15,260 research outputs found
X-ray Observations of AGN at Intermediate to High Redshift
The cores of active galactic nuclei (AGN) harbor some of the most extreme
conditions of matter and energy in the Universe. One of the major goals of
high-energy astrophysics is to probe these extreme environments in the vicinity
of supermassive black holes, which are intimately linked to the mechanisms that
produce the continuum emission in AGN. X-ray studies seek to understand the
physics responsible for the continuum emission, its point of origin, how
nuclear activity is fueled, and how supermassive black holes evolve. The key to
finding answers to these questions lies in measuring the intrinsic luminosities
and spectral shapes, the relation of these properties to other wavebands, and
how the source properties change with redshift. This article reviews X-ray
observations of AGN from redshifts of ~0.1-3 with the goal of summarizing our
current knowledge of their X-ray spectral characteristics. Results are
evaluated in terms of their robustness and are examined in the light of current
theoretical predictions of energy release via processes associated with the
accretion mechanism. A possible evolutionary scenario is discussed, along with
the importance of AGN studies at high redshift as they relate to the total
energetics of the Universe.Comment: 10 pages, 1 figure. To appear in the proceedings of the 9th Annual
October Maryland Astrophysics Conference, After the Dark Ages: When galaxies
Were Youn
Probing Dense Matter in the cores of AGN: Observations with RXTE and ASCA
Preliminary results from an X-ray spectral study of Seyfert 1 galaxies with
ASCA and RXTE are presented. From an analysis of X-ray reprocessing features of
Compton reflection and Fe K-alpha fluorescence, it is found that iron line
strength is not necessarily a good predictor of the amount of reflection. The
variability properties of Fe K-alpha and reflection do not necessarily scale
together and substantial decoupling of the behavior of the reprocessed flux
with respect to continuum variability is common. Such trends suggest the
presence of multiple and/or complex regions of dense matter in AGN cores and
that standard accretion disk models drastically oversimplify reality.Comment: 4 pages, 2, two-panel figures, latex. To appear in "Proceedings of
X-ray Astronomy '99 - Stellar Endpoints, AGN and the Diffuse Background,"
2000. G. Malaguti, G. Palumbo & N. White (eds), Gordon & Breach (Singapore
RXTE and BeppoSAX Observations of MCG -5-23-16: Reflection From Distant Cold Material
We examine the spectral variability of the Seyfert 1.9 galaxy MCG -5-23-16
using RXTE and BeppoSAX observations spanning 2 years from April 1996 to April
1998. During the first year the X-ray source brightens by a factor of ~25% on
timescales of days to months. During this time, the reprocessed continuum
emission seen with RXTE does not respond measurably to the continuum increase.
However, by the end of the second year during the BeppoSAX epoch the X-ray
source has faded again. This time, the reprocessed emission has also faded,
indicating that the reprocessed flux has responded to the continuum. If these
effects are caused by time delays due to the distance between the X-ray source
and the reprocessing region, we derive a light crossing time of between ~1
light day and ~1.5 light years. This corresponds to a distance of 0.001 pc to
0.55 pc, which implies that the reprocessed emission originates between 3x10^15
cm and 1.6x10^18 cm from the X-ray source. In other words, the reprocessing in
MCG -5-23-16 is not dominated by the inner regions of a standard accretion
disk.Comment: Accepted for publication in ApJ. 17 pages, 8 figure
The Benefits of Peer Review and a Multisemester Capstone Writing Series on Inquiry and Analysis Skills in an Undergraduate Thesis.
This study examines the relationship between the introduction of a four-course writing-intensive capstone series and improvement in inquiry and analysis skills of biology senior undergraduates. To measure the impact of the multicourse write-to-learn and peer-review pedagogy on student performance, we used a modified Valid Assessment of Learning in Undergraduate Education rubric for Inquiry and Analysis and Written Communication to score senior research theses from 2006 to 2008 (pretreatment) and 2009 to 2013 (intervention). A Fisher-Freeman-Halton test and a two-sample Student's t test were used to evaluate individual rubric dimensions and composite rubric scores, respectively, and a randomized complete block design analysis of variance was carried out on composite scores to examine the impact of the intervention across ethnicity, legacy (e.g., first-generation status), and research laboratory. The results show an increase in student performance in rubric scoring categories most closely associated with science literacy and critical-thinking skills, in addition to gains in students' writing abilities
X-ray Evidence for Multiple Absorbing Structures in Seyfert Galaxies
We have used X-ray spectra to measure attenuating columns in a large sample
of Seyfert galaxies. Over 30 of these sources have resolved radio jets,
allowing the relative orientation of the nucleus and host galaxy to be
constrained. We have discovered that the distribution of absorbing columns is
strongly correlated with the relative orientation of the Seyfert structures.
This result is inconsistent with unification models including only a torus and
is instead most readily explained if a second absorber is included: in addition
to a Compton-thick, parsec-scale torus there would also be a larger-scale
absorber with N_H < 10^{23} cm^{-2}. The second absorber is aligned with the
host galactic plane while the torus is arbitrarily misaligned.Comment: 2 pages, 1 figure, to appear in "Multiwavelength AGN Surveys"
(Cozumel, December 8-12 2003), ed. R. Maiolino and R. Mujica, Singapore:
World Scientific, 2004. Additional material may be found at
http://space.mit.edu/home/jonathan/research.htm
Ventilation of the North Atlantic Ocean during the Last Glacial Maximum: A comparison between simulated and observed radiocarbon ages
The distribution of radiocarbon during simulations of the Last Glacial Maximum with a coupled ocean-atmosphere-sea ice model is compared with sediment core measurements from the equatorial Atlantic Ceara Rise, Blake Ridge, Caribbean Sea, and South China Sea. During these simulations we introduce a perturbation of North Atlantic freshwater fluxes leading to varying strengths of the Atlantic meridional overturning. The best fit with the observations is obtained for an overturning weakened by 40% compared with today. Further, we simulate the phenomenon of an âage reversalâ found in deep sea corals, but we suggest that this indicates rather a sudden interruption of deep water formation instead of an increase in ventilation, which was suggested earlier
On the area of the symmetry orbits in symmetric spacetimes
We obtain a global existence result for the Einstein equations. We show that
in the maximal Cauchy development of vacuum symmetric initial data with
nonvanishing twist constant, except for the special case of flat Kasner initial
data, the area of the group orbits takes on all positive values. This
result shows that the areal time coordinate which covers these spacetimes
runs from zero to infinity, with the singularity occurring at R=0.Comment: The appendix which appears in version 1 has a technical problem (the
inequality appearing as the first stage of (52) is not necessarily true), and
since the appendix is unnecessary for the proof of our results, we leave it
out. version 2 -- clarifications added, version 3 -- reference correcte
Warm absorber, reflection and Fe K line in the X-ray spectrum of IC 4329A
Results from the X-ray spectral analysis of the ASCA PV phase observation of
the Seyfert 1 galaxy IC 4329A are presented. We find that the 0.4 - 10 keV
spectrum of IC 4329A is best described by the sum of a steep () power-law spectrum passing through a warm absorber plus a strong
reflection component and associated Fe K line, confirming recent results
(Madejski et al. 1995, Mushotsky et al. 1995). Further cold absorption in
excess of the Galactic value and covering the entire source is also required by
the data, consistent with the edge-on galactic disk and previous X-ray
measurements. The effect of the warm absorber at soft X-ray energies is best
parameterized by two absorption edges, one consistent with OVI, OVII or NVII,
the other consistent with OVIII. A description of the soft excess in terms of
blackbody emission, as observed in some other Seyfert 1 galaxies, is ruled out
by the data. A large amount of reflection is detected in both the GIS and SIS
detectors, at similar intensities. We find a strong correlation between the
amount of reflection and the photon index, but argue that the best solution
with the present data is that given by the best statistical fit. The model
dependence of the Fe K line parameters is also discussed. Our best fit gives a
slightly broad ( keV) and redshifted (E keV) Fe K line, with equivalent width 89 33 eV.
The presence of a weak Fe K line with a strong reflection can be reconciled if
one assumes iron underabundances or ionized reflection. We also have modeled
the line with a theoretical line profile produced by an accretion disk. This
yields results in better agreement with the constraints obtained from the
reflection component.Comment: Accepted for publication in The Astrophysical Journal, 10th February
1996 issue; 24 pages and 8 figures + 1 table tared, compressed and uuencoded
(with uufiles
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