The broadband spectral distributions of non-thermal sources, such as those of several known blazars, are well described by a log-parabolic fit. The second-degree term in these fits measures the ...curvature in the spectrum. In this paper, we investigate whether the curvature parameter observed in the spectra of the synchrotron emission can be used as a fingerprint of stochastic acceleration. As a first approach, we use the multiplicative central limit theorem to show how fluctuations in the energy gain result in the broadening of the spectral shape, introducing a curvature into the energy distribution. Then, by means of a Monte Carlo description, we investigate how the curvature produced in the electron distribution is linked to the diffusion in momentum space. To get a more generic description of the problem we turn to the diffusion equation in momentum space. We first study some 'standard' scenarios, in order to understand the conditions that make the curvature in the spectra significant, and the relevance of cooling during the acceleration process. We try to quantify the correlation between the curvature and the diffusive process in the pre-equilibrium stage, and investigate how the transition between the Klein-Nishina and the Thomson regimes, in inverse Compton cooling, determine the curvature in the distribution at equilibrium. We apply these results to some observed trends, such as the anticorrelation between the peak energy and the curvature term observed in the spectra of Mrk 421, and a sample of BL Lac objects whose synchrotron emission peaks at X-ray energies.
The 5th edition of the
Roma-BZCAT
Multifrequency Catalogue of Blazars is available in a printed version and online at the ASDC website (
http://www.asdc.asi.it/bzcat
); it is also in the NED ...database. It presents several relevant changes with respect to the past editions which are briefly described in this paper.
We present an updated version catalogue of
γ
-ray source candidates, 12Y-MST, selected using the minimum spanning tree (MST) algorithm on the 12-year
Fermi
-LAT sky (Pass 8) at energies higher than ...10 GeV. The high-energy sky at absolute Galactic latitudes above 20° has been investigated using rather restrictive selection criteria, resulting in a total sample of 1664 photon clusters, or candidate sources. Of these, 230 are new detections, that is, candidate sources without any association in other
γ
-ray catalogues. A large fraction of them have interesting counterparts, most likely blazars. We describe the main results on the catalogue selection and search of counterparts. We also present an additional sample of 224 candidate sources (12Y-MSTw), which are clusters that we extracted by applying weaker selection criteria: about 57% of them have not been reported in other catalogues.
Context.
We present the results of a new analysis of three long
Rossi
-XTE observations of the microquasar GRS 1915+105 in the
ρ
class, performed in 1997, 1999, and 2000, and characterized by ...different peak profiles. The first data set, labeled G-1, is dominated by a single peak, while in the third observation (G-3), all bursts show a clearly detectable couple of peaks. The second observation (G-2) shows an intermediate structure with a single peak and an emerging shoulder on the decay side.
Aims.
We devised a new procedure to obtain mean burst profiles in every energy channel independently of the recurrence time intervals of the bursts, variable from 45 s to 53 s in the considered observations, with the aim of investigating the different features of peaks and the eventual spectral variations.
Methods.
All the bursts were aligned at a common time bin on the decaying portion of the bursts that is stable in simultaneous light curves at different energies. An averaging algorithm was then applied without modifying the statistical properties or scaling the burst lengths. We analyzed the peak amplitude ratios and the dependence of their delays on energy. The spectral distributions were evaluated for the various components: a stable multi-temperature disk plus a power law Comptonization component was used for the baseline emission and temperature differences of peak components were evaluated with the inclusion of an additional blackbody.
Results.
In addition to the well-observed double peak (P1 and P2) pattern, we detected a third small peak (P3) in the structured G-3 light curve. This peak, differently from the other two, exhibits a fast rising and a slower exponential decay, with a
e
-folding time constant of 1.32 s. The blackbody temperatures of P2 and P3 are higher than P1 and the power law spectrum of P3 is the flattest one.
Conclusions.
The time and spectral behavior of P3 is interpreted as a signature of a relatively hot plasma outflow from the disk into the corona and its duration is consistent with the crossing timescale of the particles through the corona where electrons radiate.
ABSTRACT
The X-ray emission from the microquasar GRS 1915+105 shows, together with a very complex variability on different time-scales, the presence of low-frequency quasi-periodic oscillations ...(LFQPOs) at frequencies lower than ∼30 Hz. In this paper, we demonstrate that these oscillations can be consistently and naturally obtained as solutions of a system of two ordinary differential equations, which is able to reproduce almost all variability classes of GRS 1915+105. We modified the Hindmarsh–Rose model and obtained a system with two dynamical variables x(t), y(t), where the first one represents the X-ray flux from the source, and an input function J(t), whose mean level J0 and its time evolution is responsible of the variability class. We found that for values of J0 around the boundary between the unstable and the stable interval, where the equilibrium points are of spiral type, one obtains an oscillating behaviour in the model light curve similar to the observed ones with a broad Lorentzian feature in the power density spectrum and, occasionally, with one or two harmonics. Rapid fluctuations of J(t), as those originating from turbulence, stabilize the LFQPOs, resulting in a slowly amplitude modulated pattern. To validate the model, we compared the results with real RXTE data, which resulted remarkably similar to those obtained from the mathematical model. Our results allow us to favour an intrinsic hypothesis on the origin of LFQPOs in accretion discs ultimately related to the same mechanism responsible for the spiking limit cycle.
We describe a catalogue of γ-ray source candidates selected using the minimum spanning tree (MST) algorithm on the nine year Fermi-LAT sky (Pass 8) at energies higher than 10 GeV. The extragalactic ...sky at absolute Galactic latitudes above 20° has been investigated using rather restrictive selection criteria, resulting in a total sample of 1342 sources. Of these, 249 are new detections that have not been previously associated with γ-ray catalogues. A large portion of these candidates have interesting counterparts, which are most likely blazars. In this paper, we report the main results of the catalogue selection and search of counterparts.
We present the results of a cluster search in the
γ
-ray sky images of the Large Magellanic Cloud region by means of the Minimum Spanning Tree algorithm at energies higher than 10 GeV, using 9 years ...of
Fermi
-LAT data. Several significant clusters were found, the majority of which associated with previously known
γ
-ray sources. New significant clusters associated with the supernova remnants N 49B and N 63A are also found, and confirmed with a Maximum Likelihood analysis of the
Fermi
-LAT data.
We report the results of a new analysis of 13 wide band BeppoSAX observations of the BL Lac object Mkn 421. The data from LECS, MECS and PDS, covering an energy interval from 0.1 to over 100 keV, ...have been used to study the spectral variability of this source. We show that the energy distributions in different luminosity states can be fitted very well by a log-parabolic law $F(E)=K~(E/E_1)^{-(a+b~{\rm Log}(E/E_1))}$, which provides good estimates of the energy and flux of the synchrotron peak in the SED. In the first four short observations of 1997 Mkn 421 was characterized by a very stable spectral shape, with average values $a=2.25$ and $b=0.45$, independently of the source brightness and of the fact that the source luminosity was increasing or decreasing. In the observations of 1998 smaller values for both parameters, $a\simeq2.07$ and $b\simeq$ 0.34, were found and the peak energy in the SED was in the range 0.5–0.8 keV. The observations of May 1999 and April–May 2000 covered runs of a duration of several days and provided a very high number of events for all the instruments. The resulting spectral fits were then limited by some instrumental systematics. Also in these cases, the log-parabolic model gave a satisfactory description of the overall SED of Mkn 421. In particular, in the observations of spring 2000 the source was brighter than the other observations and showed a large change of the spectral curvature. Spectral parameters estimates gave $a\simeq1.8$ and $b\simeq$ 0.19 and the energy of the maximum in the SED moved to the range 1–5.5 keV. We give a possible interpretation of the log-parabolic spectral model in terms of particle acceleration mechanisms. An energy distribution of emitting particles with curvature close to the one observed can be explained by a simple model for statistical acceleration with the assumption that the probability for a particle to increase its energy is a decreasing function of the energy itself. A consequence of this mechanism is the existence of a linear relation between the spectral parameters a and b, well confirmed by the estimated values of these two parameters for Mkn 421.
ABSTRACT
We present the results of a cluster search in the γ-ray sky images of the Large Magellanic Cloud (LMC) region by means of the Minimum Spanning Tree (MST) and DBSCAN algorithms, at energies ...higher than 6 and 10 GeV, using 12 yr of Fermi-LAT data. Several significant clusters were found, the majority of which associated with previously known γ-ray sources. We confirm our previous detection of the supernova remnants N 49B and N 63A and found new significant clusters associated with the SNRs N 49, N 186D, and N 44. These sources are among the brightest X-ray remnants in the LMC and corresponds to core-collapse supernovae interacting with dense H ii regions, indicating that a hadronic origin of high-energy photons is the most likely process.
ABSTRACT
The complex time evolution in the X-ray light curves of the peculiar black hole binary GRS 1915+105 can be obtained as solutions of a non-linear system of ordinary differential equations ...derived from the Hindmarsh–Rose model and modified introducing an input function depending on time. In the first paper, assuming a constant input with a superposed white noise, we reproduced light curves of the classes ρ, χ, and δ. We use this mathematical model to reproduce light curves, including some interesting details, of other eight GRS 1915+105 variability classes either considering a variable input function or with small changes of the equation parameters. On the basis of this extended model and its equilibrium states, we can arrange most of the classes in three main types: (i) stable equilibrium patterns (classes ϕ, χ, α″, θ, ξ, and ω) whose light curve modulation follows the same time-scale of the input function, because changes occur around stable equilibrium points; (ii) unstable equilibrium patterns characterized by series of spikes (class ρ) originated by a limit cycle around an unstable equilibrium point; and (iii) transition pattern (classes δ, γ, λ, κ, and α′), in which random changes of the input function induce transitions from stable to unstable regions originating either slow changes or spiking, and the occurrence of dips and red noise. We present a possible physical interpretation of the model based on the similarity between an equilibrium curve and literature results obtained by numerical integrations of slim disc equations.
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