2,210 research outputs found
The Near-IR-Optical-UV Emission of BL Lacertae Objects
Near--infrared, optical and ultraviolet quasi--simultaneous observations of
11 BL Lacertae objects are reported. For all but one source the dereddened
spectral flux distribution in the Hz frequency
range can be described by a single power law f with
average spectral index = 0.88 0.42 (standard deviation) plus,
where relevant, the contribution of the host galaxy. In most cases the non
simultaneous soft X--ray fluxes obtained by the {\it Einstein Observatory} lie
on or below the extrapolation of the power law. The results are compared with
the average spectral properties of other samples of BL Lacs studied separately
in the IR--optical and in the UV bands. The implications for existing models of
the objects are shortly discussed.Comment: 23 pages, latex file, 2 figures available as postscript files
appended at the end of the latex text file, Ref. S.I.S.S.A. 31/94/
VLT Observations of the Ultraluminous X-ray Source NGC 1313 X-2
We present archive ESO VLT photometric and spectroscopic data of the
Ultraluminous X-ray source NGC 1313 X-2. The superb quality of the VLT images
reveals that two distinct objects, with R magnitudes 23.7 and 23.6, are visible
inside the Chandra error box. The two objects, separated by 0.75 arcsec, were
unresolved in our previous ESO 3.6 m+EFOSC image. We show that both are stars
in NGC 1313, the first a B0-O9 main sequence star of ~20 Msun, while the second
a G supergiant of ~10 Msun. Irrespectively of which of the two objects the
actual counterpart is, this implies that NGC 1313 X-2 is a high mass X-ray
binary with a very massive donor.Comment: 7 pages, 4 figures, accepted for publication in ApJ Letter
Neutron stars accreting the ISM: Are they fast or slow objects ?
Old neutron stars (ONSs) which have radiated away their internal and
rotational energy may still shine if accreting the interstellar medium. Rather
stringent limits from the analysis of ROSAT surveys indicate that most
optimistic predictions on ONSs observability are in excess of a factor as large
as . Here we explore two possible evolutionary scenarios that may
account for the paucity of ONSs. In the first it is assumed that the ONS
population is not too fast () and that magnetic field decay
guides the evolution. In the second, NSs move with high speed ( km
s) and preserve their magnetic field at birth. We find that according to
the former scenario most ONSs are now in the propeller phase, while in the
latter nearly all ONSs are silent, dead pulsars.Comment: 5 pages including 2 postscript figures, to appear in the proceedings
of Rome BeppoSax-RossiXTE meetin
What is the redshift of the gamma- ray BL Lac source S4 0954+65?
High signal-to-noise ratio spectroscopic observations of the BL Lac object S4
0954+65 at the alleged redshift z = 0.367 are presented. This source was
detected at gamma frequencies by MAGIC (TeV) and FERMI (GeV) telescopes during
a remarkable outburst that occurred in February 2015, making the determination
of its distance particularly relevant for our understanding of the properties
of the Extragalactic Background Light. Contrary to previous reports on the
redshift, we found that the optical spectrum is featureless at an equivalent
width limit of \sim 0.1 Ang. A critical analysis of the existing observations
indicates that the redshift is still unknown. Based on the new data we estimate
a lower limit to the redshift at z \geq 0.45.Comment: Minor comment and accepted for publication in Astronomical Journa
Stability of the replica symmetric solution for the information conveyed by by a neural network
The information that a pattern of firing in the output layer of a feedforward
network of threshold-linear neurons conveys about the network's inputs is
considered. A replica-symmetric solution is found to be stable for all but
small amounts of noise. The region of instability depends on the contribution
of the threshold and the sparseness: for distributed pattern distributions, the
unstable region extends to higher noise variances than for very sparse
distributions, for which it is almost nonexistant.Comment: 19 pages, LaTeX, 5 figures. Also available at
http://www.mrc-bbc.ox.ac.uk/~schultz/papers.html . Submitted to Phys. Rev. E
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