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Scientific Report 2007-2009
Astronomy & Astrophysics
A8. Galactic and extragalactic sources of X and Gamma rays
Cosmic sources of high-energy electromagnetic radiation
can be classified in two main classes: objects of
stellar nature, which belong to our Galaxy, and Active
Galactic Nuclei (AGN). The former objects are rotating
neutron stars (pulsars), binary systems with a collapsed
star (a neutron star or a black hole of stellar mass) and
Supernova Remnants; the latter sources are believed to
be supermassive black holes (10 6 - 10 9 solar masses) surrounded
by an accretion disk and ejecting a jet of particles
accelerated at relativistic energies.
The study of these sources had an impressive development
after the launch of the space observatory Fermi-
GST in June 2008. In the first year of operation,
the LAT (Large Area Telescope, onboard F-GST) discovered
about 1500 galactic and extragalactic sources
whose emission extends up the GeV range. The majority
of γ-ray sources with a well established counterpart
are Blazars (BL Lac objects and Flat Spectrum Radio
Quasars): these associations are mainly based on the
positional coincidence. Our group is working from a few
years in the compilation of a ”Multifrequency Catalogue
of Blazars“ (Massaro et al. 2009), also known as Roma-
BZCAT , which is a master list of sources of this class
based on an accurate study of literature and new data.
The last version of the Roma-BZCAT contains more
than 2800 objects and can be accessed at the web site of
the ASI Scientific Data Center. It is currently used by
the FGST collaboration and allowed the identification
of many new discovered γ-ray sources, particularly BL
Lac objects. The catalogue will be also printed in four
volumes: two of them, covering half of the sky, are already
issued and the last two will appear before the end
of 2010.
Figure 1: Sky distribution of the blazars in the
RomaBZCAT (from E. Massaro et al. 2009).
We have also developed a numerical code based on
the Minimal Spanning Tree (MST), a topometric algorithm
for cluster analysis, for searching γ-ray sources
in LAT sky images at energies above a few GeV. We defined
and tested the criteria for the selection of candidate
sources (Campana et al. 2008) and contributed to the
preparation of the first catalogue of LAT γ-ray sources
(1FGL, Abdo et al. 2010, submitted to ApJ). Presently,
we are currently involved in the work of preparation of
the two year LAT catalogue, which will be available before
the end of 2010. We also contributed to the study
of the high energy emission and to the analysis of multifrequency
data of some bright blazars (3C 454.3, 3C
273, PKS 1502+106, PKS 1510-089) and these results
are appearing in a number of papers.
We studied the X and γ-ray emission of some galactic
sources, in particular isolated pulsars and their Pulse
Wind Nebulae. We proposed a model for describing the
phase and spectral evolution of the Crab pulsar based
on the presence of two couples of emission components.
This model gave a successful prediction (Campana et
al. 2009) of the very high emission (> 25 GeV) of Crab
discovered by the MAGIC team.
Figure 2: Wavelet spectra of three data series of the X-ray
emission from GRS 1915+105 (from E. Massaro et al. 2010).
Another puzzling galactic source, that has been
the subject of long and detailed researches, is the
microquasar GRS 1915+105. It exhibits a very intense
X-ray emission, characterised by a very complex variability.
We are currently working on the analysis and
interpretation of a large data set of X-ray observations,
mainly performed by the BeppoSAX and Rossi-XTE
satellites. The high flux of GRS 1915+105 allow us to
investigate the instabilities of the accretion disk, which
produce long series of recurring bursts. The time and
spectral evolution of these bursts can be investigated by
means of several linear and non-linear methods useful
to describe the transitions from ragular to irregular
modes, the latter ones characterised by rapidly chanche
of the burst recurrence and shape. This source can be
also useful to investigate the onset of possible chaotic
processes in accretion disk systems.
References
1. E. Massaro et al., Astron. Astrophys. 495, 691 (2009).
2. R. Campana et al., MNRAS 383, 1166 (2008).
3. R. Campana et al., Astron. Astrophys. 499, 847 (2009).
Authors
E. Massaro, R. Campana, A. Maselli
Sapienza Università di Roma 155 Dipartimento di Fisica