Latest Results from the Milagro Observatory

Vlasios VasileiouNASA Goddard Space Flight Center &

University of Maryland, Baltimore County

Overview

The Milagro Detector

(Some of the) Latest Results• Gamma-ray Observations

– Survey of the galactic plane– Diffuse emission from the galactic plane

• Cosmic Rays– Large-scale anistropy– Discovery of two regions of excess Cosmic Rays

• Gamma-Ray Bursts– Triggered & untriggered (blind) searches

Conclusion

The Milagro Observatory

The Milagro ObservatoryThe Milagro Observatory Water-Cherenkov detector located at the

Jemez mountains near Los Alamos, New Mexico.

Elevation: 2630 m

Detector Components: Central Pond Outrigger Array

The Central PondThe Central Pond

24 Million liter reservoir of highly purified waterCovered with a light-tight cover 80m x 60m x 8m (depth) (5000 m2) 723 PMTs arranged in two layers

Air Shower Layer: 450 PMTs under 1.4 m of water

Triggering Direction Reconstruction

Muon Layer: 273 PMTs under 6m of water

Background Rejection Energy Reconstruction

80 meters50 meters

8 meters

e μ γ

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Thickness ≈ 1 m

Shower Front(Diameter ≈ 100 m)

Primary Particle

The Outrigger ArrayThe Outrigger Array

Outrigger Array• 175 Water tanks spread over 40,000 m2

• Contain water and a downwards facing PMT• Added 2003• Improved

Effective area Angular resolution Energy resolution Background rejection

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Milagro's Performance Angular reconstruction accuracy 0.3o-1.4o Most of the effective area at TeV energies

~105 m2 @ 10 TeV ~10 m2 @ 100 GeV

Median energy of triggers ~few TeV (for a Crab-like source) Performance:

Wide field of view (~2 sr) High duty cycle (~90%) Good for unbiased whole-sky searches, observations of large-scale

features & anisotropies, monitoring for transient emissions (flares, GRBs).

Crab-like source Milagro ~8σ/sqrt(year)

The TeV Sky as Seen by Milagro

The Northern Sky at 20 TeV

Crab Nebula

30°

210°

90° 65°

Cygnus Region

• 6.5 year data set (July 2000-January 2007)

ApJ 664 (2007) L91

MGRO J0634+17(Geminga)

Crab nebula

MGRO J2019+37(GeV J2020+3658)

MGRO J2226+60(Boomerang PWN)

MGRO J1908+06(GeV J1907+0557)

MGRO J2031+41(GeV J2035+4214)

MGRO J2044+36(no counterpart)

MGRO J2031+36(extension of J2019?)

MGRO J2005+33(high matter density?)

108 106

C4 J2226+60 EGRET

Geminga

Cygnus Region

GeV 1907+0557

•GeV Sources C3 J0634+17

Geminga

• Milagro has discovered 3 new sources & 4 candidate sources in the Galaxy. • 5/7 of these TeV sources have GeV counterparts (only 13 GeV counterparts in this region - excluding Crab)

• Probability = 3x10-6

Diffuse Emission from the Galactic Plane

Source Exclusion

Crab

•The signal regions (shown with the squares) were fit with a two-dimensional Gaussian plus a constant.

Diffuse Emission

Below HorizonCygnus Region

GALPROP (optimized):Sum

πo decayInverse Compton

Source Subtracted Longitude Profile by Milagro

Cygnus Region with Matter Density Contours overlaying Milagro Significance

Milagro and GALPROP predictions

Milagro

EGRET

Cygnus region Inner galaxy

Extragalactic diffuse•Bremsstrahlung•πo decay•Inverse Compton (dashed line: IC on the CMB)•Sum of the above three contributions

Profiles of the Galactic Diffuse Emission

Inner Galaxy 30o<longitude<65o

Cygnus Region 65o<longitude<85o

GALPROP Modelo decay Inverse Compton Total

• Inverse Compton component extends to higher latitudes• Pion decay due to interactions with matter mostly at low latitudes• Profile of Milagro’s measurements at Cygnus region is narrower than

GALPROP’s predictions -> possibly a stronger pion component is involved

Discovery of Two Regions of Excess Cosmic-Rays

Milagro Observes Anisotropy in 10 TeV Cosmic Rays

• Milagro’s standard point-source analysis with a 10o bin size• Results:

– Two regions of fractional excess of 6e-4 (Region A) and 4e-4 (Region B) above the cosmic ray background were detected.

• Composition:– Excesses are not gamma rays (or electrons), but charged cosmic rays (8.6σ Region A and 6.6σ

Region B). • Energy Spectrum:

– The spectra of both excesses are inconsistent with the cosmic-ray spectrum (4.6σ and 2.5σ)– Spectrum of region A: Broken power-law with index = -1.45 and break energy=9TeV.

Heliotail

Geminga

Galactic Plane

Mrk421

Crab

Cygnus region

Tibet icrc 02007

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Tibet Collaboration ICRC Merida 2007

Origin/Explanation of the Excesses Composition:

• Not photons or electrons• Neutrons from a star? Unlikely -> 10 TeV neutrons decay in 0.1pc -> much closer than

the nearest star.• Gyroradius of a 10TeV proton in a 2μG magnetic field (estimate of the local Galactic

field) is only ~0.005pc (1000AU).– Magnetic field must connect us to the source and be coherent out to it (>=100pc).

Tips:– Connection to heliosphere? Region A coincides with the direction of the heliotail.– The direction of both regions is nearly perpendicular to the expected Galactic

magnetic field direction. Multiple explanations were proposed:

• Salvati & Sacco, astro-ph:0802.2181 • Drury & Aharonian, astro-ph:0802.4403• K. Munakata for M. Amenomori AIP Conf Proc Vol 932, page 283

Large-Scale Cosmic-Ray Anisotropy

Large-Scale Cosmic Ray Anisotropy

To study anisotropies with scale larger than 10o, an alternative method was used.

Can detect effects down to the 10-4 – 10-3 level.

Measures the fractional (not absolute) anisotropy in the RA direction.

Median Energy 6 TeV

Large-Scale Cosmic-Ray Anisotropy

Galactic North Pole

•Define “central-deficit region”:•Dec: 5o-35o & RA : 160o-210o

•Symmetric around minimum at RA=188o

•Average fractional anisotropy: -2.85 ± 0.06 ± 0.08 x 10-3 (20σ)•Coincident with the Galactic North Pole

Properties of the Central-Deficit Region

07/2000 ~ Solar maximum 07/2007 ~ Solar minimum

•Mean anisotropy of central-deficit region increases with time (~ factor of 2 / 7 yrs).•Trend present in all energies•Tibet found no evidence of fluctuation: Mean anisotropy at 1997-2001 =~ 2001-2005•Average value of the anisotropy depends on the energy -> decreases at high energies

Possible Explanations Effects that can create anisotropies

• Large scale or local magnetic field configurations• Effects of the heliosphere

– Anisotropy observed to energies up to 100TeV -> CR of such high energies are not easily influenced by the heliosphere

• Diffusion of cosmic rays out of the Galactic halo– Supported by the fact that the deficit is close to the North Galactic Pole

• Contribution of discrete sources (such as supernova remnants)• Compton-Getting effect -> dipole effect due to the motion of the solar system with

respect to the CR plasma -> increase in CR flux of the order of 0.1% in the direction of motion.– Energy independent– Observed effect different than the predicted effect.

Gamma-Ray Bursts

Milagro’s Searches for GRBs• Triggered

– In coincidence with external triggers (ex. from Swift)• Using the reconstructed events (E>100 GeV)• Using the individual PMT hit-rates (E<100GeV) • Published upper limits at ICRC Merida & Santa-Fe GRB Conferences

2007• Comprehensive paper with upper limits in preparation

• Untriggered (Blind)– Search of all of the Milagro data in space, starting time, duration– Simple binned search– Careful calculation of the effective number of trials– Optimization of the bin-size versus the duration under search

Sensitivity to GRBs (Untriggered Search)

*Swift data: N. Butler et al. ApJ 2007

Fluence Sensitivity

Minimum Detectable Redshift

Blind Search for GRBs• Searched 4.6 years of Milagro data for

bursts of VHE gamma rays of duration from 0.1msec to 316sec.

• The search was also sensitive to other transient phenomena such as the last stages of primordial black hole evaporation.

• No significant events were detected. • Upper limits on the prompt VHE emission

from GRBs were set.

•Best post-trials probabilities found in each duration•Need to multiply these probabilities with an extra factor of 41 to account for the number of independent durations searched.

Setting Upper limits on VHE emission from GRBs Next step: Set upper limits on the VHE emission from GRBs

A VHE emission model that predicts -ln(1-CL) GRB detections by this search is excluded at the CL level.

• 2.3 detections -> exclude a model at the 90% Confidence Level

A simulation of the GRB population was created to estimate the number of GRB detections.

The simulation reproduced the (Eiso, z, t90) distributions of GRBs detected by Swift. • Using the Swift GRB rate and the relative FOV’s of Milagro and Swift we can

calculate the rate of (detectable by Swift) GRBs in Milagro’s FOV.• Using Milagro’s sensitivity data we can calculate the number of GRB detections by

Milagro.

Simulation Details Redshift distributions

• Detected distribution = Intrinsic distribution x detector-related selection function

• Selection function from Swift minimum detectable peak flux• Intrinsic redshift distribution

– Short GRBs• Compact-binary merger scenario• Delay between the creation of compact objects and their final merger τ follows

P(τ)=1/τ• Rate of compact-binary mergers at a redshift z calculated by integrating the Star

Formation Rate from z back to the past weighted by P(τ)– Long GRBs

• Star Formation Rate x fractional mass under some metallicity limit

Duration distribution• By Swift-detected GRBs

– This simulation can only constrain the prompt emission from GRBs

Simulation Details• Isotropic Energy distribution

– Intrinsic Eiso distribution x detector-specific selection function– Derived effective detection threshold in terms of S/sqrt(t90) from Swift data

– Intrinsic Eiso distribution• Power-law with index -1.45• Universal jet profile model

Verification of the Simulation• Fluence distribution of Swift-detected

GRBs. Top: Short GRBs, bottom: long GRBs

• These distributions depend on all the parameters involved in the simulation:

– Intrinsic Eiso, z, t90 distributions– Selection functions: minimum detectable

peak-flux, minimum detectable S/sqrt(T90)

• Excellent agreement between the simulation’s predictions and Swift’s data.

– Curves with good statistics are the simulation results (top solid, bottom dashed).

Swift data

Swift data

*Swift data: N. Butler et al. ApJ 2007

VHE Emission Model

• Simple model on the VHE emission from GRBs:– Not all GRBs have VHE emission– For the GRBs that do have VHE emission:

• Isotropic energy emitted in the 1keV-10MeV energy range is proportional to the isotropic energy emitted in the 40GeV-EVHE,max energy range (where X is a cutoff energy – results given versus various EVHE,max).

• VHE emission on a power-law – results given versus various spectral indices.

• Upper limit set on the proportionality constant R

Upper Limits on the Prompt VHE Emission by GRBs

Number of detected GRBs (by this search) versus the ratio R

Upper limit on R at the 90% Confidence Level

Upper Limits on the Prompt VHE Emission by GRBs

•Upper limits on R (90% CL) versus different spectral indices, maximum emitted energy, and upper metallicity limits.•These results are for the case that all GRBs have VHE emission.

Latest Results• 2007

– Discovery of TeV Gamma-Ray Emission from the Cygnus Region of the Galaxy• Astro-ph: 0611691 – ApJ 658 (2007) L33

– TeV Gamma-Ray Sources from a Survey of the Galactic Plane with Milagro• arXiv: 0705.0707 – ApJ 664 (2007) L91

– Milagro Constraints of the Very High Energy Emission from Short Duration Gamma-Ray Bursts• arXiv: 0705.1554 – ApJ 666 (2007) 361

• 2008– Discovery of Localized Regions of Excess 10-TeV Cosmic Rays

• arXiv:0801.3827– A Measurement of the Spatial Distribution of Diffuse TeV Gamma-Ray Emission from the Galactic Plane with Milagro

• arXiv:0805.0417 – Accepted at ApJ– The Large Scale Cosmic-Ray Anisotropy as Observed with Milagro

• arXiv:0806.2293 – Submitted to ApJ• In preparation

– Results of the triggered and untriggered GRB searches– Search for TeV Pulsation from the Crab and Geminga– Energy Spectra of Selected Gamma-Ray Sources

HAWC

TeV -rays: A New Window on the Sky

0.1 GeV

Milagro 10 TeV gamma-rayTeV gamma ray

Milagro HESS

Background and Signal

Probability distribution

AGNs

• HAWC will obtain duty factors and notify multiwavelength observers of flaring AGN in real time.

• Milagro has observed 7yr lightcurve of Mrk 421• HAWC’s increased sensitivity would result in ~10x smaller error bars

and have similar error bars on hour time scale rather than 64 days

Mila

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MJD - 500001/1/2000 1/1/2001 1/1/2002 1/1/2003 1/1/2004 1/1/2005 1/1/2006 1/1/2007

Milagro and XTE ASM 7 yr lightcurve of Mrk 421 (Smith et al. ICRC 2007)