2,199 research outputs found
On the consistency of magnetic field measurements of Ap stars: lessons learned from the FORS1 archive
CONTEXT. The ESO archive of FORS1 spectropolarimetric observations may be
used to create a homogeneous database of magnetic field measurements. However,
no systematic comparison of FORS field measurements to those obtained with
other instruments has been undertaken so far. AIMS. We exploit the FORS archive
of circular spectropolarimetric data to examine in a general way how reliable
and accurate field detections obtained with FORS are. METHODS. We examine the
observations of Ap and Bp stars, on the grounds that almost all of the
unambiguous detections of magnetic fields in the FORS1 archive are in these
kinds of stars. We assess the overall quality of the FORS1 magnetic data by
examining the consistency of field detections with what is known from previous
measurements obtained with other instruments, and we look at patterns of
internal consistency. RESULTS. FORS1 magnetic measurements are fully consistent
with those made with other instruments, and the internal consistency of the
data is excellent. However, it is important to recognise that each choice of
grism and wavelength window constitutes a distinct instrumental measuring
system, and that simultaneous field measurements in different instrumental
systems may produce field strength values that differ up to 20 %, or more.
Furthermore, we found that field measurements using hydrogen lines only yield
results that meaningfully reflect the field strength as sampled specifically by
lines of hydrogen for stars with effective temperatures above about 9000 K.
CONCLUSIONS. In general the magnetic field measurements of Ap and Bp stars
obtained with FORS1 are of excellent quality, accuracy and precision, and FORS1
provides an extremely useful example that offers valuable lessons for field
measurements with other low- resolution Cassegrain spectropolarimeters.Comment: 14 pages, 8 figures. Accepted for publication in section 13 of
Astronomy & Astrophysics on 13 October 201
The FORS1 catalogue of stellar magnetic field measurements
The FORS1 instrument on the ESO Very Large Telescope was used to obtain
low-resolution circular polarised spectra of nearly a thousand different stars,
with the aim of measuring their mean longitudinal magnetic fields. A catalogue
of FORS1 magnetic measurements would provide a valuable resource with which to
better understand the strengths and limitations of this instrument and of
similar low-dispersion, Cassegrain spectropolarimeters. However, FORS1 data
reduction has been carried out by a number of different groups using a variety
of reduction and analysis techniques. Our understanding of the instrument and
our data reduction techniques have both improved over time. A full re-analysis
of FORS1 archive data using a consistent and fully documented algorithm would
optimise the accuracy and usefulness of a catalogue of field measurements.
Based on the ESO FORS pipeline, we have developed a semi-automatic procedure
for magnetic field determinations, which includes self-consistent checks for
field detection reliability. We have applied our procedure to the full content
of circular spectropolarimetric measurements of the FORS1 archive. We have
produced a catalogue of spectro-polarimetric observations and magnetic field
measurements for about 1400 observations of about 850 different objects. The
spectral type of each object has been accurately classified. We have also been
able to test different methods for data reduction is a systematic way. The
resulting catalogue has been used to produce an estimator for an upper limit to
the uncertainty in a field strength measurement of an early type star as a
function of the signal-to-noise ratio of the observation. While FORS1 is not
necessarily an optimal instrument for the discovery of weak magnetic fields, it
is very useful for the systematic study of larger fields, such as those found
in Ap/Bp stars and in white dwarfs.Comment: Accepted for publication by A&
A new method of measuring center-of-mass velocities of radially pulsating stars from high-resolution spectroscopy
We present a radial velocity analysis of 20 solar neighborhood RR Lyrae and 3
Population II Cepheids variables. We obtained high-resolution, moderate-to-high
signal-to-noise ratio spectra for most stars and obtained spectra were covering
different pulsation phases for each star. To estimate the gamma
(center-of-mass) velocities of the program stars, we use two independent
methods. The first, `classic' method is based on RR Lyrae radial velocity curve
templates. The second method is based on the analysis of absorption line
profile asymmetry to determine both the pulsational and the gamma velocities.
This second method is based on the Least Squares Deconvolution (LSD) technique
applied to analyze the line asymmetry that occurs in the spectra. We obtain
measurements of the pulsation component of the radial velocity with an accuracy
of 3.5 km s. The gamma velocity was determined with an accuracy
10 km s, even for those stars having a small number of spectra.
The main advantage of this method is the possibility to get the estimation of
gamma velocity even from one spectroscopic observation with uncertain pulsation
phase. A detailed investigation of the LSD profile asymmetry shows that the
projection factor varies as a function of the pulsation phase -- this is a
key parameter which converts observed spectral line radial velocity variations
into photospheric pulsation velocities. As a byproduct of our study, we present
41 densely-spaced synthetic grids of LSD profile bisectors that are based on
atmospheric models of RR Lyr covering all pulsation phases.Comment: 17 pages, 16 figures, accepted for publication in MNRAS;
doi:10.1093/mnras/stx294
VHE Observations of BL Lacertae Objects: 1995-2000
The results of observations of 29 BL Lacertae objects taken with the Whipple
Observatory 10 m gamma-ray Telescope between 1995 and 2000 are presented.Comment: 4 pages to be published in the Proceedings of the 28th International
Cosmic Ray Conference (Tsukuba, Japan 2003
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&
The SED of the TeV BLLac 1ES 1426+428 after correction for the TeV--IR absorption
The recent HEGRA detection and spectrum of 1ES 1426+428 at TeV energies, once
corrected for absorption using present estimates of the diffuse extragalactic
IR background, suggest that the high energy peak of the Spectral Energy
Distribution (SED) could be much higher than the synchrotron one
(), and lie at energies above 8-10 TeV. To see if such an SED could
be accounted for, we have applied a "finite injection time" SSC model, and
present here some preliminary results. Within this model, we found the need of
an external ("ambient") contribution to the energy density of seed photons, in
order to account for both the high Compton dominance and the hard spectrum.Comment: 4 pages, 5 figures, to appear in the proceedings of the conference
"Relativistic jets in the Chandra and XMM era", Bologna, 23-27/9/02 (New
Astr. Rev.
- âŠ