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Maselli A.,Istituto di Astrofisica Spaziale e Fisica Cosmica | Massaro E.,University of Rome La Sapienza | Nesci R.,University of Rome La Sapienza | Sclavi S.,Stazione Astronomica di Vallinfreda | And 2 more authors.
Astronomy and Astrophysics | Year: 2010

Context. BL Lacertae objects with an extremely low value of the peak frequency (vs = 1013 Hz) of the synchrotron component (VLBL objects) are poorly known. Aims.We selected a small sample of these candidate objects on the basis of their radio/optical flux ratio to evaluate their variability, define the low energy part of their SED and obtain a reliable estimate of their.s through a fit with a log-parabolic law. Methods.They were the object of a ground-based observational campaign in the infrared and optical bands; data from several pointings by the Swift satellite in the UV and X-ray band were also obtained for half the sample. We complemented our data with a large number of literature data. Results. These sources showed a marked flux variability, as expected for BL Lacertae objects of the low energy peaked BL Lac objects (LBL) and intermediate BL Lac objects type; all of them have a.s value around 1013 Hz, but this is low enough only for a few sources to consider them genuine VLBL objects. We report the overall properties of individual sources and discuss some aspects of this class of rare objects, which could correspond to the transition between flat spectrum radio quasars and LBLs. © 2010 ESO. Source

Massaro E.,University of Rome La Sapienza | Ventura G.,Stazione Astronomica di Vallinfreda | Massa F.,Stazione Astronomica di Vallinfreda | Massa F.,National Institute of Nuclear Physics, Italy | And 5 more authors.
Astronomy and Astrophysics | Year: 2010

GRS 1915+105 was observed by BeppoSAX for about 10 days in October 2000. For about 80% of the time, the source was in the variability class ρ, characterised by a series of recurrent bursts. We describe the results of the timing analysis performed on the MECS (1.6-10 keV) and PDS (15-100 keV) data. The X-ray count rate from GRS 1915+105 showed an increasing trend with different characteristics in the various energy bands: in the bands (1.6-3 keV) and (15-100 keV), it was nearly stable in the first part of the pointing and increased in a rather short time by about 20%, while in the energy range (3-10 keV) the increase had a smoother trend. Fourier and wavelet analyses detect a variation in the recurrence time of the bursts, from 45-50 s to about 75 s, which appear well correlated with the count rate. From the power distribution of peaks in Fourier periodograms and wavelet spectra, we distinguished between the regular and irregular variability modes of the ρ class, which are related to variations in the count rate in the 3-10 keV range. We identified two components in the burst structure: the slow leading trail, and the pulse, superimposed on a rather stable level. Pulses are generally structured in a series of peaks and their number is related to the regularity modes: the mean number of peaks is lower than 2 in the regular mode and increases up to values higher than 3 in the irregular mode. We found that the change in the recurrence time of the regular mode is caused by the slow leading trails, while the duration of the pulse phase remains far more stable. The evolution in the mean count rates shows that the time behaviour of both the leading trail and the baseline level are very similar to those observed in the 1.6-3 and 15-100 keV ranges, while that of the pulse follows the peak number. These differences in the time behaviour and count rates at different energies indicate that the process responsible for the pulses must produce the strongest emission between 3 and 10 keV, while that associated with both the leading trail and the baseline dominates at lower and higher energies. © 2010 ESO. Source

Mineo T.,Istituto di Astrofisica Spaziale e Fisica Cosmica | Massaro E.,University of Rome La Sapienza | D'Ai A.,University of Palermo | Massa F.,Stazione Astronomica di Vallinfreda | And 6 more authors.
Astronomy and Astrophysics | Year: 2011

Context. BeppoSAX observed GRS 1915+105 on October 2000 with a long pointing lasting about ten days. During this observation, the source was mainly in the ρ class characterized by bursts with a recurrence time of between 40 and 100 s. Aims. We identify five segments in the burst structure and accumulate the average spectra of these segments during each satellite orbit. We present a detailed spectral analysis aimed at determining variations that occur during the burst and understanding the physical process that produces them. Methods. We compare MECS, HPGSPC, and PDS spectra with several models. Under the assumption that a single model is able to fit all spectra, we find that the combination of a multi-temperature black-body disk and a hybrid corona is able to give a consistent physical explanation of the source behaviour. Results. Our measured variations in KT el, τ, KT in, and R in appear to be either correlated or anti-correlated with the count rate in the energy range 1.6-10 keV. The strongest variations are detected along the burst segments: almost all parameters exhibit significant variations in the segments that have the highest fluxes (pulse) with the exception of R in, which varies continuously and reaches a maximum just before the peak. The flux of the multi-temperature disk strongly increases in the pulse and simultaneously the corona contribution is significantly reduced. Conclusions. The disk luminosity increases in the pulse and the R in-T in correlation can be most successfully interpreted in term of the slim disk model. In addition, the reduction in the corona luminosity during the bursts might represent the condensation of the corona onto the disk. © 2012 ESO. Source

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