arXiv subjects

R. Nesci

Publications and source records attributed to R. Nesci.

41 records · Page 3Linked to original sources

arXiv1999

The concave X-ray spectrum of the blazar ON 231: the signature of intermediate BL Lac objects

ON 231 was observed with BeppoSAX in May and June 1998, following an exceptional optical outburst which occured in April--May. We measured the X-ray spectrum from 0.1 up to 100 keV. In both occasions the spectrum had a concave shape, with a break detected at about 4 and 2.5 keV, respectively. We interpret the steeper component at energies below the break as due to synchrotron emission and the extremely flat component at energies above the break as due to inverse Compton emission. This is so far the best example in which both the synchrotron and the Inverse Compton component are detected simultaneously and with the same instruments in the X-ray spectrum of a blazar. We detect a decrease of the X-ray flux of about 30% below the break between the first and the second observation, and smaller variability amplitude between 4 and 10 keV. During the May observation we also detected a fast variability event with the flux below 4 keV increasing by about a factor of three in 5 hours. Above 4 keV no variability was detected. We discuss these results in the framework of synchrotron self-Compton models.

astro-ph↗

arXiv1999

Synchrotron and Inverse Compton Variability in the BL Lacertae object S5 0716+714

We report intensity variations of different spectral components in the BL Lac object S5 0716+714 detected during coordinated BeppoSAX and optical observations in 1996 and 1998. The transition between synchrotron and inverse Compton emission has been clearly detected as sharp X-ray spectral breaks at around 2-3 keV on both occasions. Correlated optical and soft X-ray variability was found during the second BeppoSAX pointing when intensive optical monitoring could be arranged. The hard (Compton) component changed by a factor of 2 between the two observations, but remained stable within each exposure. During events of rapid variability S5 0716+714 showed spectral steepening with intensity, a behaviour rarely observed in BL Lacs. We interpret these findings as the probable consequence of a shift of the synchrotron peak emission from the IR/optical band to higher energies, causing the synchrotron tail to push into the soft X-ray band more and more as the source brightens.

astro-ph↗

arXiv1999

The BeppoSAX view of the hot cluster Abell 2319

We present results from a BeppoSAX observation of the rich cluster Abell 2319. The broad band spectrum (2-50 keV) of the cluster can be adequately represented by an optically thin thermal emission model with a temperature of 9.6+/-0.3 keV and a metal abundance of 0.25+/-0.03 in solar units, and with no evidence of a hard X-ray excess in the PDS spectrum. From the upper limit to the hard tail component we derive a lower limit of ~0.04 \muG for the volume-averaged intracluster magnetic field. By performing spatially resolved spectroscopy in the medium energy band (2-10 keV), we find that the projected radial temperature and metal abundance profiles are constant out to a radius of 16 arcmin (1.4 Mpc). A reduction of the temperature of 1/3, when going from the cluster core out to 16 arcmin, can be excluded in the present data at the 99% confidence level. From the analysis of the temperature and abundance maps we find evidence of a temperature enhancement and of an abundance decrement in a region localized 6 arcmin--8 arcmin NE of the core, where a merger event may be taking place. Finally, the temperature map indicates that the subcluster located NW of the main cluster may be somewhat cooler than the rest of the cluster.

astro-ph↗

arXiv1997

Optical-IUE observations of the gamma-ray loud BL Lacertae object S5 0716+714: data and interpretation

We monitored the BL Lac object S5 0716+714 in the optical band during the period November 1994-April 1995, which includes the time of a gamma-ray observation by the Energetic Gamma Ray Experiment Telescope (EGRET) on February 14--28, 1995. The light curves in the R and B bands show fast fluctuations superimposed on longer timescale variations. The color index correlates with intensity during the rapid flares (the spectrum is flatter when the flux is higher), but it is rather insensitive to the long term trends. Over the 5 month observational period the light curve shows an overall brightening of about 1 mag followed by a fast decline. The EGRET pointing covers part of the very bright phase (V about 13.2) and the initial decline. An ultraviolet spectrum was also obtained with the International Ultraviolet Explorer (IUE) (1200-3000 A) during the EGRET observations. The variability of the optical emission of S5 0716+714 by itself sets important constraints on the magnetic field strength and on the physical processes responsible for it. Interpreting the whole electromagnetic spectrum with synchrotron self Compton models leads to the prediction of a bright gamma-ray state during the EGRET pointing. We discuss how the gamma-ray data could be used as a diagnostic of the proposed models.

astro-ph↗

arXiv1994

A ROSAT Hri Obsrevation of the Cooling Flow Cluster MS0839.9+2938

A ROSAT HRI observation of the cluster MS0839.9+2938 at z=0.194 is presented. It confirms the earlier suggestion, based on the detection of extended H$α$ emission, that the inner regions of this cluster are dominated by a cooling flow. The surface brightness distribution within the cooling radius shows structures which can be interpreted as evidence of a highly inhomogeneous space distribution of the cooling process. We note that the brightness at the barycentre of its distribution, which falls right on top of the central giant elliptical galaxy, is lower than the peaks in the structures around: we suggest that this situation is likely to arise as a consequence of photoelectric absorption by cold gas within the cooling flow, with an equivalent column density in the order of 5$\times$10$^{21}$ cm$^{-2}$ within $\sim$10" from the centre.

astro-ph↗