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TeV Gamma Ray Survey of the Northern Hemisphere with Milagro

The Milagro collaboration presents a survey of the Northern Hemisphere using the Milagro detector, a continuous TeV gamma-ray monitor with an energy threshold of 100 GeV and a median energy of ~3 TeV. The survey strategy involves examining different timescales, applying background rejection techniques, and analyzing the distribution of excesses in the sky. Results show the Crab Nebula and Mrk421 as the brightest TeV gamma-ray sources, with a previously reported hotspot appearing as the next brightest region. However, no evidence for strong flaring from any source is found on timescales > 1 week. The incorporation of outrigger detectors is expected to improve sensitivity by ~2x.

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TeV Gamma Ray Survey of the Northern Hemisphere with Milagro

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  1. TeV Gamma Ray Survey of the Northern Hemisphere with Milagro Gus Sinnis/LANL for The Milagro Collaboration

  2. The Milagro Detector A Continuous All Sky TeV Gamma Ray Monitor Energy Threshold: 100 GeV Median Energy: ~3 TeV Trigger Rate: 1.7 kHz

  3. Background Rejection in Milagro • Hadronic cosmic ray showers contain penetrating particles • Muons and hadrons • Deposit energy deep in Milagro – use bottom layer Protons Gammas

  4. Background Rejection: C Search for large pulses in small number of tubes Demand C > 2.5 Retain: 53% of Gammas 11% of Protons/Data Q = 1.7

  5. Survey Strategy • Timescales examined • 1 week, 2 week, 4 week, … DC (~128 week) • Two intervals/timescale • Shifted by ½ of timescale • Angular resolution ~0.8 degrees • Use binsize of 3 degrees • Apply compactness cut (C>2.5) to data • Oversample sky • Bin centers on an 0.1 degree grid (in RA and DEC) • Survey duration • Dec 15, 2000 – June 3, 2003

  6. Declination Mrk 421 Crab Right Ascension D.C. Map of Northern Hemisphere

  7. Distribution of Excesses: D.C. Survey

  8. Distribution of Excesses: Crab and Mrk421 Removed

  9. Crab Nebula: Excess vs. Time 1/1/2001 1/1/2002 1/1/2003

  10. Mrk421: Excess vs. Time 1/1/2001 1/1/2003 1/1/2002

  11. “Hotspot”: Excess vs. Time 1/1/2001 1/1/2002 1/1/2003

  12. Shorter Duration Results 1 week 2 weeks 4 weeks 8 weeks 16 weeks 32 weeks

  13. Median Energy vs. Declination

  14. Milagro D.C. Upper Limits

  15. Conclusions • Crab Nebula and Mrk421 were brightest TeV gamma ray sources in northern hemisphere over past 2.4 years • Previously reported hotspot still appears to be next brightest region of sky (RA 26.2 Dec 80.3 - 4.8 s) • But excess is consistent with background fluctuations • No evidence for strong flaring from any northern hemisphere source on timescales > 1 week • Outrigger detectors recently incorporated into detector • Expect ~2x improvement in sensitivity

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