7,759 research outputs found
The RR Lyrae Period - K Luminosity relation for Globular Clusters: an observational approach
The Period - metallicity - K band luminosity (PLK) relation for RR Lyrae
stars in 15 Galactic globular clusters and in the LMC globular cluster
Reticulum has been derived. It is based on accurate near infrared (K)
photometry combined with 2MASS and other literature data. The PLK relation has
been calibrated and compared with the previous empirical and theoretical
determinations in literature. The zero point of the absolute calibration has
been obtained from the K magnitude of RR Lyr whose distance modulus has been
measured via trigonometric parallax with HST. Using this relation we obtain a
distance modulus to the LMC of (m-M)_0 = 18.54 \pm 0.15 mag, in good agreement
with recent determinations based on the analysis of Cepheid variable stars.Comment: 6 pages, 4 figures, accepted for publication by MNRA
Gamma-ray emission from massive stars interacting with AGN jets
Dense populations of stars surround the nuclear regions of galaxies. In
active galactic nuclei, these stars can interact with the relativistic jets
launched by the supermasive black hole. In this work, we study the interaction
of early-type stars with relativistic jets in active galactic nuclei. A
bow-shaped double-shock structure is formed as a consequence of the interaction
of the jet and the stellar wind of each early-type star. Particles can be
accelerated up to relativistic energies in these shocks and emit high-energy
radiation. We compute, considering different stellar densities of the galactic
core, the gamma-ray emission produced by non-thermal radiative processes. This
radiation may be significant in some cases, and its detection might yield
valuable information on the properties of the stellar population in the galaxy
nucleus, as well as on the relativistic jet. This emission is expected to be
particularly relevant for nearby non-blazar sources.Comment: Accepted for publication on MNRAS (15 pages, 9 figures
Gamma rays from cloud penetration at the base of AGN jets
Dense and cold clouds seem to populate the broad line region surrounding the
central black hole in AGNs. These clouds could interact with the AGN jet base
and this could have observational consequences. We want to study the gamma-ray
emission produced by these jet-cloud interactions, and explore under which
conditions this radiation would be detectable. We investigate the
hydrodynamical properties of jet-cloud interactions and the resulting shocks,
and develop a model to compute the spectral energy distribution of the emission
generated by the particles accelerated in these shocks. We discuss our model in
the context of radio-loud AGNs, with applications to two representative cases,
the low-luminous Centaurus A, and the powerful 3C 273. Some fraction of the jet
power can be channelled to gamma-rays, which would be likely dominated by
synchrotron self-Compton radiation, and show typical variability timescales
similar to the cloud lifetime within the jet, which is longer than several
hours. Many clouds can interact with the jet simultaneously leading to fluxes
significantly higher than in one interaction, but then variability will be
smoothed out. Jet-cloud interactions may produce detectable gamma-rays in
non-blazar AGNs, of transient nature in nearby low-luminous sources like Cen A,
and steady in the case of powerful objects of FR II type.Comment: Accepted for publication in A&A (9 pages, 7 figures
Transient gamma-ray emission from Cygnus X-3
The high-mass microquasar Cygnus X-3 has been recently detected in a flaring
state by the gamma-ray satellites Fermi and Agile. In the present contribution,
we study the high-energy emission from Cygnus X-3 through a model based on the
interaction of clumps from the Wolf-Rayet wind with the jet. The clumps inside
the jet act as obstacles in which shocks are formed leading to particle
acceleration and non-thermal emission. We model the high energy emission
produced by the interaction of one clump with the jet and briefly discus the
possibility of many clumps interacting with the jet. From the characteristics
of the considered scenario, the produced emission could be flare-like due to
discontinuous clump penetration, with the GeV long-term activity explained by
changes in the wind properties.Comment: Contribution to the proceedings of the 25th Texas Symposium on
Relativistic Astrophysics - TEXAS 2010, December 06-10, Heidelberg, German
High-energy flares from jet-clump interactions
High-mass microquasars are binary systems composed by a massive star and a
compact object from which relativistic jets are launched. Regarding the
companion star, observational evidence supports the idea that winds of hot
stars are formed by clumps. Then, these inhomogeneities may interact with the
jets producing a flaring activity. In the present contribution we study the
interaction between a jet and a clump of the stellar wind in a high-mass
microquasar. This interaction produces a shock in the jet, where particles may
be accelerated up to relativistic energies. We calculate the spectral energy
distributions of the dominant non-thermal processes: synchrotron radiation,
inverse Compton scattering, and proton-proton collisions. Significant levels of
X- and gamma-ray emission are predicted, with luminosities in the different
domains up to ~ 10^{34} - 10^{35} erg/s on a timescale of about ~ 1 h. Finally,
jet-clump interactions in high-mass microquasars could be detectable at high
energies. These phenomena may be behind the fast TeV variability found in some
high-mass X-ray binary systems, such as Cygnus X-1, LS 5039 and LS I+61 303. In
addition, our model can help to derive information on the properties of jets
and clumpy winds.Comment: Proceeding of the conference "High Energy Phenomena in Massive
Stars". Jaen (Spain), 2-5 February 200
High-resolution CRIRES spectra of Terzan1: a metal-poor globular cluster toward the inner bulge
Containing the oldest stars in the Galaxy, globular clusters toward the bulge
can be used to trace its dynamical and chemical evolution. In the bulge
direction, there are ~50 clusters, but only about 20% have been subject of
high-resolution spectroscopic investigations. So far, the sample observed at
high resolution spans a moderate-to-high metallicity regime. In this sample,
however, very few are located in the innermost region (1.5\,Kpc and
). To constrain the chemical evolution enrichment of the
innermost region of Galaxy, accurate abundances and abundance patterns of key
elements based on high-resolution spectroscopy are necessary. Here we present
the results we obtained for Terzan 1, a metal-poor cluster located in the
innermost bulge region. Using the near-infrared spectrograph CRIRES at ESO/VLT,
we obtained high-resolution (R50,000) H-band spectra of 16 bright
giant stars in the innermost region () of Terzan1. Full spectral
synthesis techniques and equivalent width measurements of selected lines,
isolated and free of significant blending and/or contamination by telluric
lines, allowed accurate chemical abundances and radial velocities to be
derived. Fifteen out of 16 observed stars are likely cluster members, with an
average heliocentric radial velocity of +571.8\,km/s and mean iron
abundance of [Fe/H]=--1.260.03\,dex. For these stars we measured some
[/Fe] abundance ratios, finding average values of
[O/Fe]=+0.390.02\,dex, [Mg/Fe]=+0.420.02\,dex,
[Si/Fe]=+0.310.04\,dex, and [Ti/Fe]=+0.150.04\,dex The
enhancement (\,dex) found in the observed giant stars of Terzan1
is consistent with previous measurements on other, more metal-rich bulge
clusters, which suggests a rapid chemical enrichment.Comment: 7, pages, 6 figures, accepted for publication on A&
SN/GRB connection: a statistical approach with BATSE and Asiago Catalogues
Recent observations suggest that some types of GRB are physically connected
with SNe of type Ib/c. However, it has been pointed out by several authors that
some GRBs could be associated also with other types of core-collapse SNe (type
IIdw/IIn). On the basis of a comphrensive statistical study, which has made use
of the BATSE and Asiago catalogues, we have found that: i) the temporal and
spacial distribution of SNe-Ib/c is marginally correlated with that of the
BATSE GRBs; ii) we do not confirm the existence of an association between GRBs
and SNe-IIdw/IIn.Comment: Proceeding of the 4th workshop on Gamma Ray Bursts in the Afterglow
Era, Rome, 2004; 4 page
- …