18,181 research outputs found
The helical instability of the positive column in crossed fields and in annular plasma configurations Interim report
Helical instability of positive column in crossed fields and in annular plasma configuration
An x-band waveguide cell for study of microwave propagation through magnetoplasma technical report no. 16
Metal X-band waveguide cell for study of microwave propagation through magnetoplasm
Low intensity H-beta emission from the interstellar medium
A search for diffuse galactic H beta emission not associated with any known H II regions was conducted using a 2-inch-diamenter pressure-scanned Fabry-Perot spectrometer at the Coude focus of a 36-inch telescope. Observations were made near the directions of four pulsars. Emissions with intensities from 40,000 to 400,000 photons/sq cm sec ster (corresponding to emission measures of approximately 10 - 100) were detected in three of the directions. The data indicate an average ionization rate (assuming steady state) of approximately 10 to the minus 14th power/H-atom sec for the interstellar hydrogen in these directions and temperatures between 1000 and 10,000 K for the emitting regions. Plans were made to continue the investigation of these very faint hydrogen emission sources using a 6-inch-diameter Fabry-Perot spectrometer
An X-ray-UV correlation in Cen X-4 during quiescence
Quiescent emission from the neutron star low-mass X-ray binary Cen X-4 is
seen to be variable on timescales from hundreds of seconds to years, suggesting
that at least in this object, low-level accretion is important during
quiescence. Here we present results from recent XMM-Newton and Swift
observations of Cen X-4, where the X-ray flux (0.5 - 10 keV) varies by a factor
of 6.5 between the brightest and faintest states. We find a positive
correlation between the X-ray flux and the simultaneous near-UV flux, where as
there is no significant correlation between the X-ray and simultaneous optical
(V, B) fluxes. This suggests that while the X-ray and UV emitting regions are
somehow linked, the optical region originates elsewhere. Comparing the
luminosities, it is plausible that the UV emission originates due to
reprocessing of the X-ray flux by the accretion disk, with the hot inner region
of the disk being a possible location for the UV emitting region. The optical
emission, however, could be dominated by the donor star. The X-ray/UV
correlation does not favour the accretion stream-impact point as the source of
the UV emission.Comment: 8 pages, 3 figures, accepted for publication in MNRA
X-ray and UV correlation in the quiescent emission of Cen X-4, evidence of accretion and reprocessing
We conducted the first long-term (60 days), multiwavelength (optical,
ultraviolet, and X-ray) simultaneous monitoring of Cen X-4 with daily Swift
observations, with the goal of understanding variability in the low mass X-ray
binary Cen X-4 during quiescence. We found Cen X-4 to be highly variable in all
energy bands on timescales from days to months, with the strongest quiescent
variability a factor of 22 drop in the X-ray count rate in only 4 days. The
X-ray, UV and optical (V band) emission are correlated on timescales down to
less than 110 s. The shape of the correlation is a power law with index gamma
about 0.2-0.6. The X-ray spectrum is well fitted by a hydrogen NS atmosphere
(kT=59-80 eV) and a power law (with spectral index Gamma=1.4-2.0), with the
spectral shape remaining constant as the flux varies. Both components vary in
tandem, with each responsible for about 50% of the total X-ray flux, implying
that they are physically linked. We conclude that the X-rays are likely
generated by matter accreting down to the NS surface. Moreover, based on the
short timescale of the correlation, we also unambiguously demonstrate that the
UV emission can not be due to either thermal emission from the stream impact
point, or a standard optically thick, geometrically thin disc. The spectral
energy distribution shows a small UV emitting region, too hot to arise from the
accretion disk, that we identified as a hot spot on the companion star.
Therefore, the UV emission is most likely produced by reprocessing from the
companion star, indeed the vertical size of the disc is small and can only
reprocess a marginal fraction of the X-ray emission. We also found the
accretion disc in quiescence to likely be UV faint, with a minimal contribution
to the whole UV flux.Comment: 5 pages, 4 figures, submitted to Proc. Int. Conf. Physics at the
Magnetospheric Boundary, Geneva, Switzerland (25-28 June, 2013
Daily, multiwavelength Swift monitoring of the neutron star low-mass X-ray binary Cen X-4: evidence for accretion and reprocessing during quiescence
We conducted the first long-term (60 days), multiwavelength (optical,
ultraviolet, and X-ray) simultaneous monitoring of Cen X-4 with daily Swift
observations from June to August 2012, with the goal of understanding
variability in the low mass X-ray binary Cen X-4 during quiescence. We found
Cen X-4 to be highly variable in all energy bands on timescales from days to
months, with the strongest quiescent variability a factor of 22 drop in the
X-ray count rate in only 4 days. The X-ray, UV and optical (V band) emission
are correlated on timescales down to less than 110 s. The shape of the
correlation is a power law with index gamma about 0.2-0.6. The X-ray spectrum
is well fitted by a hydrogen NS atmosphere (kT=59-80 eV) and a power law (with
spectral index Gamma=1.4-2.0), with the spectral shape remaining constant as
the flux varies. Both components vary in tandem, with each responsible for
about 50% of the total X-ray flux, implying that they are physically linked. We
conclude that the X-rays are likely generated by matter accreting down to the
NS surface. Moreover, based on the short timescale of the correlation, we also
unambiguously demonstrate that the UV emission can not be due to either thermal
emission from the stream impact point, or a standard optically thick,
geometrically thin disc. The spectral energy distribution shows a small UV
emitting region, too hot to arise from the accretion disk, that we identified
as a hot spot on the companion star. Therefore, the UV emission is most likely
produced by reprocessing from the companion star, indeed the vertical size of
the disc is small and can only reprocess a marginal fraction of the X-ray
emission. We also found the accretion disc in quiescence to likely be UV faint,
with a minimal contribution to the whole UV flux.Comment: 19 pages, 6 figures, 4 table
Resolved Spectroscopy of the Narrow-Line Region in NGC 1068. II. Physical Conditions Near the NGC 1068 ``Hot-Spot''
The physical conditions near the optical continuum peak (``hot spot'') in the
inner narrow line region (NLR) of the Seyfert 2 galaxy, NGC 1068. Spectra were
taken with HST/STIS through the 0.1X52 arcsec slit, covering the full STIS 1200
to 10000 Angstrom waveband, and are from a region that includes the hot spot,
extending 0.2, or ~ 14 pc (for H= 75 km/sec/Mpc). Perhaps the most striking
feature of these spectra is the presence of strong coronal emission lines,
including [S XII] 7611 which has hitherto only been identified in spectra of
the solar corona. There is an apparent correlation between ionization energy
and velocity of the emission lines with respect to the systemic velocity of the
host galaxy, with the coronal lines blueshifted, most other high excitation
lines near systemic, and some of the low ionization lines redshifted. From the
results of our modeling, we find that the emission-line gas consists of three
principal components: 1) one in which most of the strong emission-lines, such
as [O III] 5007, [Ne V] 3426, C IV 1550, arise, 2) a more tenuous, highly
ionized component, which is the source of the coronal-line emission, and 3) a
component, which is not co-planar with the other two, in which the low
ionization and neutral lines, such as [N II] 6548 and [O I] 6300, are formed.
The first two components are directly ionized by the EUV-Xray continuum emitted
by the central source, while the low ionization gas is ionized by a combination
of highly absorbed continuum radiation and a small fraction of unabsorbed
continuum scattered by free electrons associated with the hot spot. The
combination of covering factor and Thomson optical depth of the high ionization
components is insufficient to scatter the observed fraction of continuum
radiation into our line-of-sight.Comment: 42 pages, Latex, includes 5 figures (postscript), to appear in the
Astrophysical Journa
Radio disappearance of the magnetar XTE J1810-197 and continued X-ray timing
We report on timing, flux density, and polarimetric observations of the
transient magnetar and 5.54 s radio pulsar XTE J1810-197 using the GBT, Nancay,
and Parkes radio telescopes beginning in early 2006, until its sudden
disappearance as a radio source in late 2008. Repeated observations through
2016 have not detected radio pulsations again. The torque on the neutron star,
as inferred from its rotation frequency derivative f-dot, decreased in an
unsteady manner by a factor of 3 in the first year of radio monitoring. In
contrast, during its final year as a detectable radio source, the torque
decreased steadily by only 9%. The period-averaged flux density, after
decreasing by a factor of 20 during the first 10 months of radio monitoring,
remained steady in the next 22 months, at an average of 0.7+/-0.3 mJy at 1.4
GHz, while still showing day-to-day fluctuations by factors of a few. There is
evidence that during this last phase of radio activity the magnetar had a steep
radio spectrum, in contrast to earlier behavior. There was no secular decrease
that presaged its radio demise. During this time the pulse profile continued to
display large variations, and polarimetry indicates that the magnetic geometry
remained consistent with that of earlier times. We supplement these results
with X-ray timing of the pulsar from its outburst in 2003 up to 2014. For the
first 4 years, XTE J1810-197 experienced non-monotonic excursions in f-dot by
at least a factor of 8. But since 2007, its f-dot has remained relatively
stable near its minimum observed value. The only apparent event in the X-ray
record that is possibly contemporaneous with the radio shut-down is a decrease
of ~20% in the hot-spot flux in 2008-2009, to a stable, minimum value. However,
the permanence of the high-amplitude, thermal X-ray pulse, even after the radio
demise, implies continuing magnetar activity.Comment: ApJ, accepted, 12 pages, 9 figure
Spin-down evolution and radio disappearance of the magnetar PSR J16224950
We report on 2.4 yr of radio timing measurements of the magnetar PSR
J16224950 using the Parkes telescope, between 2011 November and 2014 March.
During this period the torque on the neutron star (inferred from the rotational
frequency derivative) varied greatly, though much less erratically than in the
2 yr following its discovery in 2009. During the last year of our measurements
the frequency derivative decreased in magnitude monotonically by 20\%, to a
value of s, a factor of 8 smaller than when
discovered. The flux density continued to vary greatly during our monitoring
through 2014 March, reaching a relatively steady low level after late 2012. The
pulse profile varied secularly on a similar timescale as the flux density and
torque. A relatively rapid transition in all three properties is evident in
early 2013. After PSR J16224950 was detected in all of our 87 observations
up to 2014 March, we did not detect the magnetar in our resumed monitoring
starting in 2015 January and have not detected it in any of the 30 observations
done through 2016 September.Comment: 8 pages, 5 figures, submitted to Ap
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