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Armagh Observatory

RApid Temporal Survey (RATS)

This is the homepage of the RApid Time Survey (or RATS for short) home page. The main aim of this survey is to discover new stellar binary systems with orbital periods less than ~80 min. It is being led by Gavin Ramsay (Armagh Observatory) and Pasi Hakala (Tuorla Observatory) and collaborators include Ralf Napiwotzki (Herts), Harry Lehto (Tuorla), Gijs Nelemans (Nijmegen) and Steve Potter (SAAO).

The intensity of stellar objects can vary on a wide range of time-scales, ranging from seconds to months to years. A large number of projects now exist whose aim is to detect such varying sources. The reasons for this are many, but include the search for extra-solar planets and interacting binary stars. Most of these surveys are sensitive to timescales longer than a day. It is only recently that such surveys have been sensitive to shorter term timescales. For instance, the 0.3 deg x 0.3 deg survey using the Canada France Hawaii Telescope was sensitive to variations on timescales as short as ~8 min (Lott et al 2002). While the Faint Sky Variability Survey has a much larger survey area, it was sensitive to variations only as short as 24 min (Groot et al 2003, Morales-Rueda et al 2004, 2006). In principle the SuperWasp project is sensitive to variations on timescales as short as a few mins (Christian et al 2004). However, they are sensitive to relatively bright objects, V~7--15.

Why is it important that we extend the parameter search down to periods shorter than 10 min? Recently several objects have been discovered, (RX J0806+15 and RX J1914+24), in which coherent intensity variations have been detected on timescales of ~10 min or less, with the shortest being 5.4 mins (see Cropper et al 2004 for a review). It is thought that these systems are interacting white dwarf-white dwarf pairs which have no accretion disc, and the observed period represents the binary orbital period. As such, they are expected to be amongst the first sources to be detected using LISA, the planned gravitational wave observatory (Nelemans, Yungleson & Portegies Zwart 2004). If these periods can be confirmed as their orbital period, then would be at the lower end of the orbital period distribution of interacting ultra-compact binary systems with white dwarf primaries (these objects are also called AM CVn systems). For a recent review of ultra-compacts see the recent article by Gijs Nelemans which appeared in Physics Today in July 2006.

Our strategy is to use wide field camera's on 2m class telescopes and take short exposures (~30 sec) in white light of the same field of sky for 2-3 hrs. We search for variable objects in each field and for those objects which are variable (in particular those showing modulation periods less than ~80 min) followup photometry and spectroscopy are required to determine their nature.

Pilot Survey:
In our pilot dataset taken with the INT and the Wide Field Camera in Nov 2003 we discovered nearly 50 new variable objects. Many of these varied on timescales much longer than 1 hr. The results of this pilot survey have been published by Ramsay & Hakala (2005) where positions of all the variable objects are listed. However, only 4 objects showed a modulation on a timescale of 1 hour or less. Using followup data we find that RAT J0455+1305 is a pulsating (on a period of 374 sec) subdwarf B (sdB) star of the EC 14026 type. We have modelled its spectrum and determine Teff = 29200+/- 1900K and log g = 5.2+/-0.3 which locates it on the cool edge of the EC 14026 instability strip. It has a modulation amplitude which is one of the highest of any known EC 14026 star. Based on their spectra, photometric variability and their infra-red colours, we find that RAT J0449+1756, RAT J0455+1254 and RAT J0807+1510 are likely to be SX Phe stars - dwarf delta Sct stars. Our results show that our observing strategy is a good method for finding rare pulsating stars. These results have been published by Ramsay, Napiwotzki, Hakala & Lehto (2006).

Further Observations:
In addition to the pilot survey, we obtained data of 12 fields using the Wide Field Imager on the ESO 2.2m telescope in June 2005 and another 8 fields using the Wide Field Camera on the INT again in June 2005. Several of these fields included globular clusters. The analysis of the ESO data is complete and spectra have been obtained of a number of the variable objects. Details of the variable objects will be announced in due course.

You can access one of Gav's seminar talks on ultra-compact binaries and the RATS project here.

There was a week long workshop on ultra-compacts held in Nijmegen, The Netherlands, in July 2005. The presentations can be accessed from this site.

Links to other projects whose aim is to detect new ultra-compacts include the Faint Variability Survey , and the Omega White Survey.