16,445 research outputs found
The XMM-Newton slew survey in the 2-10 keV band
The XMM-Newton Slew Survey (XSS) covers a significant fraction of the sky in
a broad X-ray bandpass. Although shallow by contemporary standards, in the
`classical' 2-10 keV band of X-ray astronomy, the XSS provides significantly
better sensitivity than any currently available all-sky survey. We investigate
the source content of the XSS, focussing on detections in the 2-10 keV band
down to a very low threshold (> 4 counts net of background). At the faint end,
the survey reaches a flux sensitivity of roughly 3e-12 erg/cm2/s (2-10 keV).
Our starting point was a sample of 487 sources detected in the XMMSL1d2 XSS at
high galactic latitude in the hard band. Through cross-correlation with
published source catalogues from surveys spanning the electromagnetic spectrum
from radio to gamma-rays, we find that 45% of the sources have likely
identifications with normal/active galaxies, 18% are associated with other
classes of X-ray object (nearby coronally active stars, accreting binaries,
clusters of galaxies), leaving 37% of the XSS sources with no current
identification. We go on to define an XSS extragalactic hard band sample
comprised of 219 galaxies and active galaxies. We investigate the properties of
this extragalactic sample including its X-ray logN-logS distribution. We find
that in the low-count limit, the XSS is strongly affected by Eddington bias.
There is also a very strong bias in the XSS against the detection of extended
sources, most notably clusters of galaxies. A significant fraction of the
detections at and around the low-count limit may be spurious. Nevertheless, it
is possible to use the XSS to extract a reasonably robust sample of
extragalactic sources, excluding galaxy clusters. The differential logN-logS
relation of these extragalactic sources matches very well to the HEAO-1 A2
all-sky survey measurements at bright fluxes and to the 2XMM source counts at
the faint end.Comment: 16 pages, 13 figures, FITS table of XSS extragalactic sample
available from http://www.star.le.ac.uk/~amr30/Slew
Ground State Entropy of the Potts Antiferromagnet on Cyclic Strip Graphs
We present exact calculations of the zero-temperature partition function
(chromatic polynomial) and the (exponent of the) ground-state entropy for
the -state Potts antiferromagnet on families of cyclic and twisted cyclic
(M\"obius) strip graphs composed of -sided polygons. Our results suggest a
general rule concerning the maximal region in the complex plane to which
one can analytically continue from the physical interval where . The
chromatic zeros and their accumulation set exhibit the rather
unusual property of including support for and provide further
evidence for a relevant conjecture.Comment: 7 pages, Latex, 4 figs., J. Phys. A Lett., in pres
The X-ray properties of the merging galaxy pair NGC 4038/9 - the Antennae
We report the results of an X-ray spectral imaging observation of the
Antennae with the ROSAT PSPC. 55% of the soft X-ray flux from the system is
resolved into discrete sources, including components identified with the
galactic nuclei and large HII regions, whilst the remainder appears to be
predominantly genuinely diffuse emission from gas at a temperature ~4x10^6 K.
The morphology of the emission is unusual, combining a halo which envelopes the
galactic discs, with what appears to be a distorted, but well-collimated
bipolar outflow. We derive physical parameters for the hot gas in both diffuse
components, which are of some interest, given that the Antennae probably
represents an elliptical galaxy in the making.Comment: 15 pages plus 9 figures, uuencoded encapsulated postscript file.
Accepted for publication in MNRA
The XMM-Newton Slew Survey: Towards The Whole X-ray Sky and the Rarest X-ray Events
The data collected by XMM-Newton as it slews between pointings currently
cover almost half the entire sky, and many familiar features and new sources
are visible. The soft-band sensitivity limit of the Slew is close to that of
the RASS, and a large-area Slew-RASS comparison now provides the best
opportunity for discovering extremely rare high-variability objects.Comment: To appear in Proceedings of "X-ray Astronomy 2009: Present Status,
Multi-Wavelength Approach and Future Perspectives", Bologna, Italy, September
7-11, 2009, AIP, eds. A. Comastri, M. Cappi, and L. Angelin
Ground State Entropy of Potts Antiferromagnets on Cyclic Polygon Chain Graphs
We present exact calculations of chromatic polynomials for families of cyclic
graphs consisting of linked polygons, where the polygons may be adjacent or
separated by a given number of bonds. From these we calculate the (exponential
of the) ground state entropy, , for the q-state Potts model on these graphs
in the limit of infinitely many vertices. A number of properties are proved
concerning the continuous locus, , of nonanalyticities in . Our
results provide further evidence for a general rule concerning the maximal
region in the complex q plane to which one can analytically continue from the
physical interval where .Comment: 27 pages, Latex, 17 figs. J. Phys. A, in pres
Photoionization of Galactic Halo Gas by Old Supernova Remnants
We present new calculations on the contribution from cooling hot gas to the
photoionization of warm ionized gas in the Galaxy. We show that hot gas in
cooling supernova remnants (SNRs) is an important source of photoionization,
particularly for gas in the halo. We find that in many regions at high latitude
this source is adequate to account for the observed ionization so there is no
need to find ways to transport stellar photons from the disk. The flux from
cooling SNRs sets a floor on the ionization along any line of sight. Our model
flux is also shown to be consistent with the diffuse soft X-ray background and
with soft X-ray observations of external galaxies.
We consider the ionization of the clouds observed towards the halo star HD
93521, for which there are no O stars close to the line of sight. We show that
the observed ionization can be explained successfully by our model EUV/soft
X-ray flux from cooling hot gas. In particular, we can match the H alpha
intensity, the S++/S+ ratio, and the C+* column. From observations of the
ratios of columns of C+* and either S+ or H0, we are able to estimate the
thermal pressure in the clouds. The slow clouds require high (~10^4 cm^-3 K)
thermal pressures to match the N(C+*)/N(S+) ratio. Additional heating sources
are required for the slow clouds to maintain their ~7000 K temperatures at
these pressures, as found by Reynolds, Hausen & Tufte (1999).Comment: AASTeX 5.01; 34 pages, 2 figures; submitted to Astrophysical Journa
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Midwinter suppression of baroclinic storm activity on Mars: observations and models
We present results from assimilated analyses of observations from the Mars Global Surveyor Thermal Emission Spectrometer showing evidence for a regular suppression of baroclinic circumpolar storm activity in both hemispheres of Mars around winter solstice. General circulation model simulations are then used to elucidate the structure and possible causes of this suppression, for which the local ‘Eady growth rate’ appears to be a good predictor
Nitrification-denitrification in WSP: a mechanism for permanent nitrogen removal in maturation ponds
A pilot-scale primary maturation pond was spiked with 15N-labelled ammonia (15NH4Cl) and 15N labelled nitrite (Na15NO2), in order to improve current understanding of the dynamics of inorganic nitrogen transformations and removal in WSP systems. Stable isotope analysis of δ15N showed that
nitrification could be considered as an intermediate step in WSP, which is masked by simultaneous denitrification, under conditions of low algal activity. Molecular microbiology analysis showed that denitrification can be considered a feasible mechanism for permanent nitrogen removal in WSP, which may be supported either by ammonia-oxidising bacteria (AOB) or by methanotrophs, in addition to nitrite-oxidising bacteria (NOB). However, the relative supremacy of the denitrification process over other nitrogen removal mechanisms (e.g., biological uptake) depends upon phytoplanktonic activity
SPILC: An expert student advisor
The Lamar University Computer Science Department serves about 350 undergraduate C.S. majors, and 70 graduate majors. B.S. degrees are offered in Computer Science and Computer and Information Science, and an M.S. degree is offered in Computer Science. In addition, the Computer Science Department plays a strong service role, offering approximately sixteen service course sections per long semester. The department has eight regular full-time faculty members, including the Department Chairman and the Undergraduate Advisor, and from three to seven part-time faculty members. Due to the small number of regular faculty members and the resulting very heavy teaching loads, undergraduate advising has become a difficult problem for the department. There is a one week early registration period and a three-day regular registration period once each semester. The Undergraduate Advisor's regular teaching load of two classes, 6 - 8 semester hours, per semester, together with the large number of majors and small number of regular faculty, cause long queues and short tempers during these advising periods. The situation is aggravated by the fact that entering freshmen are rarely accompanied by adequate documentation containing the facts necessary for proper counselling. There has been no good method of obtaining necessary facts and documenting both the information provided by the student and the resulting advice offered by the counsellors
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