Kieran O’Brien

ESO, Paranal Science Operations

(UCSB, Santa Barbara, California)

Vik Dhillon (Sheffield, UK), Tom Marsh (Warwick, UK), Derek Ives (UKATC, ESO), Naidu Bezawada (UKATC)

Outline of talk

EMCCDsULTRASPECUnique applicationsGeneral advantagesLimitationsFuture upgrades and instrumentation

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EMCCDsHighest gains when low

photon flux high-TIME resolutionhigh-SPECTRAL

resolutionEffectively doubles the

aperture of the telescope (ignoring read-out time)

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ES Cet – 10sec exposureTop: avalanche modeBottom: normal read-out

mode (3e- noise)

ULTRASPEC

E2V CCD201-20 1024 x 1024 pixel, frame transfer device 536-stage electron multiplying register SDSU controller with custom high-voltage clock board Mounted on EFOSC2 on ESO 3.6m and NTT Range of grisms available, including VPH around HeII (468.6nm) and H-

alpha (656.3nm) Imaging mode also possible/used Dedicated real-time data analysis pipeline

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Unique Science - HTRAULTRASPEC on ESO 3.6m and ESO NTT Bowen-blend echo-tomography of Scorpius X-1 and 4U 1636-536 using ULTRASPEC Revealing the population of Ultra-Compact X-ray Binaries Studying brown dwarf weather with ULTRASPEC Understanding dwarf nova oscillations High-speed phase resolved spectroscopy of 4U 1822-371 A search for optical pulsations in the Anomalous X-ray Pulsar XTE J1810-197 Probing rapid multi-wavelength accretion-driven variability in the X-ray binaries GX

339-4, 4U 1957+11 and GX 9+9 High time-resolution imaging and spectroscopy time series of early GRB afterglows

with ULTRASPEC on NTT. Bowen-blend echo-tomography of EXO 0748-676 using ULTRASPEC High-speed optical spectroscopy of the Vela pulsar The rotation rates of white dwarfs in binaries Spectral eclipse mapping of accretion discs in Cataclysmic Variables

… also QUCAM on WHT High-time resolution spectroscopy of the eclipsing double degenerate SDSS

J0926+3624 Bowen blend echo-tomography of Sco X-1 using ISIS+L3CCD

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Bowen-blend echo-tomography• In Low-Mass X-ray Binaries, some of the optical emission is the result of reprocessing of X-rays into lower energy optical/IR emission• Phase-resolved optical spectroscopy reveals that the Bowen-blend fluorescence lines (~464nm) are a centred on the irradiated face of the companion star• Correlated X-ray and Bowen emission will tell us the offset (in light seconds) between the X-ray source and the companion star.• This is a function of the binary phase and will enable us to determine the inclination of the system

From O’Brien, et al. 2002

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ULTRACAM observations of 4U1636-536Munoz-Darias et al. (2009),

observed 3 simultaneous X-ray/optical bursts with the triple-beam imager ULTRACAM, mounted at the VLT, equipped with a special NB filter

Continuum contribution subtracted from the flux in the red-channel

Uncertainty in the continuum subtraction leads to uncertainty in the delay

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With fast spectroscopy we can accurately remove the continuum contribution, enabling us to measure the inclination and subsequently the Neutron Star mass.

ULTRASPEC observations of EXO 0748-676

Lightcurves show 4 simultaneous bursts (one ‘random’ example above)

Cross-correlation analysis shows different delays between continuum (right, top) and Bowen (right, middle).

However, statistics are not good enough for the continuum subtracted case (right, bottom)

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Feb ’08: ULTRASPEC on ESO 3.6m

Commissioning and Science Demonstration run

Problems with CTE and CIC, which have since been solved

ULTRASPEC observations of 4U1636-536

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General applications

Warning: spoiler!!!You cannot lose….

Always have to option of using a ‘standard’ (2-3 e-) readout port

… unless you need large mosaics

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Advantages I – duty cycle

Simulation based on 42-m ELT with an EMCCD with RON= 0e- (cf. 3.6e-), R=5000, T=11,000K, dark sky (Courtesy: Tom Marsh)

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Advantages II – Cosmic removalCosmic rays only affect an individual frame (100-102s) rather than the entire exposure, so can be removed ‘cheaply’ in terms of S/N

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Mean of 10 frames

Median of 10 frames

Advantages II – Cosmic removal

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“Real life” example:3800 second spectrum of a Quasar taken with UVES

Advantages IV – Sky subtraction

Simulation of residuals from sky-subtraction in ‘nod-and-shuffle’ mode at different nod-periods shows good agreement with measured values.Taken from Glazebrook & Bland-Hawthorn (2001)

Trapping sites limit number of shuffles (~100) and hence minimum dwell time

Significant power remains at high frequencies

Typical timescale for PCM with UVES would be ~10secs (less with improvements in CIC)

Simple beam-switching would enable extremely accurate sky removal

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Advantages III – Fringe removal

Variations of interpolated sky due to fringing are removed due to common path of sky and object

Uncertainties due to instrument flexure is reduced (removed?) as sample rate is increased

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Screen-flat from FORS 1 with UV-optimised E2V CCD44-82 detector and 600I grism

LimitationsNeed a larger format!!!

Currently only available in 1k x 1k format, which is not useful in majority of cases. Need (at least?) 2k x 4k buttable devices Multiple (8) read-out ports (10Mhz) to avoid the need to window,

especially desirable for cross-dispersed instruments

CIC a problemCIC rate of 10-3 e- pix-1 frame-1 is achievable. In photon-

counting mode this leads to effective RON of 0.1 e- pix-1 for a typical exposure This should be reduced even further as devices are better

understood.

CTE a problemCTE seen in ULTRASPEC frames but can be mitigated

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Instrument upgradesX-Shooter: An EMCCD would allow telescope nodding to

improve the sky subtraction on the IR-arm without the additional read-out noise on the UV and VIS-arms.

High resolution spectrographs (UVES, FLAMES, HIRES…)RON limited between sky linesBetter sky subtraction and cosmic removal allow longer

integration times Low resolution spectrographs (FORS, VIMOS, LRIS,

GMOS, DEIMOS…)>32 2k x 4k devices would benefit from EMCCD upgrade

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Future Instruments…

double white-dwarf system, 322second orbital period, V=21.1, 8hrs of observations with 30-sec integrations

a) FORS2 with conventional CCD on 8-m, b) with an EMCCD on 8-m, c) conventional CCD on a 42-m, d) frame-transfer CCD on a 42-m and e) an EMCCD on 42-m

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