3,268 research outputs found
BeppoSAX Observations of Mkn 421: clues on the particle acceleration ?
Mkn 421 was repeatedly observed with BeppoSAX in 1997-1998. We present
highlights of the results of the thorough temporal and spectral analysis
discussed by Fossati et al. (1999) and Maraschi et al. (1999), focusing on the
flare of April 1998, which was simultaneously observed also at TeV energies.
The detailed study of the flare in different energy bands reveals a few very
important new results: (a) hard photons lag the soft ones by 2-3 ks *a behavior
opposite to what is normally found in High energy peak BL Lacs X-ray spectra*;
(b) the flux decay of the flare can be intrinsically achromatic if a stationary
underlying emission component is present. Moreover the spectral evolution
during the flare has been followed by extracting X-ray spectra on few ks
intervals, allowing to detect for the first time the peak of the synchrotron
component shifting to higher energies during the rising phase, and then
receding. The spectral analysis confirms the delay in the flare at the higher
energies, as above a few keV the spectrum changes only after the peak of the
outburst has occurred. The spectral and temporal information obtained challenge
the simplest models currently adopted for the (synchrotron) emission and most
importantly provide clues on the particle acceleration process. A theoretical
picture accounting for all the observational constraints is discussed, where
electrons are injected at low energies and then progressively accelerated
during the development of the flare.Comment: 5 pages, LaTeX, uses aipproc.sty, 3 PostScript figures, to appear in
the Proceedings of the "Fifth Compton Symposium", held in Portsmouth (NH),
1999 Septembe
Regional and international market integration of a small open economy
This paper studies the relationship between a set of commodity prices in a small open economy like Uruguay and the corresponding international and regional prices. The empirical methodology used is the multivariate cointegration procedure based on maximum likelihood methods introduced by Johansen (1988) as well as estimations of half-life persistence indicators. In the case of cereals, the evidence suggests strong market integration between domestic and regional markets and, to some extent, also to international markets. Therefore, directly or indirectly, domestic prices are connected with the efficient price signal. Results for beef indicate strong market integration between the domestic market and the regional market, which is not so well connected with international markets. Thus, domestic price appears to be linked to a regional price that is not linked to the efficient price signal.commodity prices, law of one price, market integration, cointegration
Six years of BeppoSAX observations of blazars: a spectral catalog
We present a spectral catalog for blazars based on the BeppoSAX archive. The
sample includes 44 High-energy peaked BL Lacs (HBLs), 14 Low-energy peaked BL
Lacs (LBLs), and 28 Flat Spectrum Radio Quasars (FSRQs). A total of 168 LECS,
MECS, and PDS spectra were analyzed, corresponding to observations taken in the
period 1996--2002. The 0.1--50 keV continuum of LBLs and FSRQs is generally
fitted by a single power law with Galactic column density. A minority of the
observations of LBLs (25%) and FSRQs (15%) is best fitted by more complex
models like the broken power law or the continuously curved parabola. These
latter models provide also the best description for half of the HBL spectra.
Complex models are more frequently required for sources with fluxes F_{2-10
keV} > 10^-11 cm-2 s-1, corresponding to spectra with higher signal-to-noise
ratio. As a result, considering sources with flux above this threshold, the
percentage of spectra requiring those models increases for all the classes. We
note that there is a net separation of X-ray spectral properties between HBLs
on one side, and LBLs and FSRQs on the other, the distinction between LBLs and
FSRQs is more blurry. This is most likely related to ambiguities in the optical
classification of the two classes.Comment: 6 pages, 4 figures, 7 tables. Accepted for publication in A&
A probable pre-main sequence chemically peculiar star in the open cluster Stock 16
We used the Ultraviolet and Visual Echelle Spectrograph of the ESO-Very Large
Telescope to obtain a high resolution and high signal-to-noise ratio spectrum
of Stock 16-12, an early-type star which previous Delta-a photometric
observations suggest being a chemically peculiar (CP) star. We used spectral
synthesis to perform a detailed abundance analysis obtaining an effective
temperature of 8400 +/- 400 K, a surface gravity of 4.1 +/- 0.4, a
microturbulence velocity of 3.4 +0.7/-0.3 km/s, and a projected rotational
velocity of 68 +/- 4 km/s. We provide photometric and spectroscopic evidence
showing the star is most likely a member of the young Stock 16 open cluster
(age 3-8 Myr). The probable cluster membership, the star's position in the
Hertzsprung-Russell diagram, and the found infrared excess strongly suggest the
star is still in the pre-main-sequence (PMS) phase. We used PMS evolutionary
tracks to determine the stellar mass, which ranges between 1.95 and 2.3 Msun,
depending upon the adopted spectroscopic or photometric data results.
Similarly, we obtained a stellar age ranging between 4 and 6 Myr, in agreement
with that of the cluster. Because the star's chemical abundance pattern
resembles well that known of main sequence CP metallic line (Am) stars, the
object sets important constraints to the diffusion theory. Additional
spectroscopic and spectropolarimetric data allowed us to conclude that the
object is probably a single non-magnetic star.Comment: Accepted for publication in MNRAS; 8 pages, 5 figures, 1 tabl
Signs of strong Na and K absorption in the transmission spectrum of WASP-103b
Context: Transmission spectroscopy has become a prominent tool for
characterizing the atmospheric properties on close-in transiting planets.
Recent observations have revealed a remarkable diversity in exoplanet spectra,
which show absorption signatures of Na, K and , in some cases
partially or fully attenuated by atmospheric aerosols. Aerosols (clouds and
hazes) themselves have been detected in the transmission spectra of several
planets thanks to wavelength-dependent slopes caused by the particles'
scattering properties. Aims: We present an optical 550 - 960 nm transmission
spectrum of the extremely irradiated hot Jupiter WASP-103b, one of the hottest
(2500 K) and most massive (1.5 ) planets yet to be studied with this
technique. WASP-103b orbits its star at a separation of less than 1.2 times the
Roche limit and is predicted to be strongly tidally distorted. Methods: We have
used Gemini/GMOS to obtain multi-object spectroscopy hroughout three transits
of WASP-103b. We used relative spectrophotometry and bin sizes between 20 and 2
nm to infer the planet's transmission spectrum. Results: We find that WASP-103b
shows increased absorption in the cores of the alkali (Na, K) line features. We
do not confirm the presence of any strong scattering slope as previously
suggested, pointing towards a clear atmosphere for the highly irradiated,
massive exoplanet WASP-103b. We constrain the upper boundary of any potential
cloud deck to reside at pressure levels above 0.01 bar. This finding is in line
with previous studies on cloud occurrence on exoplanets which find that clouds
dominate the transmission spectra of cool, low surface gravity planets while
hot, high surface gravity planets are either cloud-free, or possess clouds
located below the altitudes probed by transmission spectra.Comment: Accepted for publication in A&
Ram Pressure Stripping in High-Density Environments
Galaxies living in rich environments are suffering different perturbations
able to drastically affect their evolution. Among these, ram pressure
stripping, i.e. the pressure exerted by the hot and dense intracluster medium
(ICM) on galaxies moving at high velocity within the cluster gravitational
potential well, is a key process able to remove their interstellar medium (ISM)
and quench their activity of star formation. This review is aimed at describing
this physical mechanism in different environments, from rich clusters of
galaxies to loose and compact groups. We summarise the effects of this
perturbing process on the baryonic components of galaxies, from the different
gas phases (cold atomic and molecular, ionised, hot) to magnetic fields and
cosmic rays, and describe their induced effects on the different stellar
populations, with a particular attention to its role in the quenching episode
generally observed in high density environments. We also discuss on the
possible fate of the stripped material once removed from the perturbed galaxies
and mixed with the ICM, and we try to estimate its contribution to the
pollution of the surrounding environment. Finally, combining the results of
local and high redshift observations with the prediction of tuned models and
simulations, we try to quantify the importance of this process on the evolution
of galaxies of different mass, from dwarfs to giants, in various environments
and at different epochs.Comment: Invited review for The Astronomy and Astrophysics Revie
The synchrotron peak shift during high-energy flares of blazars
A prediction for the energy shift of the synchrotron spectrum of
flat-spectrum radio quasars (FSRQs) during high-energy flares is presented. If
the -ray emission of FSRQs is produced by Comptonization of external
radiation, then the peak of the synchrotron spectrum is predicted to move to
lower energies in the flare state. This is opposite to the well-known broadband
spectral behavior of high-frequency peaked BL-Lac objects where the external
radiation field is believed to be weak and synchrotron-self Compton scattering
might be the dominant -ray radiation mechanism. The synchrotron peak
shift, if observed in FSRQs, can thus be used as a diagnostic to determine the
dominant radiation mechanism in these objects. I suggest a few FSRQs as
promising candidates to test the prediction of the external-Comptonization
model.Comment: 9 pages, including 2 figures; uses epsf.sty, rotate.sty; accepted for
ApJ Letters; minor revision
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