VHE GRBs with Milagro

Jordan Goodman Milagro Collaboration Moriond 2003

VHE GRBs with Milagro

• The Milagro Detector• Why look for VHE GRBs• Milagrito Result

– GRB 970417a• Milagro Results

– GRB010921– Detection Probabilities– Future Directions

Jordan Goodman

University of Maryland


Techniques in TeV Astrophysics

Low energy thresholdGood background rejectionSmall field of viewLow duty cycle

Good for sensitive studies of known sources.

High energy thresholdModerate background rejectionLarge field of view (~2sr)High duty cycle (>90%)

Good for all sky monitor and for investigation of transient sources.


Observing the High Energy Sky

10 9 10 11 10 10 10 1013 15 17 19

1 GeV 1 TeV 1 PeV 1 EeV

Satellites

Fly’s Eye / HiRes

Air Cherenkov

Milagro

EAS Arrays

Solar Arrays

Akeno /Auger

Milagro

•Water-Cherenkov Detector

•Threshold ~300 GeV

•Wide-angle

• /hadron Separation

•24 Hour – all year operation


Milagro Site

Located near Los Alamos, NM, USA

8650’ Elevation

60m X 80m X 8m covered pond


Milagro

Air Shower Layer

Hadron/Muon layer

2m

8" PMTs

Light-tight Cover

8 m

80m50m

450 Top Layer 8” PMTs

273 Bottom Layer 8” PMTs


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.


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

M ilagrito


Milagro


Milagro


Milagro Outriggers


Milagro Energy Response


Gamma / Hadron Separation in Milagro

Gammas (MC)

Data

Gammas (MC)


Milagro Sensitivity

Due to increasing energy threshold and decreasing sensitivity, we only look for GRB with zenith angles lessthan 45 degrees.

Energy threshold is not well defined.Even though our peak sensitivity isat a few TeV, we have substantial sensitivity at lower energies.


Milagro

EGRET at 100 MeV

Milagro at 1 TeV


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?


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.


Emission Models

Prompt Phase(Pilla & Loeb 1998)

Afterglow Phase(Sari & Esin 2001)

Luminosity of the inverse Compton component is comparable to the synchrotron luminosity.


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...).


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 t. - 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.


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.

t ~ (E/EQG

) L/c

E2-c2p2~E2(E/EQG

)- This may be modified in some quantum gravity models.

This has the important observational consequence that this will giverise to energy dependent delays between arrival times of photons.

E2 = m2c4 +p2c2 - in the Lorentz invariant case,

The expected time delay is :

This may be measurable for very high energy photons coming fromlarge distances.


Lorentz Invariance Violation

Implications for GRB observations:

Delay between the keV and VHE emission.


Quantum Gravity - Observational Consequences/issues

● Delay in arrival of high energy photons relative to lower energy. Depends on ability to measure t. - 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.

t ~ (E/EQG

) L/c


Absorption of TeV Photons

TeV

+ IR

-> e+e- -- Limits volume of observable Universe

Density of IR backgroundradiation is hard to measure dueto foreground contamination.

The density of the IR backgroundis sensitive to the epoch of galaxyformation and other details of structure formation.


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!


GRB970417

● 18 signal events with an expected background of 3.46 -> Poisson prob. 2.9e-8 (5.2). Prob. after correcting for size of search area: 2.8e-5 (4). Chance prob. of this excess in any of the 54 GRB examined for TeV emission by Milagrito: 54x2.8e-5 = 1.5e-3 (3).

Evidence for a TeV signal from GRB970417 was seen by Milagrito (a smaller, single layer prototype of Milagro)


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.


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: > 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, E

ic,max~ 4/32

e,maxE

soft , then the electron energies required to upscatter

60 keV photons to 650 GeV, e > 2000.


Lightcurves

Cross correlation betweenTeV and sub-MeV lightcurves peaks at a lagof 1 s.

Assuming Eobs

= 650 GeV, t = 4 s and z=0.1, we can obtain a constrainton E

QG which is a factor of ~70 better than previous limits (Biller 1999).


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 photonspectrum corrected for absorption on intergalacticbackground radiation.


GRB010921

Ratio of VHE to sub-MeV fluence is less than for GRB970417.

Preliminary!


Transient Search from 40s to 9months


Detection Probabilities

CDM model m=0.3 =0.7 h=0.65

Star Formation rate = 101.45z-1.52 Mž/Mpc3/year for z < 0.9 and 0.61 Mž /Mpc3/year for z ≥0.9


Maximum Milagro Detection Probabilities

L=1050

L=1051

L=1052


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 and GLAST Sensitivity

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


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<0.39/year

(Schmidt 1999)0.6% with z<0.30.6/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.


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.


Moriond 2003

Documents

VHE GRBs with Milagro