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Optical and Hygroscopic Studies of Aerosols In Simulated Planetary Atmospheres
Basic characteristics of the early Earth climate, the only known environment in the Universe in which life has been known to emerge and thrive, remain a mystery. In particular, little is understood about the Earth’s atmosphere 2.8 billion years ago. From climate models and laboratory studies, it is postulated that an organic haze, much like that found on Saturn’s largest moon Titan, covered the early Earth. This haze, generated from photolysis of carbon dioxide (CO2) and methane (CH4), may have had profound climatic consequences. Climate models of the early Earth that include this haze have had to rely upon optical properties of a Titan laboratory analog. Titan haze, though thought to be similar, is formed from a different combination of precursor gases and by different energy sources than early Earth haze.
This thesis examines the direct and indirect radiative effects of aerosol on early Earth climate by studying the optical and hygroscopic properties of a laboratory analog. A Titan analog is studied for comparison and to better understand spacecraft-retrieved haze chemical and optical properties from Titan. The properties of the laboratory analogs, generated in a flowing reactor cell with a continuum ultraviolet (UV) light source, were primarily measured using cavity ringdown aerosol extinction spectroscopy and UV-visible (UV-Vis) transmission spectroscopy.
We find that the optical properties of our early Earth analog are significantly different than those of the Titan analog from Khare et al. (1984). In both the UV and visible, when modeled as fractals, particles with the optical properties of the early Earth analog have approximately 30% larger extinction efficiencies than particles with Khare et al. (1984) values. This result implies our early Earth haze analog would provide a more efficient UV shield and have a stronger antigreenhouse effect than the Khare et al. (1984) Titan analog. Our Titan analog has significantly smaller imaginary refractive index values in the UV-Vis than Khare et al. (1984) values. These results may imply that (a) photolysis is not the dominant source of aerosol on Titan, and/or (b) the optical retrievals are dominated by the more absorbing and scattering electric discharge generated aerosol.
For the hygroscopicity studies, the optical growth of the early Earth analog at various relative humidities (RH) was measured, as well as a Titan analog for comparison. The retrieved hygroscopic parameter for the early Earth analog indicates that a humidified early Earth aerosol could have contributed to a larger antigreenhouse effect on the early Earth atmosphere than previously modeled with dry aerosol. Such effects would be important in regions where RH is greater than 50% because such high humidities are needed for significant amounts of water to be on the aerosol. The retrieved hygroscopicity parameter also indicates that the particles could activate into cloud droplets at reasonable supersaturations. In regions where the haze was dominant, it is expected that low particle concentrations, once activated into cloud droplets, would create short-lived, optically thin clouds. Such clouds, if predominant on the early Earth, would have a lower albedo than clouds today, thereby warming the planet relative to current day clouds
Potential Climatic Impact of Organic Haze on Early Earth
We have explored the direct and indirect radiative effects on climate of organic particles likely to have been present on early Earth by measuring their hygroscopicity and cloud nucleating ability. The early Earth analog aerosol particles were generated via ultraviolet photolysis of an early Earth analog gas mixture, which was designed to mimic possible atmospheric conditions before the rise of oxygen. An analog aerosol for the present-day atmosphere of Saturn's moon Titan was tested for comparison. We exposed the early Earth aerosol to a range of relative humidities (RHs). Water uptake onto the aerosol was observed to occur over the entire RH range tested (RH = 80–87%). To translate our measurements of hygroscopicity over a specific range of RHs into their water uptake ability at any RH 100%, we relied on the hygroscopicity parameter κ, developed by Petters and Kreidenweis. We retrieved κ = 0.22 ± 0.12 for the early Earth aerosol, which indicates that the humidified aerosol (RH 100%). In regions where the haze was dominant, it is expected that low particle concentrations, once activated into cloud droplets, would have created short-lived, optically thin clouds. Such clouds, if predominant on early Earth, would have had a lower albedo than clouds today, thereby warming the planet relative to current-day clouds
XMM-Newton and Swift observations of WZ Sge: spectral and timing analysis
WZ Sagittae is the prototype object of a subclass of dwarf novae, with rare
and long (super)outbursts, in which a white dwarf primary accretes matter from
a low mass companion. High-energy observations offer the possibility of a
better understanding of the disk-accretion mechanism in WZ Sge-like binaries.
We used archival XMM-Newton and Swift data to characterize the X-ray spectral
and temporal properties of WZ Sge in quiescence. We performed a detailed timing
analysis of the simultaneous X-ray and UV light curves obtained with the EPIC
and OM instruments on board XMM-Newton in 2003. We employed several techniques
in this study, including a correlation study between the two curves. We also
performed an X-ray spectral analysis using the EPIC data, as well as Swift/XRT
data obtained in 2011. We find that the X-ray intensity is clearly modulated at
a period of about 28.96 s, confirming previously published preliminary results.
We find that the X-ray spectral shape of WZ Sge remains practically unchanged
between the XMM-Newton and Swift observations. However, after correcting for
inter-stellar absorption, the intrinsic luminosity is estimated to be about
2.65X10^ 30 erg/s/cm^2 and 1.57X10^30 erg/s/cm^2 in 2003 and 2011,
respectively. During the Swift/XRT observation, the observed flux is a factor
of about 2 lower than that observed by XMM-Newton, but is similar to the
quiescent levels observed various times before the 2001 outburst.Comment: Accepted for publication on A&A.10 pages, 9 figure
X-Ray Spectra of Intermediate-Luminosity, Radio-Loud Quasars
We present new hard X-ray spectra of three radio-loud AGNs of moderately high
X-ray luminosity (L_x ~ 10^45 erg/s; PKS 2349-01, 3C 323.1, and 4C 74.26)
obtained with ASCA and BeppoSAX. The X-ray continua are described in all three
cases with a power law model with photon indices of Gamma~1.85, modified at low
energies by absorption in excess of the Galactic, which appears to be due to
neutral gas. At higher energies, an Fe Ka emission line is detected in PKS
2349-01 and 4C 74.26, and is tentatively detected in 3C 323.1. The equivalent
widths of the lines are consistent, albeit within large uncertainties, with the
values for radio-quiet AGN of comparable X-ray luminosity. The Fe Ka line is
unresolved in 4C 74.26. In the case of PKS 2349-01, however, the inferred
properties of the line depend on the model adopted for the continuum: if a
simple power-law model is used, the line is resolved at more than 99%
confidence with a full width at half maximum corresponding to approximately
50,000 km/s and a rest-frame equivalent width of 230 +/- 120 eV, but if a
Compton "reflection" model is used the line is found to be a factor of 2
weaker, for an assumed full width at half maximum of 50,000 km/s. In 4C 74.26,
a strong Compton "reflection" component is detected. Its strength suggests that
the scattering medium subtends a solid angle of 2pi to the illuminating source.
Overall, the spectral indices of these radio-loud quasars are remarkably
similar to those of their radio-quiet counterparts. On the other hand, if the
absorber is indeed neutral, as our results suggest, this would be consistent
with the typical properties of radio-loud AGNs.Comment: To appear in ApJ, v.575, Aug 10, 2002. 14 pages, including 3 tables
and 6 figures. Uses emulateapj5.st
BeppoSAX/PDS serendipitous detections at high galactic latitudes
At a flux limit of ~10^(-11) erg/cm2/s in the 20-100 keV band, the PDS
instrument on-board BeppoSAX offers the opportunity to study the extragalactic
sky with an unprecedented sensitivity. In this work we report on the results of
a search in the BeppoSAX archive for serendipitous high energy sources at high
galactic latitudes (|b| > 13 deg). We have defined a set of twelve regions in
which the PDS/MECS cross-calibration constant is higher than the nominal value.
We attribute this mismatch to the presence of a serendipitous source in the PDS
field of view.In four cases the likely high energy emitter is also present in
the MECS field of view. In these cases, we have performed a broad band spectral
analysis (1.5-100 keV) so as to understand the source spectral behaviour and
compare it with previous BeppoSAX observations when available. In eight cases
the identification of the source likely to provide the PDS spectrum is based on
indirect evidence (extrapolation to lower energies and/or comparison to
previous observations). This approach leads to the discovery of six new hard
X-ray emitting objects (PKS 2356-611, 2MASX J14585116-1652223, NGC 1566, NGC
7319, PKS 0101-649 and ESO 025-G002) and to the presentation the PDS spectrum
of NGC 3227 for the first time. In the remaining five cases we provide extra
BeppoSAX observations that can be compared with measurements which are already
published and/or in the archive.Comment: 16 pages, 9 figures, accepted for publication in A&A, main journa
XMM-Newton spectroscopy of an X-ray selected sample of RL AGNs
This paper presents the X-ray spectroscopy of an X-ray selected sample of 25
radio-loud (RL) AGNs extracted from the XBSS sample. The main goal is to study
the origin of the X-ray spectral differences usually observed between
radio-loud and radio-quiet (RQ) AGNs. To this end, a comparison sample of 53 RQ
AGNs has been also extracted from the same XBSS sample and studied together
with the sample of RL AGNs. We have focused the analysis on the distribution of
the X-ray spectral indices of the power-law component that models the large
majority of the spectra in both samples. We find that the mean X-ray energy
spectral index is very similar in the 2 samples and close to alpha_X~1.
However, the intrinsic distribution of the spectral indices is significantly
broader in the sample of RL AGNs. In order to investigate the origin of this
difference, we have divided the RL AGNs into blazars and ``non-blazars'', on
the basis of the available optical and radio information. We find strong
evidence that the broad distribution observed in the RL AGN sample is mainly
due to the presence of the blazars. Furthermore, within the blazar class we
have found a link between the X-ray spectral index and the value of the
radio-to-X-ray spectral index suggesting that the observed X-ray emission is
directly connected to the emission of the relativistic jet. This trend is not
observed among the ``non-blazars'' RL AGNs. This favours the hypothesis that,
in these latter sources, the X-ray emission is not significantly influenced by
the jet emission and it has probably an origin similar to the RQ AGNs. Overall,
the results presented here indicate that the observed distribution of the X-ray
spectral indices in a given sample of RL AGNs is strongly dependent on the
amount of relativistic beaming present in the selected sources.Comment: 18 pages, 10 figures, accepted for publication in A&
Quasars: the characteristic spectrum and the induced radiative heating
Using information on the cosmic X-ray background and the cumulative light of
active galactic nuclei at infrared wavelengths, the estimated local mass
density of galactic massive black holes (MBHs) and published AGN composite
spectra in the optical, UV and X-ray, we compute the characteristic
angular-integrated, broad-band spectral energy distribution of the average
quasar in the universe. We demonstrate that the radiation from such sources can
photoionize and Compton heat the plasma surrounding them up to an equilibrium
Compton temperature (Tc) of 2x10^7 K. It is shown that circumnuclear
obscuration cannot significantly affect the net gas Compton heating and cooling
rates, so that the above Tc value is approximately characteristic of both
obscured and unobscured quasars. This temperature is above typical gas
temperatures in elliptical galaxies and just above the virial temperatures of
giant ellipticals. The general results of this work can be used for accurate
calculations of the feedback effect of MBHs on both their immediate environs
and the more distant interstellar medium of their host galaxies.Comment: 15 pages, 5 figures. Revised version accepted for publication in
MNRA
Exploring the disc/jet interaction in the radio-loud quasar 4C +74.26 with Suzaku
We report on a 90 ks Suzaku observation of the radio-loud quasar 4C +74.26.
The source was observed in its highest flux state to date, and we find that it
brightened by about 20 per cent during the observation. We see evidence of
spectral hardening as the count rate increases and also find that the rms
variability increases with energy up to about 4 keV. We clearly detect a
broadened Fe line but conclude that it does not require any emission from
inside about 50 r_g, although a much smaller inner radius cannot be ruled out.
The large inner radius of our best fit implies that the inner disc is either
missing or not strongly illuminated. We suggest that the latter scenario may
occur if the power-law source is located high above the disc, or if the
emission is beamed away from the disc.Comment: 11 pages, 9 figures. Accepted for publication in MNRA