10,654 research outputs found
Room temperature ferromagnetic-like behavior in Mn-implanted and post-annealed InAs layers deposited by Molecular Beam Epitaxy
We report on the magnetic and structural properties of Ar and Mn implanted
InAs epitaxial films grown on GaAs (100) by Molecular Beam Epitaxy (MBE) and
the effect of Rapid Thermal Annealing (RTA) for 30 seconds at 750C. Channeling
Particle Induced X- ray Emission (PIXE) experiments reveal that after Mn
implantation almost all Mn atoms are subsbtitutional in the In-site of the InAs
lattice, like in a diluted magnetic semiconductor (DMS). All of these samples
show diamagnetic behavior. But, after RTA treatment the Mn-InAs films exhibit
room-temperature magnetism. According to PIXE measurements the Mn atoms are no
longer substitutional. When the same set of experiments were performed with As
as implantation ion all of the layers present diamagnetism without exception.
This indicates that the appearance of room-temperature ferromagnetic-like
behavior in the Mn-InAs-RTA layer is not related to lattice disorder produce
during implantation, but to a Mn reaction produced after a short thermal
treatment. X-ray diffraction patterns (XRD) and Rutherford Back Scattering
(RBS) measurements evidence the segregation of an oxygen deficient-MnO2 phase
(nominally MnO1.94) in the Mn-InAs-RTA epitaxial layers which might be on the
origin of room temperature ferromagnetic-like response observed.Comment: 16 pages, 5 figures. Acepted in J. Appl. Phy
The Chemical Composition of Cernis 52 (BD+31 640)
We present an abundance analysis of the star Cernis 52 in whose spectrum we
recently reported the napthalene cation in absorption at 6707.4 {\AA}. This
star is on a line of sight to the Perseus molecular complex. The analysis of
high-resolution spectra using a chi^2-minimization procedure and a grid of
synthetic spectra provides the stellar parameters and the abundances of O, Mg,
Si, S, Ca, and Fe. The stellar parameters of this star are found to be T_{eff}
= 8350 +- 200 K, logg= 4.2 +- 0.4 dex. We derived a metallicity of [Fe/H] =
-0.01 +- 0.15. These stellar parameters are consistent with a star of
\Msun in a pre-main-sequence evolutionary stage. The stellar spectrum is
significantly veiled in the spectral range 5150-6730 {\AA} up to almost 55 per
cent of the total flux at 5150 {\AA} and decreasing towards longer wavelengths.
Using Johnson-Cousins and 2MASS photometric data, we determine a distance to
Cernis 52 of 231 pc considering the error bars of the stellar
parameters. This determination places the star at a similar distance to the
young cluster IC 348. This together with its radial velocity, v_r=13.7+-1 km/s,
its proper motion and probable young age support Cernis 52 as a likely member
of IC 348. We determine a rotational velocity of v\sin i=65 +- 5 km/s for this
star. We confirm that the stellar resonance line of \ion{Li}{1} at 6707.8 {\AA}
is unable to fit the broad feature at 6707.4 {\AA}. This feature should have a
interstellar origin and could possibly form in the dark cloud L1470 surrounding
all the cluster IC 348 at about the same distance.Comment: Accepted for publication in The Astrophysical Journa
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Composite polymer membranes for laserinduced fluorescence thermometry
We demonstrate a modified version of laser-induced fluorescence thermometry (LIFT) for mapping temperature gradients in the vicinity of small photothermal devices. Our approach is based on temperature sensitive fluorescent membranes fabricated with rhodamine B and polydimethylsiloxane (PDMS). Relevant membrane features for LIFT, such as temperature sensitivity, thermal quenching and photobleaching are presented for a range of 25 °C to 90 °C, and their performance is evaluated upon obtaining the temperature gradients produced in the proximity of optical fiber micro-heaters. Our results show that temperature measurements in regions as small as 750 μm x 650 μm, with a temperature resolution of 1 °C, can be readily obtained
X-ray spectral variability of seven LINER nuclei with XMM-Newton and Chandra data
One of the most important features in active galactic nuclei (AGN) is the
variability of their emission. Variability has been discovered at X-ray, UV,
and radio frequencies on time scales from hours to years. Among the AGN family
and according to theoretical studies, Low-Ionization Nuclear Emission Line
Region (LINER) nuclei would be variable objects on long time scales. Our
purpose is to investigate spectral X-ray variability in LINERs and to
understand the nature of these kinds of objects, as well as their accretion
mechanism. Chandra and XMM-Newton public archives were used to compile X-ray
spectra of seven LINER nuclei at different epochs with time scales of years. To
search for variability we fit all the spectra from the same object with a set
of models, in order to identify the parameters responsible for the variability
pattern. We also analyzed the light curves in order to search for short time
scale (from hours to days) variability. Whenever possible, UV variability was
also studied. We found spectral variability in four objects, with variations
mostly related to hard energies (2-10 keV). These variations are due to changes
in the soft excess, and/or changes in the absorber, and/or intrinsic variations
of the source. Another two galaxies seem not to vary. Short time scale
variations during individual observations were not found. Our analysis confirms
the previously reported anticorrelation between the X-ray spectral index and
the Eddington ratio, and also the correlation between the X-ray to UV flux
ratio and the Eddington ratio. These results support an Advection Dominated
Accretion Flow (ADAF) as the accretion mechanism in LINERs.Comment: 35 pages, 53 figures, recently accepted pape
X-ray spectral variability of Seyfert 2 galaxies
Variability across the electromagnetic spectrum is a property of AGN that can
help constraining the physical properties of these galaxies. This is the third
of a serie of papers with the aim of studying the X-ray variability of
different families of AGN. The main purpose of this work is to investigate the
variability pattern in a sample of optically selected type 2 Seyfert galaxies.
We use the 26 Seyferts in the Veron-Cetty and Veron catalogue with data
available from Chandra and/or XMM-Newton public archives at different epochs,
with timescales ranging from a few hours to years. All the spectra of the same
source are simultaneously fitted and we let different parameters to vary in the
model. Whenever possible, short-term variations and/or long-term UV flux
variations are studied. We divide the sample in Compton-thick, Compton-thin,
and changing-look candidates. Short-term variability at X-rays is not found.
From the 25 analyzed sources, 11 show long-term variations; eight (out of 11)
are Compton-thin, one (out of 12) is Compton-thick, and the two changing-look
candidates are also variable. The main driver for the X-ray changes is related
to the nuclear power (nine cases), while variations at soft energies or related
with absorbers at hard X-rays are less common, and in many cases these
variations are accompained with variations of the nuclear continuum. At UV
frequencies nuclear variations are nor found. We report for the first time two
changing-look candidates, MARK273 and NGC7319. A constant reflection component
located far away from the nucleus plus a variable nuclear continuum are able to
explain most of our results; the Compton-thick candidates are dominated by
reflection, which supresses their continuum making them seem fainter, and not
showing variations, while the Compton-thin and changing-look candidates show
variations.Comment: Accepted for publication in A&
X-ray spectral variability of LINERs selected from the Palomar sample
Variability is a general property of active galactic nuclei (AGN). At X-rays,
the way in which these changes occur is not yet clear. In the particular case
of low ionisation nuclear emission line region (LINER) nuclei, variations on
months/years timescales have been found for some objects, but the main driver
of these changes is still an open question. The main purpose of this work is to
investigate the X-ray variability in LINERs, including the main driver of such
variations, and to search for eventual differences between type 1 and 2
objects. We use the 18 LINERs in the Palomar sample with data retrieved from
Chandra and/or XMM-Newton archives corresponding to observations gathered at
different epochs. All the spectra for the same object are simultaneously fitted
in order to study long term variations. The nature of the variability patterns
are studied allowing different parameters to vary during the spectral fit.
Whenever possible, short term variations from the analysis of the light curves
and UV variability are studied.Comment: 49 pages, accepted. arXiv admin note: text overlap with
arXiv:1305.222
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