VHE GRBs with Milagro

1. VHE GRBs with Milagro • The Milagro Detector • Why look for VHE GRBs • Milagrito Result • GRB 970417a • Milagro Results • GRB010921 • Future Directions Jordan Goodman University of Maryland Milagro Collaboration

2. Techniques in TeV Astrophysics High energy threshold poor background rejection Large field of view (~2sr) High duty cycle (>90%) Good for all sky monitor and for investigation of transient sources. Low energy threshold Good background rejection Small field of view Low duty cycle Good for sensitive studies of known sources. Milagro Collaboration

3. 1 G e V 1 T e V 1 P e V 1 E e V 19 9 1 1 1 3 1 5 1 7 1 0 1 0 1 0 1 0 1 0 1 0 Satellites Solar Arrays Air Cherenkov Milagro EAS Arrays Fly’s Eye / HiRes Akeno /Auger Observing the High Energy Sky • Milagro • Water-Cherenkov Detector • Threshold ~300 GeV • Wide-angle • g/hadron Separation • 24 Hour – all year operation Milagro Collaboration

4. Milagro Site Located near Los Alamos, NM, USA 8650’ Elevation 60m X 80m X 8m covered pond Milagro Collaboration

5. Milagro Gamma-Ray Detector • Altitude - 8692 ft • Two layers of PMTs: - Top layer used to reconstruct shower direction to ~0.7 degrees. - Bottom layer used for background rejection. • Water is used as the detection medium - allows for a large sensitive area. Milagro Collaboration

6. Milagrito A prototype for the full Milagro detector Single layer of 230 PMTs with no muon detection Milagrito operated at >250Hz from Feb 97 to April 98 (>85% livetime) More than 9 billion events - 9 Terabytes Milagro Collaboration

7. Milagro Milagro Collaboration

8. Milagro Milagro Collaboration

9. Milagro Outriggers Milagro Collaboration

10. Milagro Energy Response Milagro Collaboration

11. Gammas (MC) Data Gammas (MC) Gamma / Hadron Separation in Milagro Milagro Collaboration

12. Milagro Sensitivity Due to increasing energy threshold and decreasing sensitivity, we only look for GRB with zenith angles less than 45 degrees. Energy threshold is not well defined. Even though our peak sensitivity is at a few TeV, we have substantial sensitivity at lower energies. Milagro Collaboration

13. Milagro EGRET at 100 MeV Milagro at 1 TeV Milagro Collaboration

14. Signal map of Mrk 421 during the 2001 flare (1/17/01-4/26/01). The circle shows the position of Mrk 421 with our angular bin. The center corresponds to ~5 s Data taken in the Crab Nebula region with 6s at the position of the Crab Milagro Results Milagro Collaboration

15. High Energy Afterglow • In one GRB, EGRET observed emission above 30 MeV for more than an hour after the prompt emission. • 18 GeV photon was observed (the highest ever seen by EGRET from a GRB). • Due to Earth occultation, it is unknown for how long the high energy emission lasted. Unlike optical/X-ray afterglows, gamma-ray luminosity did not decrease with time -> additional processes contributing to high energy emission? Milagro Collaboration

16. GRB Paradigm Produce lots and lots of energy in a small region of space. Hypernova- death of a massive star merging of close compact binaries (neutron stars or black holes) (Piran 2001) Milagro Collaboration

17. Emission Models • Series of shells produced by the central engine collide, forming shocks. • Electrons accelerated at these shocks produce synchrotron radiation. • Depending on the physical parameters in the emission region, there may also be a second higher energy component due to inverse Compton emission, proton synchrotron emission, or photopion reactions. Milagro Collaboration

18. Emission Models Prompt Phase (Pilla & Loeb 1998) Afterglow Phase (Sari & Esin 2001) Luminosity of the inverse Compton component is comparable to the synchrotron luminosity. Milagro Collaboration

19. What can we learn from VHE Observations? Astrophysics: • How high in energy does the prompt GRB emission extend? Measurements of high energy cutoffs in GRB will provide information on: - particle acceleration. - Bulk Lorentz factors at each internal shock. • Is there a second emission component? What is its nature? • How common are high energy afterglows such as that seen in GRB940217? Physics: - Probe density and spectrum of IR/optical intergalactic radiation fields. - Test of Lorentz invariance at high energies (quantum gravity...). Milagro Collaboration

20. Lorentz Invariance Violation Bounds on energy dependence of the speed of light can be used to place constraints on the effective energy scale for quantum gravitational effects. E2 = m2c4 +p2c2 -in the Lorentz invariant case, E2-c2p2~E2x(E/EQG)a - This may be modified in some quantum gravity models. This has the important observational consequence that this will give rise to energy dependent delays between arrival times of photons. Dt ~ x(E/EQG)a L/c The expected time delay is : This may be measurable for very high energy photons coming from large distances. Milagro Collaboration

21. Lorentz Invariance Violation Implications for GRB observations: Delay between the keV and VHE emission. Milagro Collaboration

22. Quantum Gravity - Observational Consequences/issues Dt ~ x(E/EQG)a L/c • Delay in arrival of high energy photons relative to lower energy. Depends on ability to measure Dt. - require high luminosity. - short lived events. - instruments with large collection area. • Smearing (in time) of VHE emission. Duration of very short bursts (<1 s) will have larger duration at VHE energies and should show soft -> hard spectral evolution. Milagro Collaboration

23. Absorption of TeV Photons gTeV + gIR -> e+e- -- Limits volume of observable Universe Density of IR background radiation is hard to measure due to foreground contamination. The density of the IR background is sensitive to the epoch of galaxy formation and other details of structure formation. Milagro Collaboration

24. Measuring the Intergalactic IR Background Look for absorption features in high energy gamma-ray spectra. Need a large number of gamma-ray sources. Need sources to be distributed over a wide range of redshifts. Need the sources to be bright. Gamma-Ray Bursts are ideal test sources! Milagro Collaboration

25. GRB970417 Evidence for a TeV signal from GRB970417 was seen by Milagrito (a smaller, single layer prototype of Milagro) • 18 signal events with an expected background of 3.46 -> Poisson prob. 2.9e-8 (5.2s). Prob. after correcting for size of search area: 2.8e-5 (4s). Chance prob. of this excess in any of the 54 GRB examined for TeV emission by Milagrito: 54x2.8e-5 = 1.5e-3 (3s). Milagro Collaboration

26. GRB970417 • sub-MeV observations show a weak, soft burst. • Emission must have extended up to at least 650 GeV. - Highest energy photons ever observed from a GRB! • First evidence for existence of second emission component. Milagro Collaboration

27. Quantum Gravity - Observational Consequences • Modification of the pair production threshold -> less absorption on IR background than predicted. • Delay in arrival of high energy photons relative to lower energy. Depends on ability to measure Dt. - require high luminosity. - short lived events. - instruments with large collection area. • Smearing (in time) of VHE emission. Duration of very short bursts (<1 s) will have larger duration at TeV energies and should show soft -> hard spectral evolution. Milagro Collaboration

28. Implications of the Milagrito Observations of GRB970417 The Milagrito observation represents the highest energy photons ever observed from a GRB, and the first evidence for a second emission component. • Redshift: Opacity is ~1 for 650 GeV photons at a redshift of ~0.1. Thus z<~0.1. Implies that the burst must have been intrinsically weak at sub-MeV energies. • Bulk Lorentz Factor:G> 95 (assuming variability timescale of 1 s and that the sub-MeV spectrum turns over at 60 keV). Particle acceleration: • If the VHE emission was due to inverse Compton emission, Eic,max~ 4/3g2e,maxEsoft , then the electron energies required to upscatter 60 keV photons to 650 GeV,ge > 2000. Milagro Collaboration

29. Lightcurves Cross correlation between TeV and sub-MeV lightcurves peaks at a lag of 1 s. Assuming Eobs = 650 GeV, Dt = 4 s and z=0.1, we can obtain a constraint on EQG which is a factor of ~70 better than previous limits (Biller 1999). Milagro Collaboration

30. Inverse Compton Models For an SSC model Fic/Fsyn = sqrt(ee/eB) ~ 5 for model fits to BATSE data. However, Milagrito result implies that Fic/Fsyn >10. Can enhance IC emission if there is an external source of soft photons: - from optical flash - expect TeV emission to be slightly delayed. - from pulsar left behind from a precursor supernova which may occur days to months before the GRB. Alternatively the TeV emission may be dominated by radiation produced by a high energy population of protons. Milagro Collaboration

31. GRB010921 Constraints on TeV emission are most interesting for GRB with known redshift. • GRB010921 was detected by both the WXM and Fregate instruments on HETE, beppoSAX and IPN. • Zenith angle of 10 degrees at Milagro • Spectrum of the host galaxy measured by Palomar indicated that z=0.45 E-2.4 differential photon spectrum corrected for absorption on intergalactic background radiation. Milagro Collaboration

32. GRB010921 Preliminary! Ratio of VHE to sub-MeV fluence is less than for GRB970417. Milagro Collaboration

33. VHE Instrument Sensitivity For observations of the prompt phase of GRB, current and future high energy gamma-ray instruments (GLAST and Milagro) are very complementary. Milagro Collaboration

34. Milagro and GLAST Sensitivity For a 1 second observation, Milagro becomes more sensitive than GLAST at ~100 GeV. Milagro Collaboration

35. How many GRB will we see at TeV energies? Luminosity function at these energies is unknown! However, assuming that all are bright at TeV energies then the distance distribution of GRB will determine how many we see. (Boettcher and Dermer 1998) 9% with z<0.3 9/year (Schmidt 1999) 0.6% with z<0.3 0.6/year These predictions are only for long duration bursts and are very uncertain at low redshifts. Evidence that there may be a population of soft (Schmidt2001) and/or weak (Norris 2002) which are very close. Milagro Collaboration

36. Conclusions • VHE observations of GRB will provide a crucial piece in the puzzle to understand these enigmatic objects. • EGRET observations suggest that all prompt GRB spectra may extend out to at least 10 GeV. • Many emission models of both the prompt and afterglow phases of GRB predict VHE fluxes which are observable by the current generation of instruments. • VHE observations are much more interesting if the burst is localised and the redshift is known. SWIFT will provide a sample of such bursts. • GLAST + TeV ground based instruments will provide complete spectral coverage from 100 MeV - 50 TeV of both the prompt and afterglow phases of GRB. Milagro Collaboration