Presented by

Anna Hourihane

Searching for Dwarf Novae in Globular Clusters

Anna Hourihanea, Paul Callanana and Adrienne Coolb

aDepartment of Physics, University College Cork, IrelandbDepartment of Physics and Astronomy, San Francisco State University,

1600 Holloway Avenue, San Francisco, CA 94132, USA.

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

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)

Non-Magnetic Magnetic

SecondaryWhite DwarfPrimary

Accretion Disk

Bright Spot

Gas Stream

Accretion Column

White Dwarf Magnetic Field

CV Structure

The Current Situation

● Many CVs have been found in 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 H emission

● Paucity of observations of DN outbursts in globular clustersto date

Apparent inconsistency between field and globular cluster DNoutbursts

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 Polarsappears to stabilise disk against outbursts● Perhaps accretion rates are very low

Motivation and Aims

● To further investigate observed dearth of DN outbursts in globular clusters

● To study accretion processes in systems of knowndistances 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 intervalsfor several months

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)

● 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 circleof radius 5” enclosing the (optical) CV position. This feature may be interpreted as cyclotron resonance implying an 'intermediate polar' nature for the CV

Observations

● Data: Optical V-band images covering the period March-November 2004. Observations made with CTIO 1.3mtelescope, 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

HST Finding ChartThe CV is located at R.A. =18h36m24s.66, dec = -23o54'35”.5Close-up shows 2”x2” surroundingthe CV during quiescence (top) andoutburst maximum (bottom)

V-band image showing location

of CV

(North is up)

Anderson et al (2003)

Data Analysis Methods

● Photometry performed using the ISIS image subtractionsoftware 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-varyingconvolution kernel

● Each subtracted from reference image (stack of best ~10% of images)

● Stacked image of subtracted frames shows variable objects

● Photometry was performed on the object at the knownCV location to produce lightcurve data which was plotted in QDP.

CV shows up in stacked imageof subtracted frames

Results

Sahu et al. (2001)

● 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)

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

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 closestglobular cluster with a large known population of CVs.

References