Presented by Anna Hourihane

1. Searching for Dwarf Novae in Globular Clusters Presented by Anna Hourihane Anna Hourihanea, Paul Callanana and Adrienne Coolb aDepartment of Physics, University College Cork, Ireland bDepartment of Physics and Astronomy, San Francisco State University, 1600 Holloway Avenue, San Francisco, CA 94132, USA.

2. Outline • What are Dwarf Novae (DN): background • Current state of affairs in the search for DN • Motivation and aims • Previous work • Observations and Data Analysis • Results • Future directions

3. Background Cataclysmic Variables (CVs): • Compact binary systems comprising white dwarf primary accreting through disk from low mass companion • Undergo 'cataclysmic' eruptions from time to time • E.g. Novae, Recurrent Novae, Dwarf Novae, Magnetic CVs • In magnetic CVs the disk is partially or wholly disrupted as accreted material is funnelled along field lines Dwarf Novae (DN): • Frequent, small, short outbursts • Brightening of accretion disk due to either disk instability or increase in mass transfer rate (a binary phenomenon)

4. CV Structure Magnetic Non-Magnetic White Dwarf Magnetic Field Gas Stream Accretion Disk White Dwarf Primary Secondary Accretion Column Bright Spot

5. The Current Situation • Many CVs have been foundin the field both in quiescence and in outburst (~40% of known field CVs have DN Outbursts (Downes et al. 2001)) • In globular clusters quiescent CVs have been identified on the basis of their bright UV and low-luminosity Xray emission. Many also show strong Ha emission • Paucity of observations of DN outbursts in globular clusters to date Apparent inconsistency between field and globular cluster DN outbursts

6. Why do we see so few DN outbursts in Globular Cluster CVs? Globular Cluster CVs: • Different formation mechanism to field CVs: three-body collisions and tidal capture • Play important role in dynamical evolution of clusters due large binding energy • Strong HeII emission suggests a possible magnetic nature • Truncation of inner accretion disk in Intermediate Polars appears to stabilise disk against outbursts • Perhaps accretion rates are very low

7. Motivation and Aims • To further investigate observed dearth of DN outbursts in globular clusters • To study accretion processes in systems of known distances and metallicities • To determine the frequency, amplitudes and durations of outbursts, and constrain the rise and fall times • To probe a new timescale by observing at 2-day intervals for several months

8. We present results from a long-term monitoring program of M22 in which we observe a DN eruption of a CV in the cluster core Previous Work • Sahu (2001) originally attributed a ~3 magnitude brightening of the same object to a gravitational micro- lensing of a bulge star by a foreground cluster member • Subsequent analysis by Anderson, Cool and King (2003) revealed cluster membership (proper motion studies using archival HST data) and strongly suggested a CV nature (colour analysis and outburst characteristics)

9. Bond et al (2005) recorded two further similar outbursts between 2001 and 2004 as part of MOA microlensing survey • Webb et al (2004) observed a ~1 keV absorption feature with XMM-Newton in an X-ray source with an error circle of radius 5” enclosing the (optical) CV position. This feature may be interpreted as cyclotron resonance implying an 'intermediate polar' nature for the CV

10. Observations • Data: Optical V-band images covering the period March- November 2004. Observations made with CTIO 1.3m telescope, Chile • Sampling: Two 300s exposures every second night, and every night for the duration of the outburst • Field of View: 6x6 arcminutes centred on cluster core

11. HST Finding Chart The CV is located at R.A. = 18h36m24s.66, dec = -23o54'35”.5 Close-up shows 2”x2” surrounding the CV during quiescence (top) and outburst maximum (bottom) Anderson et al (2003) V-band image showing location of CV (North is up)

12. Data Analysis Methods • Photometry performed using the ISIS image subtraction software of Alard & Lupton • Poor quality images discarded ('seeing' worse than 1.5”) • Remainder interpolated to same positional reference grid • Convolved to same seeing using a spatially-varying convolution kernel • Each subtracted from reference image (stack of best ~10% of images)

13. CV shows up in stacked image of subtracted frames • Stacked image of subtracted frames shows variable objects • Photometry was performed on the object at the known CV location to produce lightcurve data which was plotted in QDP.

14. Results • ISIS has remarkable power in crowded fields • Outburst duration is ~15 days and V-band amplitude ~3.5 magnitudes • ISIS outputs lightcurve data in units of differential flux • Cannot perform profile photometry with DAOPhot due to extreme crowding • Calibrated using quiescent V-band magnitude of 18.77 (Anderson et al., 2003) Sahu et al. (2001)

15. Discussion • Outburst duration (~15 days) and amplitude (~3.5 MV) consistent with previous studies • Confirm CV nature of source • Appears to be DN with recurrence timescale of ~1-2 years • Recurrence time long for DN, perhaps magnetic subtype: Intermediate Polar

16. Future Directions • Need careful monitoring of outburst lightcurve morphology • Ground-based studies have a role to play • We have proposed a similar study of NGC6397 – the closest globular cluster with a large known population of CVs.

17. References