STACEE status report

1. STACEE status report Presented to SAGENAP panel, April 15, 2004 Washington, DC K. Ragan McGill University, Montreal for the STACEE collaboration • Presentation of experiment • Science issues & goals • Detector status • Recent results • Outlook/Conclusions

2. Presentation of Experiment • STACEE is based at Sandia National Labs, Albuquerque NM • uses 64 heliostats of the National Solar Thermal Test Facility (NSTTF) and custom secondary optics to capture Cherenkov light on PMT cameras Size of heliostat field well matched to Cherenkov footprint on ground: ~250 m (HESS diagram)

3. Presentation of Experiment • NSTTF is a user facility and we incur costs to be present and active there. Currently costs are approx. $100/hour for heliostat usage; • Heliostats are well maintained; time lost to malfunctioning is small (although non-zero!) • Currently retro-fitting heliostats with more rapid motors for GRB response.

4. Presentation of Experiment Three 1.9m secondaries on 260’ level have 16-PMT cameras tessellated secondary (1.9 m diameter) PMT ‘camera’ Two 1.1m secondaries on 220’ level have 8-PMT cameras

5. Presentation of Experiment • custom delay & trigger electronics designed to capture events in programmable window (8-16 ns) • full FADC waveform digitization on each channel at 1 GHz (more later) custom delay/trigger (8 channels per board) Commercial (Acqiris) FADC modules (4 channels per board)

6. Presentation of Experiment • Also dedicated atmospheric monitoring station, with: • PMT-based photometry • CCD for autoguiding • infrared radiometers • weather station • data stream: 64 waveforms containing full timing and amplitude information, plus: • GPS timestamp • PMT currents & rates • Heliostat position data • Atmospheric monitoring data

7. Presentation of Experiment Due to large mirror area, STACEE achieves lower energy threshold than imagers & is sensitive below 100 GeV This will continue to be an important niche in the early VERITAS/GLAST era Simulation of response to Crab flux: “threshold”

8. Presentation of Experiment The STACEE Collaboration: • Alberta: Gingrich + 1 MSc student • Barnard/Columbia: Mukherjee + 1 PhD student • Case Western: Covault + 2 post-docs (Driscoll, Colombo) • McGill: Hanna + Ragan + post-doc (Kildea) + 4 students (3 PhD, 1 MSc) • UCLA: Ong + research scientist (Zweerink) + 2 PhD students • UCSC/SCIPP: Williams

9. Presentation of Experiment STACEE Milestones: • 1997: SAGENAP presentation; first STACEE funding • 2000: detection of Crab nebula with 32-channel instrument at 190 GeV threshold (ApJ 547, 949 (2001)) • 2001: detection of Mrk421 flare with STACEE-48 (ApJ 579, L5 (2002)) • 2002: full STACEE-64 commissioned • 2003: WComae upper limit at ~130 GeV threshold (ApJ in press) • 2004-2006(7) Continued operations Results published in ApJ and ApJL, as well as numerous conference proceedings. Three technical papers in NIM. Four PhD’s, four MSc’s graduated; 6 more PhD’s + 2 MSc’s in pipeline; numerous undergraduates trained.

10. Science Issues and Goals STACEE targets are primarily AGNs, pulsars, GRBs; but also EGRET UnIDs. Some clear questions: • AGN: • nature of primary particles (leptonic or hadronic?) F. Halzen

11. Science Issues and Goals exploration of EBL absorption effects, primarily through multi-wavelength campaigns Absorption exp(-tau) g Energy

12. Science Issues and Goals • pulsars: • energy of cutoff of pulsed component (polar cap vs. outer gap)? • GRBs: • maximum energy of burst emission • luminosity and energy evolution during afterglow

13. Science Issues and Goals • EGRET UNIDs: • most EGRET sources are unidentified • huge potential here for study: for example, recent HEGRA observation of TeV2032 may be archetype of cosmic ray ‘nursery’ STACEE can address all of these issues with its unique sensitivity at E<100 GeV ! HEGRA (Astron. Astrophys. 393 (2000) L37

14. Detector Status Current status: • STACEE-64 fully constructed by late 2001 • FADCs fully installed in late spring 2002 • first full season with complete detector configuration: 2002-2003 • large data samples on AGNs, Crab • Crab optical pulsar observed to confirm timing • still on learning curve with FADCs: • padding to account for field darkness differences • in-situ gain measurements • advanced reconstruction techniques

15. Detector Status Crab pulsar is an exciting target in STACEE energy regime; • we took special data to observe optical pulsar and confirm absolute timing; (HST data) light pollution @ 120 Hz non-Cherenkov data filtered to remove high-freq components

16. Detector Status • digitize PMT anode current at 10 kHz, GPS time stamp periodically, fold at known ephemeris of Crab STACEE phase diagram STACEE phase diagram (simulation) Observation of optical pulsar confirms both absolute timing and optical throughput

17. Detector Status FADC information vastly more powerful than previous TDC + ADC combination, and we are still on learning curve here: One channel discriminator threshold nominal trigger position

18. Detector Status Stars (no excess expected) Eg, padding to account for field darkness differences: • excess night sky background (due to different star fields) in ON or OFF can compromise ON/OFF comparison: can mimic g signal or hide it, by promoting sub-threshold events; Average On-Off current difference

19. Detector Status • excess light leads to larger current, and to larger variance of ‘baseline’ in AC-coupled PMT traces • critical to correct for this with ‘padding’: adjustment of lower light data by addition of noise; • so make library of NSB pulses; • catalogue them as a function of variance (aka current); • use them to ‘pad’ lower-light-level data to variance/current of noisier data • FADC data are essential • it works !

20. Detector Status Schematically: off-source trace + NSB library trace on-source trace Do this channel-by-channel; then reimpose trigger and do direct on-off comparison padded trace =

21. Detector Status Advanced reconstruction techniques using full waveform information also promise superior directional and energy reconstruction: Assuming shower core at array center Using full waveform information to reconstruct shower core

22. Detector Status Both 2002-2003 and 2003-2004 seasons concentrated on detector studies, Crab, and AGN data:

23. Detector Status Details for current 2003-2004 season: • EGRET source; extreme BLL; multi-wavelength campaign with RXTE & VERITAS as well as lower l – first such comprehensive broadband campaign to our knowledge; contemporaneous data will provide input to modelers; higher z, so possible EBL information; • Standard Candle; pulsar energy limit & spectrum both worthy goals; • EGRET source, with moderate z, C&G predictions; best source available in this RA range; • Well studied, EGRET source; detected by ACT incl. STACEE; useful for instrument studies if flaring and contemporaneous data with VERITAS; • Hard EGRET spectrum; LBL; well-measured in X; detailed predictions for both electronic SSC and proton models; • XBL, not EGRET source; TeV flux predicted from X-ray data, found by ACT, • Similar to Mrk421, likely data if in flaring state. • Predictions: ~7 prompt follow-ups per year in SWIFT era. GRB @ 100 GeV would be seminal ! Active participation in multi-l campaigns, ToOs, GCN alerts

24. Detector Status GRB data acquired: Bursts from 9/01/02 through 10/31/03 with alert within 90’ of burst

25. Detector Status Other, more speculative sources possible in future: • EGRET source with photons > 30 GeV; AGN at unknown redshift; competes with Crab; • Most persistent EGRET UnID; Chandra X-ray position; postulated Geminga-like (radio-quiet) pulsar; • EGRET pulsar; SNR CTB80; • Non-EGRET; XBL; C&G TeV flux predictions • First TeV-discovered source; extended & confused region; possibly OB-association, postulated as archetype of cosmic-ray production regions; • Low z EGRET source; IBL; “3C66A-like”; not seen by ACTs; • Non-EGRET AGN, seen by ACTs; XBL; “421-like”; spectrum measurement desirable.

26. Recent Results W Comae (aka ON+231): • z=0.102 EGRET blazar with hard spectrum, consistent with power law of a=1.73 +/- 0.18; • undetected by imagers (at E>300 GeV); • extensively studied at X-ray energies; • nice test case for leptonic vs hadronic models of blazar emission: • leptonic models predict emission cuts off below ~100 GeV; • hadronic models allow emission to significantly higher energies;

27. Recent Results W Comae data: • STACEE acquired data on WComae in Mar-Jun 2003: • 34 ON-OFF pairs, 13.5 ON-source hours • after quality cuts on heliostat pointing, stability of L1 trigger rates, and DAQ performance: 10.5 hrs ON-source; • raw ON-excess at 4.6s level; but typically 1-2 mA current excess in ON data; • library padding reduces ON excess to 0.88 s, consistent with no emission; • derived 95% UL on flux is typically • ~0.5-1.0x10-10cm-2 s-1 • at threshold energy of ~130-160 GeV • result accepted for publication in ApJ, ~June 2004

28. Recent Results Resulting upper limits: proton synchrotron models, with low and high EBL absorption EGRET power-law STACEE data is starting to constrain models

29. Recent Results 1H1426+428: • z=0.129 BL Lac; • unobserved by EGRET; • detected by imagers (HEGRA, CAT, Whipple) at E>300 GeV, data consistent with power law (a = 3.5); • STACEE took 22 ON/OFF pairs in Apr-July 2003: • 10 ON-source hours in raw-data; • 7.5 ON-source hours after quality cuts (helios + L1); • no padding done (ON-OFF current diff. < 0.5 mA); • 2.9 s excess ON-source; • this data will be start of large multi-yr data set on 1H1426+428

30. Recent Results Resulting upper limit (preliminary): Petry et al. 2002

31. Recent Results These results already constrain some EBL absorption models:

32. Outlook/Conclusions • STACEE is fully operational: taking data, doing science; • We continue to improve our understanding of the data and the power of the FADC system; • Many sources still to be investigated in STACEE’s unique energy regime; • Normal operations will continue until mid-2006 to the threshold of the VERITAS/GLAST era; • Further running beyond mid-2006 will be contingent on STACEE results and evolution of VERITAS/GLAST.