VHE GRBs with Milagro Jordan Goodman University of

VHE GRBs with Milagro Jordan Goodman University of Maryland • The Milagro Detector • Why look for VHE GRBs • Milagrito Result – GRB 970417 a • Milagro Results – GRB 010921 – Future Directions Jordan Goodman Milagro Collaboration Moriond 2003

Techniques in Te. V Astrophysics Low energy threshold Good background rejection Small field of view Low duty cycle Good for sensitive studies of known sources. Jordan Goodman High energy threshold poor background rejection Large field of view (~2 sr) High duty cycle (>90%) Good for all sky monitor and for investigation of transient sources. Milagro Collaboration Moriond 2003

Observing the High Energy Sky 1 Ge. V 1 Te. V 1 Pe. V 1 Ee. V Milagro • Water-Cherenkov Detector • Threshold ~300 Ge. V 10 9 10 11 10 15 10 17 10 19 Satellites Solar Arrays Air Cherenkov • Wide-angle Milagro • g/hadron Separation EAS Arrays • 24 Hour – all year operation Jordan Goodman 10 13 Fly’s Eye / Hi. Res Akeno /Auger Milagro Collaboration Moriond 2003

Milagro Site Located near Los Alamos, NM, USA 8650’ Elevation 60 m X 8 m covered pond Jordan Goodman Milagro Collaboration Moriond 2003

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. ● Jordan Goodman Milagro Collaboration Moriond 2003

Milagrito A prototype for the full Milagro detector Single layer of 230 PMTs with no muon detection Milagrito operated at >250 Hz from Feb 97 to April 98 (>85% livetime) More than 9 billion events - 9 Terabytes Jordan Goodman Milagro Collaboration Moriond 2003

Milagro Jordan Goodman Milagro Collaboration Moriond 2003

Milagro Outriggers Jordan Goodman Milagro Collaboration Moriond 2003

Milagro Energy Response Jordan Goodman Milagro Collaboration Moriond 2003

Gamma / Hadron Separation in Milagro Gammas (MC) Data Gammas (MC) Jordan Goodman Milagro Collaboration Moriond 2003

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 Te. V, we have substantial sensitivity at lower energies. Jordan Goodman Milagro Collaboration Moriond 2003

Milagro EGRET at 100 Me. V Milagro at 1 Te. V Jordan Goodman Milagro Collaboration Moriond 2003

Milagro Results Data taken in the Crab Nebula region with 6 s at the position of the Crab Jordan Goodman 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 Milagro Collaboration Moriond 2003

High Energy Afterglow In one GRB, EGRET observed emission above 30 Me. V for more than an hour after the prompt emission. ● 18 Ge. V 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? Jordan Goodman Milagro Collaboration Moriond 2003

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) Jordan Goodman Milagro Collaboration Moriond 2003

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. ● Jordan Goodman Milagro Collaboration Moriond 2003

Emission Models Prompt Phase (Pilla & Loeb 1998) Afterglow Phase (Sari & Esin 2001) Luminosity of the inverse Compton component is comparable to the synchrotron luminosity. Jordan Goodman Milagro Collaboration Moriond 2003

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 GRB 940217? Physics: - Probe density and spectrum of IR/optical intergalactic radiation fields. - Test of Lorentz invariance at high energies (quantum gravity. . . ). Jordan Goodman Milagro Collaboration Moriond 2003

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. E 2 = m 2 c 4 +p 2 c 2 - in the Lorentz invariant case, E 2 -c 2 p 2~E 2 x(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. The expected time delay is : Dt ~ x(E/EQG)a L/c This may be measurable for very high energy photons coming from large distances. Jordan Goodman Milagro Collaboration Moriond 2003

Lorentz Invariance Violation Implications for GRB observations: Delay between the ke. V and VHE emission. Jordan Goodman Milagro Collaboration Moriond 2003

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. Jordan Goodman Milagro Collaboration Moriond 2003

Absorption of Te. V Photons g. Te. V + g. IR -> 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. Jordan Goodman Milagro Collaboration Moriond 2003

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! Jordan Goodman Milagro Collaboration Moriond 2003

GRB 970417 Evidence for a Te. V signal from GRB 970417 was seen by Milagrito (a smaller, single layer prototype of Milagro) 18 signal events with an expected background of 3. 46 -> Poisson prob. 2. 9 e-8 (5. 2 s). Prob. after correcting for size of search area: 2. 8 e-5 (4 s). Chance prob. of this excess in any of the 54 GRB examined for Te. V emission by Milagrito: 54 x 2. 8 e-5 = 1. 5 e-3 (3 s). ● Jordan Goodman Milagro Collaboration Moriond 2003

GRB 970417 sub-Me. V observations show a weak, soft burst. ● Emission must have extended up to at least 650 Ge. V. - Highest energy photons ever observed from a GRB! ● First evidence for existence of second emission component. ● Jordan Goodman Milagro Collaboration Moriond 2003

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 Te. V energies and should show soft -> hard spectral evolution. Jordan Goodman Milagro Collaboration Moriond 2003

Implications of the Milagrito Observations of GRB 970417 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 Ge. V photons at a redshift of ~0. 1. Thus z<~0. 1. Implies that the burst must have been intrinsically weak at sub-Me. V energies. Bulk Lorentz Factor: G > 95 (assuming variability timescale of 1 s and that the sub-Me. V spectrum turns over at 60 ke. V). ● Particle acceleration: ● If the VHE emission was due to inverse Compton emission, Eic, max~ 4/3 g 2 e, max. Esoft , then the electron energies required to upscatter 60 ke. V photons to 650 Ge. V, ge > 2000. Jordan Goodman Milagro Collaboration Moriond 2003

Lightcurves Cross correlation between Te. V and sub-Me. V lightcurves peaks at a lag of 1 s. Assuming Eobs = 650 Ge. V, 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). Jordan Goodman Milagro Collaboration Moriond 2003

Inverse Compton Models For an SSC model Fic/Fsyn = sqrt(ee/e. B) ~ 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 Te. V emission to be slightly delayed. - from pulsar left behind from a precursor supernova which may occur days to months before the GRB. Alternatively the Te. V emission may be dominated by radiation produced by a high energy population of protons. Jordan Goodman Milagro Collaboration Moriond 2003

GRB 010921 Constraints on Te. V emission are most interesting for GRB with known redshift. GRB 010921 was detected by both the WXM and Fregate instruments on HETE, beppo. SAX 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. Jordan Goodman Milagro Collaboration Moriond 2003

GRB 010921 Preliminary! Ratio of VHE to sub-Me. V fluence is less than for GRB 970417. Jordan Goodman Milagro Collaboration Moriond 2003

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. Jordan Goodman Milagro Collaboration Moriond 2003

Milagro and GLAST Sensitivity For a 1 second observation, Milagro becomes more sensitive than GLAST at ~100 Ge. V. Jordan Goodman Milagro Collaboration Moriond 2003

How many GRB will we see at Te. V energies? Luminosity function at these energies is unknown! However, assuming that all are bright at Te. V 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 (Schmidt 2001) and/or weak (Norris 2002) which are very close. Jordan Goodman Milagro Collaboration Moriond 2003

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 Ge. V. 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 + Te. V ground based instruments will provide complete spectral coverage from 100 Me. V - 50 Te. V of both the prompt and afterglow phases of GRB. Jordan Goodman Milagro Collaboration Moriond 2003