in accretion flows. In particular, the measurement of an MeV tail in the hard state of Cyg X-1 by McConnell et al. implies the magnetic field strength in this source to be at most an order of magnitude below equipartition.Comment: 10 pages, 9 postscript figures. Accepted to MNRA

Cyclotron-Synchrotron: harmonic fitting functions in the non-relativistic and trans-relativistic regimes

The present work investigates the calculation of absorption and emission cyclotron line profiles in the non-relativistic and trans-relativistic regimes. We provide fits for the ten first harmonics with synthetic functions down to 10^(-4) of the maximum flux with an accuracy of 20 per cent at worst. The lines at a given particle energy are calculated from the integration of the Schott formula over the photon and the particle solid angles relative to the magnetic field direction. The method can easily be extended to a larger number of harmonics. We also derive spectral fits of thermal emission line plasmas at non-relativistic and trans-relativistic temperatures extending previous parameterisations.Comment: 11 pages, 11 figures, accepted for publication in Astronomy & Astrophysic

Effects of the COVID-19 lockdown on mental health in a UK student sample

BACKGROUND: The COVID-19 pandemic and the resulting restrictions placed upon society have had a profound impact on both physical and mental health, particularly for young people. AIMS: The current study assesses the impact of COVID-19 on student mental health. METHOD: Four hundred and thirty four first year Undergraduate students completed a battery of self-report questionnaires (PHQ-P, GAD-7 and SAS-SV) to assess for depression, anxiety and mobile phone addiction respectively with data being collected over a 2 year period. The data from each year was compared (216 and 218 students respectively). RESULTS: A MANOVA revealed that COVID-19 had a significant impact on self-reported levels of depression, anxiety and smartphone addiction—which all significantly increased from the 2020 to the 2021 group. The percentage of students who had a score which warranted a classification of clinical depression increased from 30 to 44%, and for anxiety increased from 22 to 27%—those students who showed a comorbidity across the two rose from 12 to 21%. Smartphone addiction levels rose from 39 to 50%. Correlational analysis showed a significant relationship between Smartphone usage and depression and anxiety. CONCLUSIONS: This research suggests that COVID-19 has had a major impact upon student mental health, and smartphone addiction. The importance of identifying predictive factors of depression and anxiety is emphasised, and suggestions for intervention are discussed

X-ray and gamma-ray spectra and variability of the black-hole candidate GX 339-4

We analyse five observations of the X-ray binary GX 339-4 by the soft gamma-ray OSSE detector on board CGRO simultaneous with either Ginga or RXTE observations. The source was bright during four of them, with the luminosity of L ~ 10^{37} erg/s and the spectrum typical for hard states of accreting black holes, and it was in an off state during the fifth one, with L ~ 10^{35} erg/s. Our broad-band spectral fits show the mean electron energy of electrons in the Comptonizing plasma decreasing with increasing luminosity within the hard (bright) state. For the observation with the best statistics at soft gamma-rays, approximately 1/4 of energy in the Comptonizing plasma is probably carried by non-thermal electrons. Then, considering the efficiency of Comptonized hybrid synchrotron emission allows us to obtain an upper limit on the strength of the magnetic field in the X-ray source. Furthermore, this synchrotron emission is capable of producing the optical spectrum observed in an optically-high state of GX 339-4. In the off state, the hard X-ray spectrum is consistent with being dominated by bremsstrahlung. The unusually strong Fe K alpha line observed by the PCA during that state is found not to be intrinsic to the source but to originate mostly in the Galactic diffuse emission.Comment: 12 pages, 7 figures (2 in colour). Accepted for publication in MNRA

Establishing a framework for dynamic risk management in 'intelligent' aero-engine control

The behaviour of control functions in safety critical software systems is typically bounded to prevent the occurrence of known system level hazards. These bounds are typically derived through safety analyses and can be implemented through the use of necessary design features. However, the unpredictability of real world problems can result in changes in the operating context that may invalidate the behavioural bounds themselves, for example, unexpected hazardous operating contexts as a result of failures or degradation. For highly complex problems it may be infeasible to determine the precise desired behavioural bounds of a function that addresses or minimises risk for hazardous operation cases prior to deployment. This paper presents an overview of the safety challenges associated with such a problem and how such problems might be addressed. A self-management framework is proposed that performs on-line risk management. The features of the framework are shown in context of employing intelligent adaptive controllers operating within complex and highly dynamic problem domains such as Gas-Turbine Aero Engine control. Safety assurance arguments enabled by the framework necessary for certification are also outlined

An intense state of hard X-ray emission of Cyg X-1 observed by INTEGRAL coincident with TeV measurements

We present INTEGRAL light curves and spectra of the black-hole binary Cyg X-1 during a bright event that occurred in 2006 September, and which was simultaneous with a detection at 0.15-1 TeV energies by the MAGIC telescope. We analyse the hard X-ray emission from 18 to 700 keV with the INTEGRAL data taken on 2006 September 24-26. We present the light curves and fit the high energy spectrum with various spectral models. Despite variations in the flux by a factor of ~2 in the the 20-700 keV energy band, the shape of the energy spectrum remained remarkably stable. It is very well represented by an e-folded power law with the photon index of 1.4 and a high energy cut-off around 130-140 keV. The spectrum is also well described by thermal Comptonisation including a moderate reflection component, with a reflectionamplitude R around 0.4. The temperature of the hot Comptonising electrons is ~70 keV and their Thomson optical depth is ~2.5. These spectral properties are typical of those observed in the low/hard state. This shows that Cyg X-1 may stay in the low hard state at least up to the flux level of 2 Crab, which corresponds to ~2-3 percent of the Eddington luminosity. It is the first time a persistent high-mass black-hole binary is observed at a few percent of the Eddington luminosity with a stable low/hard state spectrum over a period of a few days. The TeV detection coincides with the peak of a small X-ray flare just after a very fast rise in hard X-ray flux. In contrast, the source remained undetected by MAGIC at the peak of a larger X-ray flare occurring one day later and corresponding to the maximum of the X-ray luminosity of the whole outburst. We do not find any obvious correlation between the X-ray and TeV emission.Comment: 8 pages, 8 figures, accepted for publication in Astronomy and Astrophysics (major changes following the referee's comments

1E 1740.7-2942: temporal and spectral evolution from INTEGRAL and RXTE observations

We present results of the monitoring of the black hole candidate 1E 1740.7-2942 with INTEGRAL, in combination with simultaneous observations by RXTE. We concentrate on broad-band spectra from INTEGRAL/IBIS and RXTE/PCA instruments. During our observations, the source spent most of its time in the canonical low/hard state with the measured flux variation within a factor of two. In 2003 September the flux started to decline and in 2004 February it was below the sensitivity level of the INTEGRAL and RXTE instruments. Notably, during the decline phase the spectrum changed, becoming soft and typical of black hole binaries in the intermediate/soft state.Comment: 5 pages, 4 figures, accepted for publication in A&A; typos corrected in the abstrac

The assessment of the ionizing radiation dose received by patients during some diagnostic X-ray examinations carried out on the basis of the working procedures in the health care entities

Background. X-ray examination is a popular and universally used injury and disease diagnostic method. A distinctive X ray examination feature is that it can be done quickly which is extremely important in case of the need for rapid diagnosis of patients in life threatening condition. Another advantage of the X-ray examinations is also relatively low cost of carry. However, X-ray examination involve adverse health effects. During the examination the patient is subjected to ionizing radiation that might have impact on his health. Objective. The aim of this study has been to determine and assess the size of the entrance surface doses (ESD) received by patients during selected X-ray examinations performed on the basis of the medical working procedures available in healthcare entities in Masovian Voivodeship in Poland. Materials and Method. The examinations were conducted for 71 X-ray units located in the Masovian Voivodeship. Measurements of doses received by the patients were based on our own validated test methods. Results. It was found that the range applied to the high voltage in healthcare entities does not always coincide with the values specified in the standard procedures. It was found in the skull projection radiography AP and LAT that the recorded values were from range 60 to 82 kV (the average value of 74 kV) while in accordance with a standard procedure they should be in the range from 65 to 75 kV. Only in case of cervical spine radiography in the AP projection, the LAT exposure conditions were matching with the standard obligatory procedures in Poland. The consequence of selecting exposure conditions are significant differences in the size of the doses the patient receive during the same medical procedures. The greatest range of ESD doses was found during radiography of the thoracic spine in the projection AP and LAT. The projection LAT measured values were in the range of 523 to 10550 μGy (average value 2175 μGy). Conclusions. It is necessary to update immediately the standard procedures and to develop detailed guidelines for the preparation of working procedures in X-ray rooms.Wprowadzenie. Badania rentgenowskie są powszechnie stosowaną metodą diagnozowania urazów i schorzeń występujących u pacjentów. Największą zaletą tych badań jest szybkość ich wykonania, co jest niezwykle istotne w przypadku konieczności postawienia szybkiej diagnozy medycznej u pacjentów w ciężkim stanie zagrażającym ich życiu. Badania rentgenowskie niosą jednakże ze sobą także negatywne skutki zdrowotne. W trakcie ich przeprowadzania pacjent poddany jest działaniu promieniowania jonizującego, które nie jest obojętne dla zdrowia. Cel. Celem badań było oszacowanie wielkości wejściowej dawki powierzchniowej (ESD) jaką otrzymują pacjenci podczas wybranych badań rentgenowskich przeprowadzonych w oparciu o procedury robocze obowiązujące w placówkach ochrony zdrowia na terenie województwa mazowieckiego. Materiał i metody. Zbadano 71 aparatów rentgenowskich wykorzystywanych w placówkach ochrony zdrowia na terenie województwa mazowieckiego. Pomiary dawek otrzymywanych przez pacjentów podczas badań rentgenowskich przeprowadzono własnymi zwalidowanymi metodami badawczymi. Wyniki. Stwierdzono, iż zakres wartości wysokiego napięcia dobieranego przez personel medyczny na aparatach rentgenowskich w placówkach ochrony zdrowia nie zawsze pokrywa się z wartościami określonymi w procedurach wzorcowych. Stwierdzono, iż w radiografii czaszki w projekcji AP i LAT wartość wysokiego napięcia była od 60 do 82 kV (wartość średnia 74 kV), zaś zgodnie z procedurą wzorcową powinny mieścić się w zakresie od 65 do 75 kV. Najlepszą zgodność pomiędzy wartościami wysokiego napięcia stosowanymi w pracowniach rtg a tymi określonymi w procedurach wzorcowych obowiązujących w Polsce stwierdzono w przypadku radiografii kręgosłupa szyjnego. Konsekwencją tak dobieranych warunków ekspozycji są znaczące różnice w wielkości dawek promieniowania otrzymywanych przez pacjentów dla tych samych procedur medycznych. Największą rozpiętość dawek ESD stwierdzono w trakcie radiografii kręgosłupa piersiowego w projekcji AP oraz LAT. W projekcji LAT zmierzone wartości były w przedziale od 523 do 10550 μGy (wartość średnia 2175 μGy). Wnioski. Niezbędne jest jak najszybsze uaktualnienie procedur wzorcowych oraz opracowanie szczegółowych wytycznych dotyczących przygotowywania procedur roboczych w pracowniach rentgenowskich