In this paper, we explore a possible route of black hole seed formation that appeals to a model by Davies, Miller & Bellovary who considered the case of the dynamical collapse of a dense cluster of ...stellar black holes subjected to an inflow of gas. Here, we explore this case in a broad cosmological context. The working hypotheses are that (i) nuclear star clusters form at high redshifts in pre-galactic discs hosted in dark matter haloes, providing a suitable environment for the formation of stellar black holes in their cores, (ii) major central inflows of gas occur on to these clusters due to instabilities seeded in the growing discs and/or to mergers with other gas-rich haloes and (iii) following the inflow, stellar black holes in the core avoid ejection due to the steepening to the potential well, leading to core collapse and the formation of a massive seed of ≲ 1000 M⊙. We simulate a cosmological box tracing the build-up of the dark matter haloes and their embedded baryons, and explore cluster evolution with a semi-analytical model. We show that this route is feasible, peaks at redshifts z ≲ 10 and occurs in concomitance with the formation of seeds from other channels. The channel is competitive relative to others, and is independent of the metal content of the parent cluster. This mechanism of gas-driven core collapse requires inflows with masses at least 10 times larger than the mass of the parent star cluster, occurring on time-scales shorter than the evaporation/ejection time of the stellar black holes from the core. In this respect, the results provide upper limit to the frequency of this process.
ABSTRACT
We consider the largest observed sample including all intermediate-frequency peaked (IBL) and high-frequency peaked (HBL) flaring blazars above 100 GeV up to redshift z = 0.6. We show that ...the best-fitting regression line of the emitted spectral indices Γem(z) is a concave parabola decreasing as z increases, thereby implying a statistical correlation between the {Γem(z)} distribution and z. This result contradicts our expectation that such a distribution should be zindependent. We argue that the above correlation does not arise from any selection bias. We show that our expectation naturally emerges provided that axion-like particles (ALPs) are put into the game. Moreover, ALPs can also explain why flat spectrum radio quasars emit up to 400 GeV, in sharp contradiction with conventional physics (CP). So, the combination of the two very different but consistent results – taken at face value – leads to a hint at an ALP with mass $m = {\cal O} (10^{-10} \, {\rm eV})$ and two-photon coupling in the range $2.94 \times 10^{- 12} \, {\rm GeV}^{- 1} \lt g_{a \gamma \gamma } \lt 0.66 \times 10^{- 10} \, {\rm GeV}^{- 1}$. As a bonus, the Universe would become considerably more transparent above energies $E \gtrsim 1 \, {\rm TeV}$ than dictated by CP. Our prediction can be checked not only by the new generation of observatories like CTA, HAWC, GAMMA-400, LHAASO, TAIGA-HiSCORE, and HERD, but also thanks to the planned laboratory experiments ALPS II (upgraded), STAX, IAXO and with other techniques now being developed by Avignone and collaborators.
Dedication: We wish to dedicate the present work to the memory of our dear friend Nanni Bignami.
ABSTRACT
We examine a model for the observed temporal variability of powerful blazars in the γ-ray band in which the dynamics is described in terms of a stochastic differential equation, including ...the contribution of a deterministic drift and a stochastic term. The form of the equation is motivated by the current astrophysical framework, accepting that jets are powered through the extraction of the rotational energy of the central supermassive black hole mediated by magnetic fields supported by a so-called magnetically arrested accretion disc. We apply the model to the γ-ray light curves of several bright blazars and we infer the parameters suitable to describe them. In particular, we examine the differential distribution of fluxes (dN/dFγ) and we show that the predicted probability density function for the assumed stochastic equation naturally reproduces the observed power-law shape at large fluxes $\mathrm{ d}N/\mathrm{ d}F_{\gamma } \propto F_{\gamma }^{-\alpha }$ with α > 2.
Transparency of the Universe to gamma-rays De Angelis, A; Galanti, G; Roncadelli, M
Monthly Notices of the Royal Astronomical Society,
07/2013, Letnik:
432, Številka:
4
Journal Article
Recenzirano
Odprti dostop
Using the most recent observational data concerning the extragalactic background light and the radio background for a source at an arbitrary redshift in the range z
s ≤ 3, we compute the energy E
0 ...of an observed γ-ray photon in the range 10 ≤ E
0 ≤ 1013 GeV such that the resulting optical depth τγ(E
0, z
s) takes the values 1, 2, 3 and 4.6 corresponding to an observed flux dimming of e
−1 0.37, e
−2 0.14, e
−3 0.05 and e
−4.6 0.01, respectively. Below a distance D 8 kpc, we find that τγ(E
0, DH
0/c) < 1 for any value of E
0. In the limiting case of a local Universe (z
s 0), we compare our result with the one derived in 1997 by Coppi and Aharonian. The present achievement is of paramount relevance for the planned ground-based detectors like Cherenkov Telescope Array, High Altitude Water Cherenkov Experiment and Hundred Square-km Cosmic ORigin Explorer.
The aim of this paper was to determine changes of the bioelectrical impedance vector (BIVA) throughout a soccer season and to ascertain whether vector changes are associated with endurance ...performance changes.
Eighteen professional male soccer players (age=21.8±3.0 years, height=1.8±0.07 m, mass=7.2±6.5 kg) participated in the study. BIVA was conducted serially on 8 occasions throughout one soccer season. Endurance performance (Yo-Yo test) was assessed before the first training session of the preseason training, after the pre-season training and at the end of the season.
Vector length shortened (p<0.05) during pre-season training and was associated with improvements in endurance performance (r=0.569, p=0.034). Vector length and phase-angle increased at mid-season compared to post pre-season training (p<0.05). Vector length at end-season was lower compared to mid-season (p<0.05). No further changes in endurance performance occurred.
Bioimpedance vector variations from baseline indicate that fluid-gains occur during the pre-season training, possibly due to plasma volume expansion and enhanced glycogen storage, accompanied by improvements in endurance performance. The vector migration and the increase in phase angle during the competitive season indicate fluid-loss and an increase in body cell mass without effects on performance. At the very end of the season, when training volume and intensity are reduced, body fluid increases again.
Atrial fibrillation in athletes Mascia, G; Perrotta, L; Galanti, G ...
International journal of sports medicine,
05/2013, Letnik:
34, Številka:
5
Journal Article
Recenzirano
Many studies have shown a relationship between long-term endurance sport practice and atrial fibrillation. Inflammation, anatomic remodelling, alterations in the autonomic system and neurohormonal ...activation are all possible explanations for the increased prevalence of this arrhythmia in athletes. Atrial fibrillation may determine disabling symptoms like palpitations and impaired physical performance, compromising eligibility for competitive activities, but exclusion from sport is not necessary for all athletes. Limited data are available on drug therapy and recently ablation resulted to be a particularly attractive option for young athletes with paroxysmal atrial fibrillation. The purpose of this review is to discuss mechanisms, clinical features, management of atrial fibrillation in competitive athletes, including criteria for eligibility and disqualification in sport practice.
Competitive sports induce gradual cardiac adaptations in young athletes. During growth, changes occur in their body composition. The purpose of this study is to provide the left ventricular ...parameters indexed for body composition during young athletes’ growth 220 young athletes (110 females and 110 males) aged from 8 to 19 years old were enrolled. An accurate body composition analysis and echocardiography were performed. The left ventricular parameters were then indexed to the body surface area formula with the data related to body composition (fat-free mass and body cellular mass). The left ventricular and body composition parameters increased continuously during growth and no differences between the sexes were found before puberty. Higher fat mass was found in females from 12 years old (Fat Mass Index: Female = 4.8 ± 1.8 kg/m
2
, Male = 3.6 ± 0.9 kg/m
2
;
p
< 0.05). Cardiac differences started at 13 years old, with a greater left ventricular mass in males (Female = 110.9 ± 20.2 g, Male = 128.7 ± 23.7 g;
p
< 0.05). The indexing of the left ventricle to the body composition parameters increased the age of onset of these cardiological differences to 14 years old with fat-free mass (Female = 91.8 ± 18.7 g/m
2
, Male = 105.0 ± 19.5 g/m
2
;
p
< 0.05), or to 15 years old with body cell mass (Female = 124.3 ± 17.9 g/m
2
, Male = 145.8 ± 28.5 g/m
2
;
p
< 0.05). Differences between the sexes appear to start after puberty. The above indexing was used in order to normalize the differences between the sexes according to body composition. This study reports the reference values for age and gender of the left ventricular parameters indexed for metabolically active mass.
The bicuspid aortic valve (BAV) is the most common congenital cardiac malformation. A literature search was performed using the key words ‘bicuspid aortic valve’, ‘pathophysiology’, ‘exercise’ and ...‘training’. BAV is the result of a complex developmental process where several genes seem to lead to abnormal valvulogenesis. Complications associated with BAV include aortic stenosis (AS) and regurgitation, infective endocarditis and aortic dilation and dissection. Moreover, BAV may be associated with other cardiovascular anomalies, mainly aortic coarctation. There is greater awareness of BAV in the young population who practice sport, with an increasing interest on the impact of regular and competitive exercise on athletes with BAV. The early identification of BAV through pre-participation screening is of paramount importance, and the justification of the more appropriate diagnostic methods is still an area of debate. A normally functioning BAV usually does not represent a limit for practising sport. The stress of regular and intense exercise on an abnormal aortic valve may favour its early deterioration and accelerate the development of complications. Therefore, athletes with BAV warrant regular follow-up, which should include echocardiographic assessment at least every year. The eligibility for participation and ability to continue to practise competitive sports in athletes with BAV cannot be generalized, but needs to be individualized depending on age, severity of lesions and type of sport. Further studies are required to elucidate the impact of physical training and competitive sports on the natural course of the BAV.
Although athletic participation lowers cardiovascular risk and improves quality of life, it may represent a hazard in high-risk group athletes such as those with cardiac abnormalities receiving an ...implantable cardioverter defibrillator (ICD). ICD sports participants are exposed to the potential risk of inappropriate shocks due to sinus tachycardia and other supraventricular arrhythmias during exertion as well as device injury. The safety of athletic participation of ICD-patients is not completely defined and ICD efficacy in interrupting malignant arrhythmias during intense exercise is partly unknown. This explains difficulties in current recommendations made by physicians, given the associated potentially ischemic, autonomic and metabolic conditions. The scope of this review is to underline specific considerations including potential risks and recommendations for athletic participation in this patient-group.
In this study, the impact of regular training on left ventricle morphology in a group of athletes with bicuspid aortic valve (BAV) was evaluated.
Longitudinal cohort study. A group of competitive ...athletes with BAV was followed up with a yearly standard echocardiographic examination for 5 years.
Sport Medicine Centre, University of Florence, Pre-Participation Protocol Study.
A group of 88 consecutive athletes diagnosed with BAV was identified in the period January to December 1999, and 30 of these completed a 5-year follow-up. They were compared with a group of 56 athletes with a normal tricuspid valve (TAV).
BAV athletes showed significant progressive increase in left ventricular dimensions and aortic diameters at four levels. The values were within the range of the general and non-athletic BAV populations. In TAV athletes, the aortic and left ventricle dimensions did not increase significantly and remained within physiological range.
Left ventricular dimensions in competitive BAV athletes remain within the normal range. There is, however, a significant progressive increase in the BAV group compared with the TAV group. These results are in agreement with data obtained in previous studies on the non-athletic BAV population. Sports activity does not have an additional effect on cardiac morphology in athletes with asymptomatic BAV associated with mild regurgitation, for at least 5 years.
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