Atmospheric Cerenkov telescopes are used to detect electromagnetic showers from primary gamma rays of energy ~300 GeV - ~10 TeV and to discriminate these from cascades due to hadrons using the Cerenkov images. The geomagnetic field affects the development of showers and is shown to diffuse and distort the images. When the component of the field normal to the shower axis is sufficiently large (> 0.4 G) the performance of gamma ray telescopes may be affected, although corrections should be possible.

P. M. Chadwick K. Lyons T. J. L. McComb K. J. Orford J. L. Osborne S. M. Rayner I. D. Roberts S. E. Shaw K. E. Turver

The X-ray selected BL Lac PKS 2155-304 has been observed using the University of Durham Mark 6 very high energy gamma ray telescope during 1998. We find no evidence for TeV emission during these recent observations when the X-ray flux was observed to be low. We have reconsidered our measurements made in 1997 November when PKS 2155-304 was in a bright X-ray state and extended X-ray and GeV gamma ray observations were made as part of a multiwavelength campaign. Comparisons are made of the VHE emission during this time with the available data from other wavelengths.

P. M. Chadwick K. Lyons T. J. L. McComb K. J. Orford J. L. Osborne S. M. Rayner S. E. Shaw K. E. Turver

Cen X-3 is a well-studied high-mass accreting X-ray binary and a variable source of high energy gamma rays from 100 MeV to 1 TeV. The object has been extensively monitored with the University of Durham Mark 6 telescope. Results of observations, including those taken in 1998 and 1999, are reported. There is no evidence for time variability in all the VHE data. There is also no evidence for correlation of the VHE flux with the X-ray flux detected by BATSE and RXTE/ASM. A search for periodic emission, at or close to the X-ray spin period, in the VHE data yielded a 3 sigma upper limit to the pulsed flux of 2.0 x 10^-12 cm^-2 s^-1.

P. M. Chadwick K. Lyons T. J. L. McComb K. J. Orford J. L. Osborne S. M. Rayner S. E. Shaw K. E. Turver

A programme of detailed simulations of the response of the Durham Mark 6 atmospheric Cherenkov telescope is in progress. The effective collecting area for triggering by gamma-ray showers after application of selection criteria is derived as a function of energy. An initial result from the larger events in the 1996 and 1997 observations of the BL Lac PKS 2155-304 is that the time averaged flux above 1.5 TeV was (6.7 +/- 2.2) x 10^-8 m^-2 s^-1.

P. M. Chadwick K. Lyons T. J. L. McComb K. J. Orford J. L. Osborne S. M. Rayner S. E. Shaw K. E. Turver

Atmospheric Cerenkov telescopes are used to detect electromagnetic showers from primary gamma rays of energy > 300 GeV and to discriminate these from cascades due to hadrons using the shape and orientation of the Cerenkov images. The geomagnetic field affects the development of showers and diffuses and distorts the images. When the component of the field normal to the shower axis is sufficiently large (> 0.4 G) the performance of gamma ray telescopes may be affected.

P. M. Chadwick K. Lyons T. J. L. McComb K. J. Orford J. L. Osborne S. M. Rayner S. E. Shaw K. E. Turver

ROSAT observations of the Large Magellanic Cloud (LMC) have revealed a large diffuse X-ray arc around the point source RX J053335-6854.9. The relative locations of the diffuse and point sources suggest that they might originate from a common supernova explosion. We have analyzed the physical properties of the diffuse X-ray emission and determined that it is most likely a supernova remnant in a low-density medium in the LMC. We have also analyzed the X-ray and optical observations of RX J053335-6854.9 and concluded that it is a foreground dMe star in the solar neighborhood. Therefore, despite their positional coincidence, these two X-ray sources are physically unrelated.

J. D. Lowry Y. -H. Chu M. A. Guerrero R. A. Gruendl S. L. Snowden R. C. Smith

In an experiment to simulate the conditions in high optical power advanced gravitational wave detectors such as Advanced LIGO, we show that strong thermal lenses form in accordance with predictions and that they can be compensated using an intra-cavity compensation plate heated on its cylindrical surface. We show that high finesse ~1400 can be achieved in cavities with internal compensation plates, and that the cavity mode structure can be maintained by thermal compensation. It is also shown that the measurements allow a direct measurement of substrate optical absorption in the test mass and the compensation plate.

C. Zhao J. Degallaix L. Ju Y. Fan D. G. Blair B. J. J. Slagmolen M. B. Gray C. M. Mow Lowry D. E. McClellandl D. J. Hosken D. Mudge A. Brooks J. Munch P. J. Veitch M. A. Barton G. Billingsley

We develop an unsupervised machine learning algorithm for the automated discovery and identification of traveling waves in spatio-temporal systems governed by partial differential equations (PDEs). Our method uses sparse regression and subspace clustering to robustly identify translational invariances that can be leveraged to build improved reduced order models (ROMs). Invariances, whether translational or rotational, are well known to compromise the ability of ROMs to produce accurate and/or low-rank representations of the spatio-temporal dynamics. However, by discovering translations in a principled way, data can be shifted into a coordinate systems where quality, low-dimensional ROMs can be constructed. This approach can be used on either numerical or experimental data with or without knowledge of the governing equations. We demonstrate our method on a variety of PDEs of increasing difficulty, taken from the field of fluid dynamics, showing the efficacy and robustness of the proposed approach.

Ariana Mendible Steven L. Brunton Aleksandr Y. Aravkin Wes Lowrie J. Nathan Kutz

Historical experimental testing of high-altitude nuclear explosions (HANEs) are known to cause severe and detrimental effects to radio frequency signals and communications infrastructure. In order to study and predict the impact of HANEs, tractable computational approaches are required to model the complex physical processes involved in the detonation wave physics. Modern reduced-order models (ROMs) can enable long-time and many-parameter simulations with minimal computational cost. However, translational and scale invariances inherent to this type of wave propagation problem are known to limit traditional ROM approaches. Specifically, dimensionality reduction methods are typically ineffective in producing low-rank models when invariances are present in the data. In this work, an unsupervised machine learning method is used to discover coordinate systems that make such invariances amenable to traditional dimensionality reduction methods. The method, which has previously been demonstrated on one-dimensional translations, is extended to higher dimensions and additional invariances. A surrogate HANE system, i.e. a HANE-ROM, with one detonation wave is captured well at extremely low-rank. Two detonation-waves are also considered with various amounts of interaction between the waves, with improvements to low-rank models for multiple wave quantities with limited interaction.

Ariana Mendible Weston Lowrie Steven L. Brunton J. Nathan Kutz

In this paper, we classify the possible group structures on the set of $R$-valued points of an abelian variety, where $R$ is any real closed field. We make use of a family of abelian varieties that, in effect, allows one to quantify over all abelian varieties of a fixed dimension and degree of polarization in a first-order fashion.

Nathanial Lowry