Recent Observations of the with the Solar Tower Atmospheric Cerenkov Effect Experiment

Donald D. Driscoll, Jr.1 for the STACEE Collaboration

1Department of Physics Case Western Reserve University

APS Division of Particles and Fields 2006

Don Driscoll (STACEE) STACEE Observations of Draco DPF 2006 1 / 23 The STACEE Collaboration

J. Ball, J.E. Carson, A. Jarvis, R. A. Ong, J. Zweerink University of California at Los Angeles

D. S. Hanna, J. Kildea, T. Lindner, C. Mueller, K. Ragan McGill University

P. Fortin, R. Mukherjee Barnard College

C. E. Covault, D. Driscoll Case Western Reserve University

D. M. Gingrich University of Alberta and TRIUMF

D. A. Williams Santa Cruz Institute for Particle Physics

Don Driscoll (STACEE) STACEE Observations of Draco DPF 2006 2 / 23 Outline

1 Background

2 The Solar Tower Atmospheric Cerenkov Experiment Atmospheric Cerenkov Effect STACEE at the NSTTF

3 Recent Observations by STACEE The Crab Active Galactic Nuclei Gamma-Ray Bursts Annihilation

4 STACEE Observations of Draco Data Summary Flux Limits Dark Matter Limits

Don Driscoll (STACEE) STACEE Observations of Draco DPF 2006 3 / 23 Indirect Detection of Dark Matter

If dark matter particles can self-annihilate, they can decay into gamma-rays. The method is complementary to both direct detection and accelerator experiments, which probe elastic cross-section. Indirect detection is very sensitive to astrophysics! While we might not measure the absolute fluxes well, we can measure the WIMP mass by the shape of the spectrum. The Draco dwarf spheroidal galaxy is a strong candidate for dark-matter induced gamma-rays, and there is a possible detection by the CACTUS experiment.

Don Driscoll (STACEE) STACEE Observations of Draco DPF 2006 4 / 23 The Atmospheric Cerenkov Effect

Gamma-rays strike the atmosphere at typical height of about 12 km above sea level. The resulting cascade of electromagnetic particles travels through the atmosphere near the speed of light in vacuo. The speed of the emitted light (in air) is lower than the speed of the particles, so a conical wavefront forms simliar to a “sonic boom,” The roughly spherical wavefront of bluish light forms a circle with an appproximate diameter of 250 meters when it strikes the ground.

Don Driscoll (STACEE) STACEE Observations of Draco DPF 2006 5 / 23 STACEE at the NSTTF

Don Driscoll (STACEE) STACEE Observations of Draco DPF 2006 6 / 23 STACEE at the NSTTF

STACEE Specifications National Solar Tower Test Facility was completed in 1978. It is not an operational power facility - currently only used for research (not at night!). 64 of 222 heliostats used gives us a total of 2400 m2 of mirror surface. Light from each heliostat is reflected off of a secondary mirror onto a single PMT. STACEE is a wavefront-sampling Cerenkov Telescope.

Don Driscoll (STACEE) STACEE Observations of Draco DPF 2006 7 / 23 STACEE at the NSTTF

STACEE Optical Schematic

Don Driscoll (STACEE) STACEE Observations of Draco DPF 2006 8 / 23 default The Crab

The Crab is the brightest steady source of gamma-rays in the northern sky, making it the “Standard Candle” for gamma-ray astronomy.

Well-established source for STACEE: First Detection: 6.8σ (STACEE-32: Oser et al., 2001, ApJ, 547:949) 2002-2003 Season (7.2 hrs.) - 5.1σ (Kildea, ICRC 2005) 2003-2004 Season (14.0 hrs.) - 5.5σ (Kildea, ICRC 2005) Significance (Li & Ma, 1983, ApJ, 272:317): ( + )− σ = √ON−OFF = √S B B ≈ √S ON+OFF (S+B)+B 2B

Don Driscoll (STACEE) STACEE Observations of Draco DPF 2006 9 / 23 default Grid Ratio Discrimination

Gamma/Hadron Separation – Grid Ratio 13 Compares H/W of “Grid Ratio” cut pioneered by CELESTE summed pulse (Bruel,P., et al., Proc. of Frontier Science 2004, Physics & Astrophysics in Space) between timing 0.45 aligned at Gamma rays Protons maximum and 200 0.4 meters off-target. 0.35 Showers from 0.3 cosmic-rays are 0.25

less sensitive to Normalised Population 0.2 pointing offsets 0.15 than showers from 0.1 gamma-rays. 0.05 Grid ratio cut gives almost a factor of 2 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 increase in [(H/W) ]/[(H/W) ] 200m max significance! SimJ. Kildea,ulated “Observationsdata of the Crab Nebula and Pulsar with STACEE,” ICRC 2005 • — Grid ratio is a good gamma/hadron separation parameter for STACEE

Don Driscoll (STACEE) STACEE Observations of Draco DPF 2006 10 / 23 default Active Galactic Nuclei

STACEE has observed many sources near 100 GeV.

Source Type Year (Livetime) Notes 3C 66A LBL 2003 (23 hrs) Bramel, Ap.J. 629 (2005) 2005 (18 hrs) OJ 287 LBL 2004 (6 hrs) Mrk 421 HBL 2003 (16 hrs) Carson, Forthcoming (2006) 2004 (12 hrs) (5.8σ detection, energy spectrum) 1ES 1218+304 HBL 2005 (11 hrs) W Comae LBL 2003 (10.5 hrs) Scalzo, Ap.J. 607 (2004) 2004 (4.6 hrs) Mukherjee, ICRC 2005 2005 (5.1 hrs) Mukherjee, ICRC 2005 1ES 1426+428 HBL 2003 (10 hrs) Mukherjee, ICRC 2005 2004 (20 hrs) Mrk 501 HBL 2005 (7 hrs) 1ES 1741+196 HBL 2004 (3 hrs) BL Lac LBL 2005 (2 hrs)

R.Mukherjee, “STACEE observations of Active Galactic Nuclei,” HEAD 2006

Don Driscoll (STACEE) STACEE Observations of Draco DPF 2006 11 / 23 GRB Observations around 100 GeV with STACEE B. Jarvis,a D.A. Williams,b J. Ball,a D. Bramel,c J. Carson,a C. E. Covault,d D.D. Driscoll,d P. Fortin,e D. M. Gingrich,f,g D. Hanna,e J. Kildea,e T. Lindner,e C. Mueller,e R. Mukherjee,c R. A. Ong,a K. Ragan,e R. A. Scalzo,h J. Zweerinka a Department of Physics and Astronomy, University of California, Los Angeles, California, USA, 90025 b Santa Cruz Institute for Particle Physics, University of California, Santa Cruz, California, USA, 95064 c Department of Physics, Columbia University, New York, New York, USA, 10027 d Department of Physics, Case Western Reserve University, Cleveland, Ohio, USA, 44106 e Department of Physics, McGill University, Montreal, QC H3A 2T8, Canada f Centre for Subatomic Research, University of Alberta, Edmonton, AB T6G 2N5, Canada g TRIUMF, Vancouver, BC V6T 2A3, Canada h Lawrence Berkeley National Laboratory, Berkeley, California, USA, 94720 Presenter: Alex Jarvis

The STACEE Telescope GRB Observing Strategy

The Solar Tower Atmospheric Cherenkov Effect Experiment (STACEE) is a Observing gamma-ray bursts is a high priority for STACEE. Burst alerts from the GRB High Sources showerfront-sampling atmospheric Cherenkov telescope. It was constructed Coordinates Network (GCN) are monitored with a computer program which alerts at a previously existing facility, the National Solar Thermal Test Facility STACEE operators if a burst is visible from the STACEE site by updating a web page and One of the advantages of using high-energy gamma rays for astronomy is that they are (NSTTF) at Sandia National Laboratories outside Albuquerque, New Mexico. initiating an audio alert. The operators are also required to carry a cell phone or pager relatively insensitive to the attenuating effects of dust as they travel from their source to The NSTTF is located at 34.96o N, 106.51o W and is 1700 m above sea level. that receives burst alerts. The standard operating procedure is to immediately retarget . Unfortunately there is another effect that can extinguish the signal from distant The facility has 220 heliostats mirrors, designed to track the sun across the the detector upon receiving a burst alert. STACEE observes any burst location that sources of gamma rays: pair production. As high-energy photons traverse the sky, each with 37 m2 area. STACEE uses 64 of these heliostats. ascends above 30 degrees elevation within 12 hours of the burst. This period may be they encounter extragalactic background light (EBL) photons. When the center-of- extended to 24 hours for particularly interesting bursts. momentum energies of the two photons are greater than twice electron rest mass, When a high-energy gamma ray strikes the Earth’s atmosphere it produces electron-positron pair production can occur. The extent of the attenuation depends on an electromagnetic cascade. The front of this electromagnetic shower the density of sufficiently energetic EBL photons and the distance traveled. The figure produces a wide, shallow pool of Cherenkov light. below [2] is essentially a plot of the horizon redshift as a function of gamma-ray energy, NEW! Socket (0.1-2 sec) with each curve indicating a different degree of attenuation. For each of the values of ! shown, the corresponding curve indicates the conditions where the attenuation is e-!. Pager (5-180 Heliostat The intensity of the EBL was Swift sec) Control Room recently found to be near the Localization (~20 sec) minimum of the expected GCN range, extending the horizon for high-energy observations Email (5-30 sec) somewhat. However, the On-site email Data Acq. median redshift of the GRBs server Control detected by Swift (~2.6) is Room much higher than that of the In the summer of 2004, the slewing speeds of the STACEE heliostats were more than pre-Swift era (~1.0). In light doubled through motor upgrades. The heliostats can now re-target from zenith to any of this discovery, very few direction above 45° elevation in less than about one minute. In the fall of 2005, we set up a GRBs are expected to be socket connection with the GCN for receiving burst alerts, which should improve our detectable by atmospheric response time by up to 30 seconds. Follow-up observations within 100s of some bursts Cherenkov telescopes for all should now be attainable but the most extreme models.

The heliostats can be aimed in order to reflect like from these Cherenkov GRB Observations showers to the solar tower. STACEE employs five secondary mirrors to focus the light onto photomultiplier tube (PMT) cameras. The light from each Since the fall of 2002, STACEE has made follow-up observations of 18 bursts. More than half of those were made since heliostat is detected by a separate PMT and the waveform of the PMT signal the Swift GRB Observatory[3] came online in the winter of 2005. Swift provides fast, accurate burst localizations, which is recorded by a Flash ADC. Cherenkov events from the direction of interest have allowed STACEE to make observations within minutes of some bursts. Satellite observatories produce GRB alerts GRB 050607 are selected using a custom made delay and trigger system[1]. with localizations approximately once every 3 days. STACEE requires clear, dark skies to operate effectively and has historically operated with a duty cycle of about 5%. We are also constrained to observe targets above 30° in elevation. Our most rapid follow-up to date, was an observation of Based on these constraints we would expect fast follow-up observations for a few bursts per year. GRB 050607. Data acquisition began 191 seconds STACEE Performance after the initial detection of the burst by Swift, despite The Table below shows lists the GRB follow-up observations made so far by STACEE. In order to determine integral an operator error which delayed the run by about one flux limits, a spectrum must be assumed and combined with the energy-dependent effective area of the detector. Early minute. This observation was also made before the As with all atmospheric Cherenkov telescopes, the probability that a gamma time X-ray afterglows typically well fit by absobed power-laws with photon indexes roughly near 2[4]. The table gives the inception of our socket connection to the GCN, which ray will be detected by STACEE increases with the energy of the gamma integrated flux between 100 and 1000 GeV assuming power-law spectra with photon indexes of 2.0 and 3.0. The spring could have brought the alert to us 30 seconds earlier. In ray. One advantage that STACEE has is its large mirror area, which allows of 2005 was a particulary fortunate period, producing 3 observations within 6 minutes of their respective bursts, and addition to being our fastest follow-up, this burst for the detection of gamma rays as low in energy as 50 GeV for sources another 20 minutes after the burst at high elevation. In comparison, the 2005-06 observing season much less fruitful occurred at a position where STACEE has a relatively near zenith. The detector can also be triggered by hadronic showers. The with no observations within the first 4 hours after any bursts. We have not seen a statistically significant excess of high- low energy threshold. isotropic cosmic-ray background produces an event rate of several Hertz energy photons in any of our follow-up observations. and is one of the limiting factors in the sensitivity of the detector. Some STACEE observations of GRB 050607 coincided with cosmic-ray rejection techniques are employed, and other potential Average Integral Flux (0.1 – 1 TeV) Approx. an early flare in the X-Ray afterglow[5] detected by techniques are being explored. Time to Initial Preliminary 95% CL Upper Limit GRB Live-time On Comments defaultSwift’s X-Ray Telescope (XRT). The figure below Target (min) Elevation Significance (10-10 ergs cm-2 s-1) Source (min) shows the X-ray (0.2-10 keV) photon rate as a function The Figure below shows rough effective area curves for STACEE (before N ~ E-2.0 N ~ E-3.0 Gamma-Rayof time for the period betwe Burstsen 100 and 500 seconds hadronic shower rejection cuts) for 2 different sky positions. The effective 021112 217 73° 66 0.2 after the burst trigger. Upper limits on the rate of high- area is determined from simulations and is defined as the fraction of 030324 123 31° 0 na Analysis complicated by bright star at edge of FOV energy gamma rays detected by STACEE, starting 200 simulated showers which trigger the (simulated) detector multiplied by the 030501 369 47° 0 na Unstable rates due to instrumental problem seconds after the trigger, are also shown in red. area over which those showers were scattered. As the figure shows, the position of the source can have a strong effect on the energy threshold of 031220 310 59° 0 na Unstable rates due to weather STACEE actively monitors the STACEE. The intensity of the Cherenkov light pool decreases as the 040422 95 35° 20 -1.6 1.5 3.63 elevation of the source decreases. However, the light pool gets stretched 040916 104 46° 27 1.1 4.85 7.81 GRB Coordinates Network. into a larger, more eccentric, ellipse, increasing the effective area for 041016 142 51° 16 0.0 22.5 35.3 sufficiently bright showers. Because of the layout of the STACEE heliostats, 050209 146 56° 22 1.1 5.31 7.26 Response times of less than 1 the azimuth of the source also affects our sensitivity. Swift XRT rate 050402 3.8 49° 18 1.3 13.5 30.8 050408 640 43° 20 -1.0 7.75 19.8 minute are now possible. 050412 5.7 54° 9 1.0 14.6 17.0 050509B 20 83° 25 0.8 2.05 1.83 Since the fall of 2002, 1 050509A 480 53° 15 0.1 3.75 8.93 - s

t STACEE has made follow-up n

050607 3.2 62° 19 1.3 1.41 1.33 u ) o 2 C m

( 060122 385 2.61 14.4

observations of 18 bursts, but a e

r 060206 260 1.39 1.52 A 060323 876 1.69 2.38 no excess has been seen. ve i t c 060526 1.30 1.79 STACEE high-energy limits e f f

E Past observing season References (2005-2006) has been unlucky [1] D.M. Gingrich et al., Presented at the 2004 IEEE Nuclear Science Symposium; astro-ph/0506613. [2] J.R. Primack et al., Proc. AIP Conf. 745, 23 (2005); astro-ph/0502177. Time since Swift trigger (s) (no bursts under 5 hours). [3] N. Gehrels et al., ApJ 611, 1005 (2004). GRB 050607 (left) was our Gamma Ray Energy (GeV) A.Jarvis, “GRB Observations around 100 GeV with STACEE,” fastest response time (191 HEAD 2006 seconds).

Don Driscoll (STACEE) STACEE Observations of Draco DPF 2006 12 / 23 default Dark Matter Annihilation

See Tyler, “Particle Dark Matter Constrains from the Draco ” (Phys.Rev.D 66, 023509, 2002) χχ¯ ⇒ π0 decays to gamma-rays. Models and experiments currently favor WIMP with Mχ ∼ 50 GeV − 10 TeV . A candidate target needs to be nearby and have a high concentration of dark matter. Draco has one of the highest known Mass-to-Light ratios (M/L ∼ 450 M /L ) It is a satellite of our own (79 kpc). Stellar velocities indicate that Draco has relaxed to an isothermal sphere (ρ ∼ r −2) which has a high central density.

Don Driscoll (STACEE) STACEE Observations of Draco DPF 2006 13 / 23 default SDSS Image of Draco

Robert Lupton and the Sloan Digital Sky Survey Consortium Don Driscoll (STACEE) STACEE Observations of Draco DPF 2006 14 / 23 default Draco Data Summary

35 “pairs” of data were taken in May-June 2006 with ∼ 10 hours of livetime after cuts. Raw triggers (time cuts only): 177,498 - 177,273 = 225 (4.8 Hz) After grid ratio cut: 1576 - 1587 = -11 (−0.19σ) Gamma Rate: −0.02 ± 0.09 γ/min These results are model-independent, but any interpretation needs both: 1 A measure of the the detector response. 2 The source spectrum of the object being studied!

Don Driscoll (STACEE) STACEE Observations of Draco DPF 2006 15 / 23 default Effective Area

Effective Area Curves for Draco

] 80000 2 Time Cuts Only Measures the 70000 Time cuts + a grid ratio cut

probability that a 60000 shower will trigger 50000 given the projection Effective Area [m

on the ground. 40000

Grid ratio cut 30000 trades sensitivity at high energies for 20000

gamma-hadron 10000 separation. 00 200 400 600 800 10001200140016001800 2000 Energy [GeV]

(curves based on CORSIKA simulations)

Don Driscoll (STACEE) STACEE Observations of Draco DPF 2006 16 / 23 default Detector Response

Power-law spectra Response Curve (eff.area * spectrum) for E-2.2 Spectrum ] -1 are “standard” for 0.08 Time Cuts Only GeV

gamma-ray -1 Time Cuts + grid ratio cut astronomy. 0.07 “Energy threshold” 0.06

is defined as peak A(E)*F(E) [s 0.05 of detector 0.04 response. Time cuts + grid 0.03 ratio cut (green) 0.02

used to maximize 0.01 response at lower 0 energies. 0 200 400 600 800 1000 1200 1400 Energy [GeV]

Don Driscoll (STACEE) STACEE Observations of Draco DPF 2006 17 / 23 default Modelled Flux Spectra

Differential Gamma-Ray Spectra for Different DM Production Models

4 ] 10 -1

s 3 Power-law -2 10

cm 2 spectrum based on γ 10 emission from 10

. dN/dx [ 1

Self-annihilation 10-1 spectra will have a 10-2 sharp cutoff at Mχ. 10-3

-4 -2.2 Magnitude depends 10 Power-law Spectrum (E ) Neutral Pion Decay (Tyler, astro-ph/0203242) -5 on DM distribution, 10 b-b Final State: "Softest" Spectrum τ+-τ- Final State: "Hardest" Spectrum -6 distance. 10 (Profumo and Kamionkowski, astro-ph/0601249) 10-7 10-2 10-1 1 x = (Eγ/mχ) [GeV]

Don Driscoll (STACEE) STACEE Observations of Draco DPF 2006 18 / 23 default Comparison with Other Experiments

−3.1 dΦ −6  E  −2 −1 −1 EGRET [PK]: dE ∼ 1.6x10 GeV cm s GeV CACTUS [BH]: an ACT like STACEE Preliminary results hinted at in conference talks. “Results” found in theory papers from non-CACTUS authors. ∼ 60σ excess (30,000 events in 7 hours with E > 50 GeV ) ∼ 20σ excess (7,000 events with E > 100 GeV ) No signal seen above 150 GeV. WHIPPLE [H]: Φ(E > 400 GeV ) < 0.03 Crab HESS [PK]: −2.2 dΦ −8  E  −2 −1 −1 dE (E > 780 GeV ) < 1.0x10 GeV cm s GeV Profumo & Kamionkowski, astro-ph/0601249 Bergstr¨om& Hooper, astro-ph/0512317 Hall, ICRC 05, astro-ph/0507448

Don Driscoll (STACEE) STACEE Observations of Draco DPF 2006 19 / 23 default Flux Comparison

Comparison of Flux Limits for an E-2.2 Spectrum -9 ]

-1 10

GeV -10 CACTUS 50 GeV Signal -1 10 s EGRET (E -2 Crab Spectrum 10-11 -3.1 ) E -2.2 Spectrum dN/dE [cm 10-12 CACTUS 150 GeV Limit 10-13 STACEE

10-14 WHIPPLE (0.03 Crab) HESS

10-15

10-16 102 103 Energy [GeV]

Don Driscoll (STACEE) STACEE Observations of Draco DPF 2006 20 / 23 default Dark Matter Limits

Flux normalized Gamma-ray Flux Limits from Draco (B-B) ]

to detector -1

s STACEE Limit -2 response from 10-4 CACTUS 60-σ Excess (E>50 GeV)

cm σ

γ CACTUS 20- Excess (E>100 GeV)

shape of [ γ -5 CACTUS Limit (E>150 GeV) φ 10 spectrum. -6 50 GeV and 100 10 GeV CACTUS 10-7

excesses have 10-8 both upper and 10-9 lower limits. -10 Black box 10 indicates where 10-11 50 100 150 200 250 300 350 400 the two excess Mχ [GeV] curves cross. Our measurements favor a lower-mass WIMP.

Don Driscoll (STACEE) STACEE Observations of Draco DPF 2006 21 / 23 default Dark Matter Limits

-2 Draco Dark Matter Limits Using a SIS Halo Profile (nχ=Ar , BB) ] Normalized to -1 STACEE Limit s 3 10-23 CACTUS 60-σ Excess (E>50 GeV) CACTUS 20-σ Excess (E>100 GeV) distance and single -24 [cm 10 CACTUS Limit (E>150 GeV) χ χ EGRET limit (Tyler) isothermal sphere 10-25 v> σ core model (Tyler). < 10-26 -27 Also dependent on 10 -28 accuracy of mass, 10 10-29 halo density, and 10-30 central density. 10-31 -32 EGRET limits 10 -33 based on absence 10 10-34 102 103 104 of Draco from 1 mχ [GeV] GeV catalog. CACTUS excess “crossing point” excluded by EGRET limit

Don Driscoll (STACEE) STACEE Observations of Draco DPF 2006 22 / 23 default Summary

STACEE does not see Draco with a sensitivity of about 0.07 times the Crab. The STACEE observations of Draco are competetive with other experiments for a power-law spectrum. The unofficial (?!) CACTUS result would only be possible (self-consistent) for a low-mass (Mχ ∼ 50 − 200 GeV ) WIMP.

Outlook 2006-2007 is the last STACEE observing season. VERITAS is now online and GLAST is soon to follow Further Draco observations are planned. Improved analysis techniques could find “hidden treasures” in past data!

Don Driscoll (STACEE) STACEE Observations of Draco DPF 2006 23 / 23