11,984 research outputs found
Chandra LETGS spectroscopy of the Quasar MR2251-178 and its warm absorber
We present an analysis of our Chandra Low Energy Transmission Grating
Spectrometer (LETGS) observation of the quasar MR2251-178. The warm absorber of
MR2251-178 is well described by a hydrogen column density, N_H~2x10^21 cm^-2,
and an ionization parameter log(xi)~0.6. We find in the spectrum weak evidence
for narrow absorption lines from Carbon and Nitrogen which indicate that the
ionized material is in outflow. We note changes (in time) of the absorption
structure in the band (0.6-1) keV (around the UTAs plus the OVII and OVIII
K-edges) at different periods of the observation. We measure a (0.1-2) keV flux
of 2.58x10^-11 ergs cm^-2 s^-1. This flux implies that the nuclear source of
MR2251-178 is in a relatively low state. No significant variability is seen in
the light curve. We do not find evidence for an extra cold material in the line
of sight, and set an upper limit of N_H~1.2x10^20 cm^-2. The X-ray spectrum
does not appear to show evidence for dusty material, though an upper limit in
the neutral carbon and oxygen column densities can only be set to N_CI~2x10^19
cm^-2 and N_OI~9x10^19 cm^-2, respectively.Comment: 42 pages, 12 figures, Accepted in Apj. Typo in abstract (ver2): "We
do not find evidence for an extra...
The Galactic plane at faint X-ray fluxes - II. Stacked X-ray spectra of a sample of serendipitous XMM-Newton sources
We have investigated the X-ray spectral properties of a sample of 138 X-ray
sources detected serendipitously in observations of the Galactic
plane, at an intermediate to faint flux level. We divide our sample into 5
subgroups according to the spectral hardness of the sources, and stack (i.e.
co-add) the individual source spectra within each subgroup. As expected these
stacked spectra show a softening trend from the hardest to the softest
subgroups, which is reflected in the inferred line-of-sight column density. The
spectra of the three hardest subgroups are characterized by a hard continuum
plus superimpose Fe-line emission in the 6--7 keV bandpass. The average
equivalent width (EW) of the 6.7-keV He-like Fe-K line is
170 eV, whereas the 6.4-keV Fe-K fluorescence line from neutral
iron and the 6.9-keV H-like Fe-Ly line have EWs of 89 eV
and 81 eV respectively, i.e. roughly half that of the 6.7-keV
line. The remaining subgroups exhibit soft thermal spectra. Virtually all of
the spectrally-soft X-ray sources can be associated with relatively nearby
coronally-active late-type stars, which are evident as bright near-infrared
(NIR) objects within the X-ray error circles. On a similar basis only a
minority of the spectrally-hard X-ray sources have likely NIR identifications.
The average continuum and Fe-line properties of the spectrally-hard sources are
consistent with those of magnetic cataclysmic variables but the direct
identification of large numbers of such systems in Galactic X-ray surveys,
probing intermediate to faint flux levels, remains challenging.Comment: 14 pages, 9 figures, 4 tables, accepted for publication in MNRA
Diffusion of muonium and hydrogen in diamond
Jump rates of muonium and hydrogen in diamond are calculated by quantum
transition-state theory, based on the path-integral centroid formalism. This
technique allows us to study the influence of vibrational mode quantization on
the effective free-energy barriers Delta F for impurity diffusion, which are
renormalized respect to the zero-temperature classical calculation. For the
transition from a tetrahedral (T) site to a bond-center (BC) position, Delta F
is larger for hydrogen than for muonium, and the opposite happens for the
transition from BC to T. The calculated effective barriers decrease for rising
temperature, except for the muonium transition from T to BC sites. Calculated
jump rates are in good agreement to available muon spin rotation data.Comment: 4 pages, 3 figure
Masgomas-4: Physical characterization of a double-core obscured cluster with a massive and very young stellar population
The discovery of new, obscured massive star clusters has changed our
understanding of the Milky Way star-forming activity from a passive to a very
active star-forming machine. The search for these obscured clusters is strongly
supported by the use of all-sky, near-IR surveys.
The main goal of the MASGOMAS project is to search for and study unknown,
young, and massive star clusters in the Milky Way, using near-IR data. Here we
try to determine the main physical parameters (distance, size, total mass, and
age) of Masgomas-4, a new double-core obscured cluster.
Using near-IR photometry (, , and ) we selected a total of 21
stars as OB-type star candidates. Multi-object, near-IR follow-up spectroscopy
allowed us to carry out the spectral classification of the OB-type candidates.
Of the 21 spectroscopically observed stars, ten are classified as OB-type
stars, eight as F- to early G-type dwarf stars, and three as late-type giant
stars. Spectroscopically estimated distances indicate that the OB-type stars
belong to the same cluster, located at a distance of
kpc. Our spectrophotometric data confirm a very young and massive stellar
population, with a clear concentration of pre-main-sequence massive candidates
(Herbig Ae/Be) around one of the cluster cores. The presence of a surrounding
HII cloud and the Herbig Ae/Be candidates indicate an upper age limit of 5 Myr.Comment: Accepted for publication in A&
Environmental Policies and Mergersâ Externalities
A Cournot oligopolistic setting model of trade is characterized by local and foreign firms competing in the presence of pollution quota and tax. Local firms are foreign-owned (FDI) and repatriate their profits. First, we analyze the impact on welfare given by the merger of the local firms, as a response to external firmsâ competition and pollution abatement costs. Second, when merger is welfare decreasing, we study the best response of the government in order to compensate this negative externality. Finally, we compare the pollution quota and tax in order to determine their efficiency as a policy instrument.environmental policies, mergers, emission permits
Comment on "Two Phase Transitions in the Fully frustrated XY Model"
The conclusions of a recent paper by Olsson (Phys. Rev. Lett. 75, 2758
(1995), cond-mat/9506082) about the fully frustrated XY model in two dimensions
are questioned. In particular, the evidence presented for having two separate
chiral and U(1) phase transitions are critically considered.Comment: One page one table, to Appear in Physical Review Letter
Characterization of the known T type dwarfs towards the Sigma Orionis cluster
(Abridged) A total of three T type candidates (SOri70, SOri73, and
SOriJ0538-0213) lying in the line of sight towards Sigma Orionis were
characterized by means of near-infrared photometric, astrometric, and
spectroscopic studies. H-band methane images were collected for all three
sources and an additional sample of 15 field T type dwarfs using LIRIS/WHT.
J-band spectra of resolution of ~500 were obtained for SOriJ0538-0213 with
ISAAC/VLT, and JH spectra of resolution of ~50 acquired with WFC3/HST were
employed for the spectroscopic classification of SOri70 and 73. Proper motions
with a typical uncertainty of +/-3 mas/yr and a time interval of ~7-9 yr were
derived. Using the LIRIS observations of the field T dwarfs, we calibrated this
imager for T spectral typing via methane photometry. The three SOri objects
were spectroscopically classified as T4.5+/-0.5 (SOri73), T5+/-0.5
(SOriJ0538-0213), and T7 (SOri70). The similarity between the
observed JH spectra and the methane colors and the data of field ultra-cool
dwarfs of related classifications suggests that SOri70, 73, and
SOriJ053804.65-021352.5 do not deviate significantly in surface gravity in
relation to the field. Additionally, the detection of KI at ~1.25 microns in
SOriJ0538-0213 points to a high-gravity atmosphere. Only the K-band reddish
nature of SOri70 may be consistent with a low gravity atmosphere. The proper
motions of SOri70 and 73 are measurable and are larger than that of the cluster
by >3.5 sigma. The proper motion of SOriJ0538-0213 is consistent with a null
displacement. These observations suggest that none of the three T dwarfs are
likely Sigma Orionis members, and that either planetary-mass objects with
masses below ~4 MJup may not exist free-floating in the cluster or they may lie
at fainter near-infrared magnitudes than those of the targets (this is H>20.6
mag), thus remaining unidentified to date.Comment: Accepted for publication in A&A (2014), corrected typo
Higgs mediated lepton flavor violating tau decays and in effective theories
The size of the branching ratios for the and decays induced by a lepton flavor violating Higgs
interaction is studied in the frame of effective field theories.
The best constraint on the vertex, derived from the know
measurement on the muon anomalous magnetic moment, is used to impose the upper
bounds and , which are more stringent than current experimental
limits on this class of transitions.Comment: 6 pages, 3 figure
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