NSF is currently funding the Experimental Astrophysics Group at the Space Sciences Laboratory of UC Berkeley to carry out a 3-year plan of high time-resolution astronomical observations with BVIT on the 10m SALT. Here is a brief overview of some of the topics that are being pursued:-
Optical Pulsars & Isolated Neutron Stars (Lead: Dr. Barry Welsh UCB/SSL)
Although over ~ 1000 fast rotating neutron stars (pulsars) have been observed at radio, gamma-ray and X-ray wavelengths, due to their inherent faintness there are only 8 known optical counterparts to these enigmatic objects, with pulse periods (and associated phase profiles) having been obtained for only 3 of them at visible wavelengths (Cocke et al. 1969, Wallace et al. 1977, Middleditch & Pennypacker 1985). Some ofthe observed pulsar optical emissionis of a thermal natureand is connected tothe hot neutron starsurface or to thepolar cap which is bombarded and heated bythe secondary electrons (orpositrons). However, there is alsoan appreciable non-thermal componentof the optical emission, which may be caused by synchrotron emission generated in theoptical range (Malov & Machabeli 2001). Analysis of the Crab pulsar’s visible spectrum has shown that energy-dependentanomalies exist in this emission andthat their presence cast doubtson the validity ofall currently proposed emission models (Romani et al 2001).
Sporadic, large-amplitude and short-duration (micro-second) bursts some 100 – 10,000 times more energetic than regular pulses have been reported from radio observations of several pulsars such as the Crab (Bhat et al. 2008) and PSR J1820-30A (Knight et al. 2005). These bursts are thought to be due to changes in the coherence of the radio emission, with the pulse profiles being composed of many superposed nanosecond structures. Such tiny timescales imply tremendous brightness temperatures of ~ 1038 K, making nanopulses the brightest pulses in the Universe (Hankins et al. 2003), and thus challenge our understanding of pulsar magnetospheric physics. Visible observations of the Crab pulsar show that the optical pulses recorded during a giant radio pulse period are on average 3% brighter than normal optical pulses (Shearer et al 2003), but no `giant’ optical pulses have yet been detected for this, or any other, pulsar. The optical pulses that are coincident with the giant radio pulses show slightly enhanced intensity, suggesting that the coherent (radio) and incoherent (optical) emissions produced in the pulsar’s magnetosphere are linked. Thus, the optical emission may be a reflection of the increased plasma density that causes the giant radio pulse event.
Bright, sporadic but recurrent short bursts of radio emission have recently been detected from a remarkable new class of neutron star, the Rotating Radio Transients, (RRATs) (McLaughlin et al 2006). About ~20 RRATs have been detected, and they are all characterized by short radio bursts of 2 to 30 ms duration, with peak fluxes from 100 mJy to 10Jy, and with average time intervals between (recurrent) bursts ranging from 3 min to 3 hours. Such bursts of emission may be similar to those widely observed from millisecond magnetars in gamma-rays and X-rays. Similar optical emission bursts from RRAT PSRJ1819-1458 have been searched for using the ULTRACAM fast CCD photometer without success (Dhillon et al 2006). However, this null result is not surprising given the limited 1800s of observation for that project. Obtaining a ratio of the optical-to-radio flux of these RRAT bursts would place meaningful limits on the spectral slope of any proposed emission mechanism from a plausible magnetar. The BVIT detector is well-suited for optical pulsar observations, as demonstrated by the B-band light curve of the (33 milli-sec) Crab pulsar obtained during the BVIT commissioning phase on the 1m Nickel telescope of the Lick Observatory in 2006 (Siegmund et al 2008a).
Our proposed pulsar observations with the upgraded BVIT on SALT will focus on 3 main aspects of their associated visible emission. Firstly we shall derived accurate pulse periods, phases and period derivatives for both PSR B0540-69 and the Vela pulsar recorded in at least 2 photometric bands. Secondly, we shall search for fast “giant” pulses during these extended observations of both pulsars. Thirdly, we shall record visible emission at the radio position of the RRATs (often unknown to < 10 arc sec) listed in Table 1 (McLaughlin et al 2006, Keane et al 2009) in order to search for their elusive sporadic optical bursts during several periods of extended observation (t >> 4000sec). Typically, between 5 to 50 radio pulses/hr have been detected from these RRATS. Based on the calculations used by ULTRACAM for the optical null detection of PSRJ1819-1458 (Dhillon et al 2006), if we assume a BVIT integration time of 3.5ms for a random 5mJ source pulse detected over a sky area of 9 sq.arc sec in 1 hour, we can place limiting magnitudes of B=18.2, V=17.7 & R = 17.3 for a 6-sigma (10-7 deviation from the mean) detection of a pulse from the approximate sky position of an RRAT source. If a potential source of visible emission is identified, we would propose follow-up low resolution spectral observations (on Gemini South or the Keck) to attempt to confirm the nature of the (magnetar?) emitting source.
Gamma-ray bursters are another class of transient phenomenon suited for visible observations with BVIT. These strong and fast flaring outbursts are normally discovered in the high-energy regime (e.g. using the SWIFT satellite) with most optical follow-up observations being performed using instruments with integration times exceeding tens of seconds, which are therefore unable to resolve any fast variability. However, Stefanescu et al. (2008) have observed extremely bright and rapid optical flaring in the Galactic transientSWIFT J195509.6+261406, with optical light curves (obtained with a photomultiplier tube) being similar to the high-energy light curves of soft gamma-ray repeaters and anomalous X-ray pulsars. Both types of source are thought to be neutron stars with extremely high magnetic fields (i.e. magnetars). This suggests that similar processes are in operation, but with strong emission in the optical, unlike in the case of other known magnetars. We note that most observational data on magnetars has been gained in the high-energy regime, and the implications for optical emission have been somewhat neglected. An intriguing model of an optical magnetar proposes optical ion cyclotron emission (Beloborodov et al. 2007). In this model, coherent microwave and radio emission emitted near the neutron star is absorbed higher in the magnetosphere by ions at their cyclotron resonance, and then re-emitted in the optical nearer to the poles, where the ion cyclotron cooling and transit times become comparable. However, tests of such a model are limited by the current complete lack of observations of such phenomena in the visible regime. We shall use the SWIFT satellite gamma-ray burst alert system (GCN circular web-page at http://gcn.gsfc.nasa.gov/gcn3_archive.html#tc1) to obtain (when possible) rapid follow-up observations of these objects with BVIT in order to record the Fourier power spectral density of any optical transient burst phenomena. These data will then be compared with g-ray and X-ray observations in order to constrain physical parameters such as the magnetar rotation period and the size of the emitting light cylinder (Stefanescu et al 2008).
| Pulsar/RRAT name | R.A. (2000) | Decl (2000) | V mag |
| Vela pulsar | 08 35 21 | -45 10 35 | 23.6 |
| PSR B0540-69 | 05 40 11 | -69 19 54 | 22.4 |
| RRAT J1819-1458 | 18 19 34 | -14 58 05 | > 19.9 |
| RRAT J0847-4316 | 08 47 57 | -43 16 57 | unknown |
| RRAT J1317-5759 | 13 17 46 | -57 59 31 | unknown |
| RRAT J1841-14 | 18 41.0 | -14 18.0 | unknown |
| RRAT J1554-52 | 15 54.0 | -52 10.0 | unknown |
| RRAT J1854+03 | 18 54 09 | +03 04 00 | unknown |
| RRAT J1444-6026 | 14 44 06 | -60 26 09 | unknown |
| RRAT J1826-1419 | 18 26 42 | -14 19 22 | unknown |
| RRAT J1913+1330 | 19 13 18 | +13 30 33 | unknown |
Table 1: List of proposed pulsars and RRATs for BVIT on SALT
Stellar Transits & Occultations (SAAO Lead: Dr. Amanda Gulbis)
To date, more than 400 planets have been discovered orbiting stars outside of our solar system. Nearly five dozen of these extrasolar planets have orbital inclinations such that the planet is seen to transit in front of the star as viewed from Earth. Parameters that can be derived from a transit light curve include planet-to-star radius, inclination of the planet’s orbit, stellar limb darkening, and timing (which, over multiple observations, can be used to detect perturbations due to other planets and possibly even moons) (e.g. Seager and Mallén-Ornelas 2003; Holman and Murry 2005). Very high-quality photometry of secondary eclipses, during the planetary occultation, can be used to place an upper limit on planetary albedo (Rowe et al. 2006). Knowledge of these parameters is critical to characterizing individual planets (their sizes, masses, internal structure, and atmospheric information) as well as understanding each extrasolar planetary system as a whole. The accuracy of transit photometry is ultimately limited by photon noise; therefore, BVIT on SALT (a high-speed, low-noise detector on a large telescope) provides an optimum configuration for deriving highly accurate parameters from transit light curves.
The SALT plateau is home to the southern hemisphere sites for SuperWASP (Wide Angle Search for Planets) and KELT (the Kilodegree Extremely Little Telescope). KELT, designed to find transiting planets around bright stars using a large field of view, is currently being deployed (Pepper et al. 2008). SuperWASP has to date discovered at least 18 transiting extrasolar planets. The most recent WASP planets include (i) the first planet to orbit in the opposite direction of the star’s rotation, which also has the lowest density of any known, transiting, hot Jupiter (Anderson et al. 2009), and (ii) a planet that is so large and close to its parent star that it is locked in what the press refer to as a “tango of death” (Hellier et al. 2009). Strong observational constraints are required to learn more about the characteristics of these unusual planets. The BVIT is ideally positioned to make critical (high S/N and high time-resolution) follow-up observations of newly discovered extrasolar planets from these projects. We will work with SALT consortium members, such as Saurabh Jha (Rutgers Univ.) and Stefan Dreizler (Univ. of Göttingen), who have significant expertise in extrasolar planet observations and have attempted transit observations with SALTICAM, a CCD based high-speed (< 10Hz) imager.
Also in conjunction with our collaborators in South Africa, we shall observe stellar occultations by small bodies in the solar system. We will focus on Kuiper Belt Objects (KBOs), since little is known about these distant and faint bodies. KBOs are the frozen leftovers from the formation period of the outer solar system, with the total mass in the Kuiper Belt being estimated to be 0.03 – 0.3 Earth masses and more than 1000 KBOs having thus far been detected. Thisregion is likelyto have been shaped by the orbital migration of the giant planets and perhaps other massivebodies; therefore, KBO sizes, spatialdistributions, and masses are important keys to understandingthe evolution our solar system as well as other planetarydisks. One intriguing possibility is that some KBOs could retain tenuous atmospheres that might be detectable through stellar occultation observations (Elliot and Kern 2003). Simple loss and retention models for volatile surface ices predict volatile retention on Pluto and Triton (known to have atmospheres) as well as on large KBOs such as Eris, Sedna, and possibly Haumea, Makemake, and Quaoar (Schaller and Brown, 2007). Another possibility is the serendipitous discovery of binaries, which are predicted to be commonplace by KBO formation models (Astakhov et al. 2005). High S/N and high-timing precision of stellar occultations, such as those afforded by BVIT, are required for the most accurate determination of KBO orbital geometries(to hundredths of an arcsec), size constraints (to a few km at the distance of Pluto), and spatial probing of the structure of any atmospheres (at the microbar level) (e.g. Gulbis et al. 2006, 2008, Elliot et al. 2007).
Most instruments that are optimized for stellar occultation observations are constrained by small fields of view, limited readout rates, and detector read noise. For example, the Portable Occultation Eclipse and Transit System (Gulbis et al. 2008) utilizes a 512 x 512 CCD with a readout rate of ~0.3 s (full frame, unbinned), read noise of ~5 electrons per pixel, and a pre-selected readout rate. The BVIT is ideally suited to occultation observations since the detector has no associated read noise and records photon data in a continuous stream, allowing the optimal time binning for highest S/N to be determined post-observation.
Table 2: Sample list of proposed KBO observations with BVIT on SALT
| Date and midtime (UT) | Body | Star Mag | Shadow Velocity (km/s) | SNR | Closest Approach (arcsec) | SALT Visibility Window (UT) | Scientific Motivation |
| 2010-08-12 21:52 | Pluto | 16.0 | 14.7 | 140 | 0.24 | 21:26–22:30 | monitor atmospheric evolution |
| 2011-06-07 02:31 | Pluto | 15.2 | 22.0 | 220 | 0.11 | 02:04–03:07 | monitor atmospheric evolution |
| 2011-07-25 02:00 | Charon | 15.7 | 21.0 | 140 | 0.24 | 01:50–02:53 | investigate non-sphericity |
| 2012-07-10 18:54 | Charon | 15.4 | 23.4 | 160 | 0.02 | 18:48–19:50 | investigate non-sphericity |
| 2013-06-08 22:18 | Quaoar | 12.1 | 25.0 | 940 | 0.27 | 22:07–23:28 | determine size, detect & characterize any atmosphere, search for companions |
aStellar candidates were found by using object ephemerides from JPL with M.I.T. Ephemeris Correction Model offsets applied
Although the viewing geometry of SALT is restrictive, this 10m telescope has already proven to be successful at observing occultations and eclipses by small bodies in the outer solar system. Christou et al. (2009) using SALT have obtained best-fit albedo ratios for several Uranian satellites. In Table 2, we have listed the most promising predicted KBO occultations viewable from SALT during the requested funding period. This list considers events for only the eight largest KBOs in angular size, including Pluto. Note that frequent stellar occultation observations by Pluto are particularly important given its evolving atmosphere (e.g. Elliot et al. 2007) and the imminent arrival of NASA’s New Horizons spacecraft in 2015. The current astrometric accuracy on the orbits of most KBOs is larger than the angular extent of the Earth at their orbital distances. Therefore, Table 2 is representative of events we are likely to observe. We will work with the M.I.T. Planetary Astronomy Laboratory (of which collaborator Gulbis is a member) as they generate increasingly accurate ephemerides for the large, known KBOs in order to plan BVIT observations. As single-chord observations can only place lower limits on object size, we will coordinate BVIT observations in conjunction with other sites to obtain multiple chords when feasible.
X-Ray Transient Sources (SAAO Lead: Prof. P. Charles)
Low-mass X-ray binaries (LMXBs) are systems in which a low-mass companion star transfers material onto a neutron star or a black hole. Most of the systems have orbital periods of a few hours to days and contain ordinary hydrogen-rich donor stars. X-ray transients (XRTs), or X-ray novae, are a sub-set of LMXRBs that undergo significant X-ray, optical and radio outbursts separated by years or even longer periods of quiescence. In their low X-ray state such sources are usually associated with an optically bright counterpart, during which fast flickering on time-scales as short as 20ms has been previously observed (Motch et al. 1985, Hynes et al 2003). In their “thermal” state (see Remillard & McClintock 2006), soft X-ray emission generated in the accretion disk dominates. These eruptive phases of very high luminosities are followed by long periods in quiescence, and such cycles are likely to have their origin in accretion disc instabilities associated with hydrogen ionization. While the X-ray timing properties of LMXBs are well established, little is known about short-time-scale non-orbital variations in the optical. On time-scales < 1min, the variability of the optical continuum or Hα emission in LMXBs and its correlation with X-rays is still largely unexplored; such studies are hampered by the faintness of most systems, even when in outburst. A still unanswered question concerning XRT black hole candidates is the physical origin of their rapid (milli-sec) variable optical emission. However, it is now widely accepted that emission lines in such systems form in the rotating accretion disc flow, with the strongest supporting evidence being in the form of radial velocity curves and Doppler tomograms (Charles & Coe 2006).
Previous optical studies of XRTs in quiescence have revealed an ellipsoidal modulated continuum signal (from the secondary star) superposed with prominent short-lived flares (Hynes et al 2003, Motch et al 1983). Although there is a gross correlation between UV, X-ray and optical variability in such sources, the peaks in the optical region generally lag those observed in the X-ray regime and the complex nature of the cross-correlation function of their respective flux intensities now argues strongly against reprocessing of X-rays from an accretion disk as a source for the observed optical emission (Spruit & Kanbach 2002, Gandhi et al 2008). However, in total contradiction note that in some cases the optical emission has been seen to actually lead the X-ray emission, which (being counter-intuitive) has led to many new scenarios (such as synchrotron emission from a jet, Gandhi et al 2008) being forwarded. In the case of Sco X-1 changes in the UV flux are tracked by changes in the Johnson B magnitude, and thus a linear relationship exists between the mass loss rate, dM/dt and visible B-band flux (Willis et al 1980).
The presence of many quasi-periodic oscillations (QPOs) and breaks in the frequency power density spectrum (PDS) of several LMXRBs is thought to indicate the characteristic time-scale at some transition radius in the disk. Motch et al (1983) found a 50% rms variability when the source was in an optically bright state, while Gandhi et al (2008) found only a 15% rms variability in the optically faint “hard” state.
In order to progress further on identifying the possible origin of the optical emission we will require simultaneous real-time X-ray data (such as the NASA RXTE), since a testable prediction from the relativistic outflow model is that the strength of the positive cross correlation function signal should be related to the prominence of the jet. As the source goes from a thermal to a “hard” state we would expect the jet to increase in strength, as should the related optical fractional variability rms of the data.
TABLE 3: Potential X-ray transient targets for BVIT
| XRT Name | R.A. (2000) | Decl (2000) | V mag |
| Cir X-1 | 15 20 41 | -57 10 00 | 21.4 |
| V616 Mon | 06 22 45 | -00 20 45 | 18.2 |
| V801 Ara | 16 40 56 | -53 45 05 | 17.5 |
| UY Vol | 07 48 33 | -67 45 00 | 17.2 |
| LMC X-2 | 05 20 28 | -71 57 53 | 18.0 |
| LMC X-3 | 05 38 56 | -64 04 56 | 17.2 |
| V395 Car | 09 22 35 | -63 17 41 | 15.5 |
| V821 Ara | 17 02 50 | -48 47 23 | 20.0 |
| IL Lup | 15 47 08 | -47 40 11 | 16.6 |
| GU Mus | 11 26 27 | -68 40 32 | 20.1 |
| BW Cir | 13 58 10 | -64 44 05 | 20.5 |
Although the availability of the RXTE satellite is not guaranteed beyond 2010, the SALT scientists (led by Prof. Phil Charles) have an on-going collaboration with the soon-to-be-launched ASTROSAT mission (Seetha et al 2006) such that simultaneous X-ray and BVIT observations of XRTs will be carried out. However, even without any X-ray data much can still be learned from high-time resolution visible observations of XRTs, especially in their quiescent state (Zurita et al 2003). Although the origin of their visible variability remains uncertain (with optical emission from an advective region, magnetic reconnection events in the disk and flickering from the accretion impact point being likely emission candidates), measurement of the PDS of optical QPO flare events can place meaningful limits on the size of the advection dominated accretion flow region (Shabaz et al. 2005). We plan to observe the XRT systems listed in Table 3, towards which X-ray QPOs have been previously observed, using an H-a filter with BVIT. We note that HST/FOS spectral observations of the Cir X-1 system have revealed a prominent H-alpha line superposed on an otherwise featureless continuum, such that that the emission line seen in this (and other) X-ray binary systems accounts for ~ 50% of the optical flux between 5000 and 6800Å (Mignani et al 1997). The H-alpha line emission is thought to be entirely ascribable to the accretion disk, and is characterized by a remarkable asymmetry. Our proposed BVIT observations will directly monitor the time-variable physical conditions of the gas associated with the accretion disk over time-scales << 1 sec, with our main goal being how the properties of H-alpha emission vary, if at all, with the accretion ‘state’ (i.e. thermal or hard) and luminosity of such systems. Fender et al (2009) have recently found evidence for an anti-correlation between the H-alpha equivalent width and overall X-ray luminosity of several of these systems, in particular, when in they are in the ‘hard’ X-ray state. By observing a large (~10) sample of the (normally under-observed) southern hemisphere XRTs and deriving their respective PDS in quiescence, we hope to be able to determine which types of optical variability signatures dominate this class of object’s emission, which in turn will isolate any contributions from variations in their inner disk flows and hence probe their physical nature. We note that the low duty cycle of CCD data acquisition schemes introduces large amounts of spurious power in a Fourier transform that can cover several frames.
Magnetic Cataclysmic Variables (Polars) (SAAO Lead: Dr.Stephen Potter)
Cataclysmic variables (CVs) are semi-detached binary systems in which a white dwarf primary star accretes matter via Roche Lobe overflow from a secondary star (which is typically a red dwarf). Most CVs accrete material onto the primary via an accretion disk, with a “bright spot” of enhanced emission in the shock-heated area where the gas stream from the secondary star hits the disk. In some CV systems the white dwarf magnetic field is sufficiently strong to be able to disrupt the accretion flow, such that the accreted material flows down the field lines, either directly from the gas stream (polars) or via a truncated disk (Intermediate polars). In polars the strong magnetic field of the white dwarf primary causes it to rotate synchronously with the orbital motion.
In January 2009 we observed the polar UZ For (mv ~ 18.0) with the proto-type BVIT on SALT. The CV system has a 126.5-min orbital period, close to the lower edge of the 2–3 h ‘period gap’, which is a sparsely populated region in the orbital period distribution of cataclysmic variables. The ballistic accretion stream from the secondary star is thought to be funneled along two sets of magnetic field lines that eventually impact on two regions near the magnetic poles of the white dwarf. Our new data shows that the eclipse lasts for ~ 469s and it resolves the previously known two `emission bumps’ located approximately half-way through the ingress and egress of the eclipse. These two features are where the eclipse of the two separate accretion regions has been spatially resolved. One new and intriguing feature of these data is that the total eclipse of UZ For is not completely flat, which suggests that we may be observing the eclipse of the photosphere of the white dwarf itself (i.e. see the small rise in flux at ~ 930 sec). This has not been detected before (see the superconducting tunnel junction observations from Perryman et al. 2001 for comparison: right panel of Fig. 8). If confirmed, this would be very important since it can place very tight constraints on the size of the white dwarf and hence lead to accurate estimates of the stellar mass and size of the companion in this system. Note that our B filtered observations are of superior quality compared to the white light observations of Perryman et al. (2001).
Our proposed program of BVIT observations of polars on SALT will be aimed at mapping the size, shape and brightness distributions of the accretion regions on the surface of the white dwarf in ~ 10 eclipsing systems (see Table 4). In practice, this is equivalent to indirectly imaging regions approximately the size of Iceland on a white dwarf that is Earth sized! The method of data analysis for obtaining these polar system parameters has been fully demonstrated on SALT for the eclipsing polar SDSS J015543.40+002807.2 by O‘Donoghue et al (2006). Briefly this analysis entails modeling of the gradients of the slopes of the ingresses and egresses of the 2 emission regions. Furthermore, measurements of the duration of the eclipse of the centre of the primary Roche lobe will yield a relation between two key orbital parameters: q, the ratio of the mass of the secondary to that of the primary, and i, the inclination of the observer with respect to the binary orbital plane (Horne 1985).
An additional science goal in the study of Polars is investigating the nature of Quasi Periodic Oscillations (QPOs). At present QPOs are thought to come in 2 flavors. The ~ 1- 5 s ones probably arise as oscillations occurring in the shock at the accretion region. By observing them in different systems that possess different magnetic field strengths we will gain a better understanding of the physics behind the shocks by looking at the effect that cyclotron cooling has on QPOs. Also, by looking at the same polar system when in different emission states we can investigate how the accretion rate affects the frequency and nature of QPOs. The addition of simultaneous multi-color information will also enable us to obtain the broad spectroscopic (and thus temperature) profiles of the QPOs. BVIT data also allows the study of these rapid a (QPOs) on time-scales <0.5 sec. A detailed Fourier analysis of our UZ For observations reveals flickering type variations on sub-second to several minute time-scales. Our collaborators at SALT are currently in the process of publishing a detailed paper on the eclipse characteristics of this system.
| Polar CV Name | R.A. (2000) | Decl (2000) | V mag |
| FL Cet | 01 55 43 | 00 28 07 | 15.5 |
| WW Hor | 02 36 11 | -52 19 14 | 18.4 |
| HY Eri | 05 01 46 | -03 59 21 | 17.5 |
| V1309 Ori | 05 15 41 | 01 04 41 | 15.5 |
| MN Hya | 09 29 07 | -24 05 05 | 17.0 |
| V895 Cen | 14 29 27 | -38 04 10 | 16.5 |
| V2301 Oph | 18 00 36 | 08 10 13 | 16.5 |
| CTCV J1928-5001 | 19 28 33 | -50 01 34 | 18.0 |
| V1432 Aql | 19 40 11 | -10 25 26 | 15.5 |
| HU Aqr | 21 07 58 | -05 17 39 | 15.3 |
| SDSS J205017-053626 | 20 50 17 | -05 36 26 | 16.0 |
Table 4: Proposed list of Polar CV systems for BVIT on SALT
The origin of the few minute-long QPOs (the second flavour) is still unknown. Theories involving feedback mechanisms through irradiation of the secondary and/or some mechanism at the magnetic coupling region have been proposed. By observing several systems covering a large parameter space of magnetic field, accretion rate, mass ratios, white dwarf mass etc, we can search for possible correlations that may indicate the physical mechanism at work for these longer period eruptive events. An unanswered question concerning the emission of QPOs is whether they arise in the continuum or in emission lines. By obtaining simultaneous continuum (or photo-polarimetry) and line (H-alpha) photometry we can directly compare the QPO flux modulations. The photo-polarimetry observations are sensitive to emission from the accretion hot spot, the H-alpha observations are sensitive to emission from the reprocessing sites and the BVR (grism) photometry records emission from the system as a whole. In order to perform such studies we shall make use of the photo-polarimeter on the 1.9m Radcliffe telescope (located next to the SALT facility) to carry out simultaneous measurements with the BVIT system. We note that our SAAO collaborators have recently discovered the first evidence of polarized QPOs in a polar (Potter et al. 2009)
Other Variable Objects of Interest
An important result from NASA’s on-going Kepler mission is that the majority of stars seem to be variable at a level > 10-4 over periods typically less than weeks. However, although the “long” term variability of stars is currently being measured by the Kepler satellite and ground based surveys such as the Large Synpotic Survey Telescope (LSST), the very short-term (t < 1 sec) variability of stars as a function of their spectral type is still not well categorized or understood. As such, BVIT plans to:
1) Carry out a survey of short-term variability ( t << 1 sec) versus spectral type for targets recorded over a 100 minute time-frame. By using suitable Sloan Sky Survey footprints we shall simultaneously observe many targets of different spectral types, such that their short-term variability characteristics can be measured and posted to an on-line catalogue of stellar spectral type versus (short-term) variability characteristics.
2) We shall observe several exo-planet transits at high time-resolution to fully characterize the entry and exit light curves, thus providing a far higher accuracy timing for these transit periods than previous observations. By observing with BVIT through a narrow-band H-alpha filter the we shall be able to greatly improve the timing precision of the current predictions of the transit entry and exit periods, which have all been gained using CCD systems at lower timing cadence. By re-observing the exo-planet transits over several orbits with BVIT and comparing each of these derived transit periods (with accuracies < 1 sec), we may be able to comment on whether the transits are caused by single or multi-object exoplanet systems.