461,389 research outputs found
Multiwavelength study of Cygnus A II. X-ray inverse-Compton emission from a relic counterjet and implications for jet duty-cycles
The duty-cycle of powerful radio galaxies and quasars such as the prototype
Cygnus A is poorly understood. X-ray observations of inverse-Compton scattered
Cosmic Microwave Background (ICCMB) photons probe lower Lorentz-factor
particles than radio observations of synchrotron emission. Comparative studies
of the nearer and further lobes, separated by many 10s of kpc and thus 10s of
thousands of years in light-travel time, yield additional temporal resolution
in studies of the lifecycles. We have co-added all archival Chandra ACIS-I data
and present a deep 200 ks image of Cygnus A. This deep image reveals the
presence of X-ray emission from a counterjet i.e. a jet receding from Earth and
related to a previous episode of jet activity. The non-thermal X-ray emission,
we interpret as ICCMB radiation. There is an absence of any discernible X-ray
emission associated with a jet flowing towards Earth. We conclude that: (1) The
emission from a relic jet, indicates a previous episode of jet activity, that
took place earlier than the current jet activity appearing as synchrotron radio
emission. (2) The presence of X-ray emission from a relic counterjet of Cygnus
A and the absence of X-ray emission associated with any relic approaching jet
constrains the timescale between successive episodes of jet activity to ~10^6
years. (3) Transverse expansion of the jet causes expansion losses which shifts
the energy distribution to lower energies. (4) Assuming the electrons cooled
due to adiabatic expansion, the required magnetic field strength is
substantially smaller than the equipartition magnetic field strength. (5) A
high minimum Lorentz factor for the distribution of relativistic particles in
the current jet, of a few 10^3, is ejected from the central nucleus of this
active galaxy. Abridged.Comment: Accepted for publication by MNRAS, 8 pages Dates in Table 1 correcte
NGC 2992 in an X-ray high state observed by XMM: Response of the Relativistic Fe K Line to the Continuum
We present the analysis of an XMM observation of the Seyfert galaxy NGC 2992.
The source was found in its highest level of X-ray activity yet detected, a
factor higher in 2--10 keV flux than the historical minimum. NGC
2992 is known to exhibit X-ray flaring activity on timescales of days to weeks,
and the XMM data provide at least factor of better spectral resolution
in the Fe K band than any previously measured flaring X-ray state. We find that
there is a broad feature in the \sim 5-7 keV band which could be interpreted as
a relativistic Fe K emission line. Its flux appears to have increased
in tandem with the 2--10 keV continuum when compared to a previous Suzaku
observation when the continuum was a factor of lower than that during
the XMM observation. The XMM data are consistent with the general picture that
increased X-ray activity and corresponding changes in the Fe K line
emission occur in the innermost regions of the putative accretion disk. This
behavior contrasts with the behavior of other AGN in which the Fe K
line does not respond to variability in the X-ray.Comment: 30 pages, 6 figures, Accepted to Ap
Influence of the stop/start system on CO2 emissions of a diesel vehicle in urban traffic
This paper presents measurements of CO2 emission and efficiency of stop/start technology on a diesel vehicle in urban traffic. Two four-wheel-drive diesel vehicles with on-board exhaust emission and vehicle activity measurement systems were tested in two urban driving circuits representative of downtown Madrid. The vehicles had similar turbocharged and intercooled diesel engines fulfilling the same Euro 4 emissions regulation; but one had an improved engine incorporating stop/start technology. CO2 emission reduction of more than 20% for the car equipped with the stop/start system was obtained. Regardless of the variability in driving style, the grade and type of streets, traffic congestion, and the engine operating temperature, the car equipped with the stop/start system has intrinsically a lower CO2 emission factor
Dynamic Activity Analysis Model Based Win-Win Development Forecasting Under the Environmental Regulation in China
Porter Hypothesis states that environmental regulation may lead to win-win opportunities, that is, improve the productivity and reduce the undesirable output simultaneously. Based on directional distance function, this paper proposes a novel dynamic activity analysis model to forecast the possibilities of win-win development in Chinese Industry between 2009 and 2049. The evidence reveals that the appropriate energy-saving and emission-abating regulation will result in both the improvement in net growth of potential output and the steadily increasing growth of total factor productivity. This favors Porter Hypothesis.Dynamic Activity Analysis Model, Energy-Saving and Emission-Abating, Environmental Regulation, Win-Win Development
Forever Young: High Chromospheric Activity in M subdwarfs
We present spectroscopic observations of two halo M subdwarfs which have H
alpha emission lines. We show that in both cases close companions are the most
likely cause of the chromospheric activity in these old, metal-poor stars. We
argue that Gl 781 A's unseen companion is most likely a cool helium white
dwarf. Gl 455 is a near-equal-mass M subdwarf (sdM) system. Gl 781 A is rapidly
rotating with v sin i = 30 km/s. The properties of the chromospheres and X-ray
coronae of these systems are compared to M dwarfs with emission (dMe). The
X-ray hardness ratios and optical chromospheric lines emission ratios are
consistent with those seen in dMe stars. Comparison to active near-solar
metallicity stars indicates that despite their low metallicity ([m/H] = -1/2),
the sdMe stars are roughly as active in both X-rays and chromospheric emission.
Measured by L_X/L_bol, the activity level of Gl 781 A is no more than a factor
of 2.5 subluminous with respect to near-solar metallicity stars.Comment: 16 pages including 1 figure, AASTeX, to appear in May 1998 A.
Far-Ultraviolet Activity Levels of F, G, K, and M dwarf Exoplanet Host Stars
We present a survey of far-ultraviolet (FUV; 1150 - 1450 Ang) emission line
spectra from 71 planet-hosting and 33 non-planet-hosting F, G, K, and M dwarfs
with the goals of characterizing their range of FUV activity levels,
calibrating the FUV activity level to the 90 - 360 Ang extreme-ultraviolet
(EUV) stellar flux, and investigating the potential for FUV emission lines to
probe star-planet interactions (SPIs). We build this emission line sample from
a combination of new and archival observations with the Hubble Space
Telescope-COS and -STIS instruments, targeting the chromospheric and transition
region emission lines of Si III, N V, C II, and Si IV.
We find that the exoplanet host stars, on average, display factors of 5 - 10
lower UV activity levels compared with the non-planet hosting sample; this is
explained by a combination of observational and astrophysical biases in the
selection of stars for radial-velocity planet searches. We demonstrate that UV
activity-rotation relation in the full F - M star sample is characterized by a
power-law decline (with index ~ -1.1), starting at rotation periods
>~3.5 days. Using N V or Si IV spectra and a knowledge of the star's bolometric
flux, we present a new analytic relationship to estimate the intrinsic stellar
EUV irradiance in the 90 - 360 Ang band with an accuracy of roughly a factor of
~2. Finally, we study the correlation between SPI strength and UV activity in
the context of a principal component analysis that controls for the sample
biases. We find that SPIs are not a statistically significant contributor to
the observed UV activity levels.Comment: ApJS, accepted. 33 pages in emulateapj, 13 figures, 10 table
A Correlation Between Stellar Activity and Hot Jupiter Emission Spectra
We present evidence for a correlation between the observed properties of hot
Jupiter emission spectra and the activity levels of the host stars measured
using Ca II H & K emission lines. We find that planets with dayside emission
spectra that are well-described by standard 1D atmosphere models with water in
absorption (HD 189733, TrES-1, TrES-3, WASP-4) orbit chromospherically active
stars, while planets with emission spectra that are consistent with the
presence of a strong high-altitude temperature inversion and water in emission
orbit quieter stars. We estimate that active G and K stars have Lyman alpha
fluxes that are typically a factor of 4-7 times higher than quiet stars with
analogous spectral types, and propose that the increased UV flux received by
planets orbiting active stars destroys the compounds responsible for the
formation of the observed temperature inversions. In this paper we also derive
a model-independent method for differentiating between these two atmosphere
types using the secondary eclipse depths measured in the 3.6 and 4.5 micron
bands on the Spitzer Space Telescope, and argue that the observed correlation
is independent of the inverted/non-inverted paradigm for classifying hot
Jupiter atmospheres.Comment: 9 pages, 5 figures, accepted for publication in ApJ. The updated
paper includes spectra for ten additional systems and a new section
discussing the connection between chromospheric activity and UV flu
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