377 research outputs found
4U 1626-67 as seen by Suzaku before and after the 2008 torque reversal
Aims. The accretion-powered pulsar 4U 1626-67 experienced a new torque
reversal at the beginning of 2008, after about 18 years of steadily spinning
down. The main goal of the present work is to study this recent torque reversal
that occurred in 2008 February.
Methods. We present a spectral analysis of this source using two pointed
observations performed by Suzaku in 2006 March and in 2010 September.
Results. We confirm with Suzaku the presence of a strong emission-line
complex centered on 1 keV, with the strongest line being the hydrogen-like Ne
Ly-alpha at 1.025(3) keV. We were able to resolve this complex with up to seven
emission lines. A dramatic increase of the intensity of the Ne Ly-alpha line
after the 2008 torque reversal occurred, with the equivalent width of this line
reaching almost the same value measured by ASCA in 1993. We also report on the
detection of a cyclotron line feature centered at ~37 keV. In spite of the fact
that an increase of the X-ray luminosity (0.5-100 keV) of a factor of ~2.8
occurred between these two observations, no significant change in the energy of
the cyclotron line feature was observed. However, the intensity of the ~1 keV
line complex increased by an overall factor of ~8.
Conclusions. Our results favor a scenario in which the neutron star in 4U
1626-67 accretes material from a geometrically thin disk during both the
spin-up and spin-down phases.Comment: 7 pages, 5 figures and 2 tables. Accepted in A&
Earth Occultation Imaging of the Low Energy Gamma-Ray Sky with GBM
The Earth Occultation Technique (EOT) has been applied to Fermi's Gamma-ray
Burst Monitor (GBM) to perform all-sky monitoring for a predetermined catalog
of hard X-ray/soft gamma-ray sources. In order to search for sources not in the
catalog, thus completing the catalog and reducing a source of systematic error
in EOT, an imaging method has been developed -- Imaging with a Differential
filter using the Earth Occultation Method (IDEOM). IDEOM is a tomographic
imaging method that takes advantage of the orbital precession of the Fermi
satellite. Using IDEOM, all-sky reconstructions have been generated for ~sim 4
years of GBM data in the 12-50 keV, 50-100 keV and 100-300 keV energy bands in
search of sources otherwise unmodeled by the GBM occultation analysis. IDEOM
analysis resulted in the detection of 57 sources in the 12-50 keV energy band,
23 sources in the 50-100 keV energy band, and 7 sources in the 100-300 keV
energy band. Seventeen sources were not present in the original GBM-EOT catalog
and have now been added. We also present the first joined averaged spectra for
four persistent sources detected by GBM using EOT and by the Large Area
Telescope (LAT) on Fermi: NGC 1275, 3C 273, Cen A, and the Crab
Recent activity of the Be/X-ray binary system SAX J2103.5+4545
Aims. We present a multiwavelength study of the Be/X-ray binary system SAX
J2103.5+4545 with the goal of better characterizing the transient behaviour of
this source.
Methods. SAX J2103.5+4545 was observed by Swift-XRT four times in 2007 from
April 25 to May 5, and during quiescence in 2012 August 31. In addition, this
source has been monitored from the ground-based astronomical observatories of
El Teide (Tenerife, Spain), Roque de los Muchachos (La Palma, Spain) and Sierra
Nevada (Granada, Spain) since 2011 August, and from the TUBITAK National
Observatory (Antalya, Turkey) since 2009 June. We have performed spectral and
photometric temporal analyses in order to investigate the different states
exhibited by SAX J2103.5+4545.
Results. In X-rays, an absorbed power law model provided the best fit for all
the XRT spectra. An iron-line feature at ~6.42 keV was present in all the
observations except for that taken during quiescence in 2012. The photon
indexes are consistent with previous studies of SAX J2103.5+4545 in high/low
luminosity states. Pulsations were found in all the XRT data from 2007
(2.839(2) mHz; MJD 54222.02), but not during quiescence. Both optical outbursts
in 2010 and 2012 lasted for about 8/9 months (as the one in 2007 probably did
and the current one in 2014 might do) and were most probably caused by mass
ejection events from the Be star that eventually fed the circumstellar disc.
All of these outbursts started about 3 months before the triggering of the
X-ray activity, and about the same period before the maximum of the H_alpha
line equivalent width (in emission) was reached at only ~ -5 \AA. In this work
we found that the global correlation between the BV variability and the X-ray
intensity was also observed at longer wavelengths in the IR domain.Comment: 11 pages, 7 figures, and online material (2 tables). Submitted to A&A
in 2014 Januar
Consideraciones acerca de los contenidos y metodologías en los cursos de Química en la Educación Secundaria en Bogotá (Colombia)
La formulación de un nuevo currículo para los cursos de química general, impartidos en la Universidad Nacional de Colombia, nos llevó a indagar acerca de la química que se enseña en la secundaria y a analizar los contenidos en su coherencia y relación con los cursos universitarios. Los resultados muestran una gran intención de los profesores por impartir una química relacionada con la vida, pero muy abundante en contenidos formales y muy relacionada con los cursos universitarios de química. Las preferencias entre diversos temas no se relacionan directamente y se prevé que se privilegia una enseñanza teórica y preparatoria para la química universitaria. Esto motiva a pensar en un currículo alejado de presiones, como exámenes de ingreso o de cursos universitarios
Long-term pulse profile study of the Be/X-ray pulsar SAX J2103.5+4545
Aims. We present the first long-term pulse profile study of the X-ray pulsar
SAX J2103.5+4545. Our main goal is to study the pulse shape correlation either
with luminosity, time or energy.
Methods. This Be/X-ray binary system was observed from 1999 to 2004 by RXTE
PCA, and by INTEGRAL from 2002 to 2005, during the Performance and Verification
(PV) phase and the Galactic Plane Scan survey (GPS). X-ray pulse profiles were
obtained in different energy ranges. The long-term spectral variability of this
source is studied. The long-term flux, frequency and spin-up rate histories are
computed. A new set of orbital parameters are also determined.
Results. The pulse shape is complex and highly variable either with time or
luminosity. However, an energy dependence pattern was found. Single, double,
triple or even quadruple peaks pulse profile structure was obtained. It was
confirmed that SAX J2103.5+4545 becomes harder when the flux is higher. The new
orbital solution obtained is: P_orb= 12.66528+-0.00051 days, e = 0.401+-0.018,
w = 241.36+-2.18 and a_xsin i = 80.81+-0.67 lt-s.Comment: 9 pages, 11 figures, submitted to Astronomy & Astrophysic
One world, one health, one virology of the mysterious labyrinth of Coronaviruses: the canine coronavirus affair
The described pictures underline the ability of CoVs of driving genetic evolution, to undergoes recombination, and to easy cross interspecies barriers. This potential high genetic recombination ability ensures the proliferation of new strains that may have selective advantages over parental genomes.9 In this aspect, the newly identified CCoV-Hupn-2018 should lead researchers to pay a special attention to the mechanisms of recombination among CoVs, in addition to the onset of variants as a result of mutations. Continuous monitoring of these viruses are required because (without saying as Cassandra…!!!) recombination observed in CCoVs may represent a dramatic warning for SARS-CoV-2
Orbital Decay and Evidence of Disk Formation in the X-ray Binary Pulsar OAO 1657-415
OAO 1657-415 is an eclipsing X-ray binary wind-fed pulsar that has exhibited
smooth spin-up/spin-down episodes and has undergone several torque reversals
throughout its long history of observation. We present a frequency history
spanning nearly 19 years of observations from the Burst and Transient Source
Experiment (CGRO/BATSE) and from the Gamma-Ray Burst Monitor (Fermi/GBM). The
analysis suggests two modes of accretion: one resulting in steady spin-up
during which we believe a stable accretion disk is present and one that results
in what appears to be a random walk in spin frequency where an unstable
accretion disk forms alternating in direction ("flip flop"). Orbital elements
of the pulsar system are determined at several intervals throughout this
history. With these ephemerides, statistically significant orbital decay
() is established
suggesting a transition between wind-fed and disk-mediated accretion
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