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 EMCCDs ULTRASPEC Unique applications General advantages Limitations Future upgrades and instrumentation 14th October 2009Detectors for Astronomy 2009, ESO Garching2

EMCCDs Highest gains when low photon flux high-TIME resolution high-SPECTRAL resolution Effectively doubles the aperture of the telescope (ignoring read-out time) 14th October 2009Detectors for Astronomy 2009, ESO Garching3 ES Cet – 10sec exposure Top: avalanche mode Bottom:normal read-out mode (3e - noise)

ULTRASPEC E2V CCD 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 14th October 2009Detectors for Astronomy 2009, ESO Garching4

Unique Science - HTRA ULTRASPEC on ESO 3.6m and ESO NTT Bowen-blend echo-tomography of Scorpius X-1 and 4U 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 A search for optical pulsations in the Anomalous X-ray Pulsar XTE J Probing rapid multi-wavelength accretion-driven variability in the X-ray binaries GX 339-4, 4U 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 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 J Bowen blend echo-tomography of Sco X-1 using ISIS+L3CCD 14th October 2009Detectors for Astronomy 2009, ESO Garching5

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 th October 2009Detectors for Astronomy 2009, ESO Garching6

ULTRACAM observations of 4U Munoz-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 14th October 2009Detectors for Astronomy 2009, ESO Garching7 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 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) 14th October 2009Detectors for Astronomy 2009, ESO Garching8 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 4U th October 2009Detectors for Astronomy 2009, ESO Garching9

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 14th October 2009Detectors for Astronomy 2009, ESO Garching10

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) 14th October 2009Detectors for Astronomy 2009, ESO Garching11

Advantages II – Cosmic removal Cosmic rays only affect an individual frame ( s) rather than the entire exposure, so can be removed ‘cheaply’ in terms of S/N Raw ULTRASPEC frames – 10 x 60secs 14th October 2009Detectors for Astronomy 2009, ESO Garching12 Mean of 10 frames Median of 10 frames

Advantages II – Cosmic removal 14th October 2009Detectors for Astronomy 2009, ESO Garching13 “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 14th October 2009Detectors for Astronomy 2009, ESO Garching14

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 14th October 2009Detectors for Astronomy 2009, ESO Garching15 Screen-flat from FORS 1 with UV- optimised E2V CCD44-82 detector and 600I grism

Limitations Need 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 problem CIC rate of 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 problem CTE seen in ULTRASPEC frames but can be mitigated 14th October 2009Detectors for Astronomy 2009, ESO Garching16

Instrument upgrades X-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 lines Better 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 14th October 2009Detectors for Astronomy 2009, ESO Garching17

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 14th October 2009Detectors for Astronomy 2009, ESO Garching18