Phonon sidebands of color centers in hexagonal boron nitride

Low temperature photoluminescence spectra of a color center in hexagonal boron nitride are analyzed. The acoustic phonon sideband can be described by a deformation coupling proportional to strain to a phonon bath that is effectively two dimensional. The optical phonon band is described by Frohlich coupling to the LO-branches, and a deformation coupling proportional to lattice displacement for the TO-branch. The resonances expressed in the optical band vary from defect to defect, in some emitters, coupling to out-of-plane polarized phonons is reported.Comment: 7 pages, 4 fig

Searching for I band variability in stars in the M/L spectral transition region

We report on I band photometric observations of 21 stars with spectral types between M8 and L4 made using the Isaac Newton Telescope. The total amount of time for observations which had a cadence of <2.3 mins was 58.5 hrs, with additional data with lower cadence. We test for photometric variability using the Kruskal-Wallis H-test and find that 4 sources (2MASS J10224821+5825453, 2MASS J07464256+2000321, 2MASS J16262034+3925190 and 2MASS J12464678+4027150) were found to be significantly variable at least on one epoch. Three of these sources are reported as photometrically variable for the first time. If we include sources which were deemed marginally variable, the number of variable sources is 6 (29 percent). No flares were detected from any source. The percentage of sources which we found were variable is similar to previous studies. We summarise the mechanisms which have been put forward to explain the light curves of brown dwarfs.Comment: Accepted MNRAS 6 page

Spectroscopic observations of the eclipsing Polar MN Hya (RX J0929--24)

We present low--medium resolution optical spectroscopy of the eclipsing AM Her system MN Hya (RX J0929--24). We determine the magnetic field strength at the primary accretion region of the white dwarf to be 42MG from the spacing of cyclotron features visible during ~0.4--0.7. From spectra taken during the eclipse we find that the secondary has a M3--4 spectral type. Combined with the eclipse photometry of Sekiguchi, Nakada & Bassett and an estimate of the interstellar extinction we find a distance of ~300--700pc. We find unusual line variations at phase ~0.9: Halpha is seen in absorption and emission. This is at the same point in the orbital phase that a prominent absorption dip is seen in soft X-rays.Comment: accepted by MNRAS, 7 pages, 5 fig

Suppression of X-rays during an optical outburst of the helium dwarf nova KL Dra

KL Dra is a helium accreting AM CVn binary system with an orbital period close to 25 mins. Approximately every 60 days there is a 4 mag optical outburst lasting ~10 days. We present the most sensitive X-ray observations made of an AM CVn system during an outburst cycle. A series of eight observations were made using XMM-Newton which started shortly after the onset of an optical outburst. We find that X-rays are suppressed during the optical outburst. There is some evidence for a spectral evolution of the X-ray spectrum during the course of the outburst. A periodic modulation is seen in the UV data at three epochs -- this is a signature of the binary orbital or the super-hump period. The temperature of the X-ray emitting plasma is cooler compared to dwarf novae, which may suggest a wind is the origin of a significant fraction of the X-ray flux.Comment: Accepted by MNRA

Swift observations of the 2015 outburst of AG Peg -- from slow nova to classical symbiotic outburst

Symbiotic stars often contain white dwarfs with quasi-steady shell burning on their surfaces. However, in most symbiotics, the origin of this burning is unclear. In symbiotic slow novae, however, it is linked to a past thermonuclear runaway. In June 2015, the symbiotic slow nova AG Peg was seen in only its second optical outburst since 1850. This recent outburst was of much shorter duration and lower amplitude than the earlier eruption, and it contained multiple peaks -- like outbursts in classical symbiotic stars such as Z And. We report Swift X-ray and UV observations of AG Peg made between June 2015 and January 2016. The X-ray flux was markedly variable on a time scale of days, particularly during four days near optical maximum, when the X-rays became bright and soft. This strong X-ray variability continued for another month, after which the X-rays hardened as the optical flux declined. The UV flux was high throughout the outburst, consistent with quasi-steady shell burning on the white dwarf. Given that accretion disks around white dwarfs with shell burning do not generally produce detectable X-rays (due to Compton-cooling of the boundary layer), the X-rays probably originated via shocks in the ejecta. As the X-ray photo-electric absorption did not vary significantly, the X-ray variability may directly link to the properties of the shocked material. AG Peg's transition from a slow symbiotic nova (which drove the 1850 outburst) to a classical symbiotic star suggests that shell burning in at least some symbiotic stars is residual burning from prior novae.Comment: Accepted by MNRAS 23 June 2016. Manuscript submitted in original form 5 April 201