Recent Results of the Milagro Experiment
To date, our most important published observational results with Milagro have
been:
 The first detection of TeV gamma rays from the Galactic plane [1],
 The mapping of the diffuse Galactic gamma-ray emission at TeV energies,
including the detection of the Cygnus Region at high-significance (over 10) [2]
 The discovery of a new (slightly extended) source (MGRO J2019+37) of TeV
gamma rays embedded in the Cygnus Region [3],
 The possible detection of a gamma-ray burst with our prototype instrument
Milagrito [4].
In addition, we have detected TeV gamma rays from the active galaxies Mrk 501 [5],
Mrk 421 [6] plus the Crab Nebula [7], set stringent upper limits on the prompt TeV
emission from several gamma ray bursts [8], and performed the most sensitive survey
of the northern hemisphere at TeV energies [6].
In this document we will review these results, plus show new results on two
more newly discovered sources that have been presented at meetings. In addition,
we show four more potential sources that are detected with >4.5, slightly less than
the 5 post trials threshold we set for discovery.
It should be noted that the most exciting discoveries from Milagro have been
published since our 2005 review. Figure 1 shows the significant improvement that we
have made in the last view years with the addition of the outrigger counters and
improved gamma-hadron separation and data analysis.
Cygnus Region
MRK 421
Crab Nebula
Figure 1. A northern sky seen in TeV gamma rays using Milagro data. The top panel shows the sky
as observed before the outriggers and the bottom panel after the completion of the outrigger array. 
SURVEY OF THE GALACTIC PLANE AT 12 TEV WITH MILAGRO AND THE
DISCOVERY OF MGRO J1909+06 AND MGRO J2033+42
This analysis of approximately 6 years of Milagro [9] data shows that an excess of
TeV gamma rays also exist along the plane. Most of the activity occurs in the Cygnus
region of the Galaxy, as seen in Figures 1 & 2. (The Cygnus region is also where
EGRET saw the strongest gamma ray emission outside the galactic center which is not
visible from the northern hemisphere). This is also seen in a previous analysis of the
plane using 3 years of data in [10], where the median energy is 3.5 TeV. In the past
year, the sensitivity of the Milagro experiment has been substantially increased through
improvements in gamma/hadron separation and data analysis. We now use a
gamma/hadron separation parameter, A4, which is capable of rejecting background with
high efficiency at high energies. We the improved sensitivity with the introduction of an
analysis technique which weights events based on the likelihood that they are due to
gamma rays. This method is equivalent to the Likelihood ratio method in the large N
limit. These two improvements result in an increase in sensitivity of ~2.5x and an
increase in median energy from to ~12 TeV. Note that the peak sensitivity of the
Milagro detector is well above that of ACTs.
Figure 2. Significance map of the Galactic plane visible to Milagro. There are 7
interesting spots of TeV gamma-ray emission with > 4.5 standard deviations within |b| <
5°. Four of these spots are in the Cygnus Region of the Galaxy from l  [65°, 85°].
MGRO J1909+06
MGRO J2019+37
Longitude Profile
|b|<2°
MGRO J2033+42
GALPROP optimized to fit EGRET
Cygnus Region
below horizon
Figure 3. Milagro’s measurement of the longitudinal profile of the galaxy with
|b|<2°. The overall excess in the Cygnus region can be seen as well as the
three new sources discovered by Milagro. The red line is the GALPROP fit to
EGRET data. This is the first measurement of this type above 1 GeV.
We compute the fluxes reported here by assuming a Crab-like source spectrum with
a spectral index of -2.62 taking into account the changing exposure of Milagro with
respect to the declination. This information is applied to the data to give absolute fluxes
for gamma-ray sources and diffuse fluxes from different regions of the Galaxy. The flux
are scaled to the differential HEGRA measured Crab flux extrapolated to 12 TeV [11].
Table 1 gives the flux from different longitude ranges in the plane. The Cygnus
Figure 4 – The Cygnus region as seen by Milagro shown with contours of expected
0 produced gamma-rays s predicted from the GALPROP model with no sources.
The crosses are unidentified EGRET sources.
Region is the most luminous source of TeV gamma rays visible to Milagro with a
significance of 10.5 standard deviations above the isotropic background. The flux
measurement is 8.7 standard deviations above the GALPROP prediction for diffuse
emission, where GALPROP is fit to agree with the EGRET diffuse GeV measurements
for the plane [12]. This large discrepancy indicates that the VHE emission from the
plane is not dominated by diffuse emission from cosmic ray interactions with matter, but
an additional hard component likely associated with localized sources.
TABLE 1. Gamma-Ray Emission from the Galactic Plane
Object
Galactic
Plane
Position Box (deg)
|b| < 2°
30 < l < 110
30 < l < 65
65 < l < 85
85 < l < 110
136 < l < 216
Significance

7.8
5.6
10.5
0.4
0.9
Flux1 (x10-12)
(TeV-1cm-2s-1sr 1)
GALPROP (x10-12)
(TeV-1cm-2s-1sr-1)
3.0 ± 0.3
4.1 ± 0.6
3.8 ± 0.3
< 1.3 (95% CL)
< 1.4 (95% CL)
1.6
2.5
1.2
0.7
0.3
1
errors are statistical; add a 20% error due to the systematic uncertainty in the HEGRA Crab flux used to
normalize the data
Gamma-Ray Source Discovery
A search for localized sources of TeV emission from the plane was conducted by
identifying spots with more than 4.5 standard deviations (pre-trials) above the
background. Seven candidate sources are seen, excluding the Crab Nebula, for
Galactic latitudes |b| < 5°. One candidate is seen at Galactic longitude l ~ 106°, 4
sources are seen in the Cygnus Region, and two more are observed near l ~ 40°. The
source MGRO J2019+37 is the brightest spot in the Cygnus Region with a flux of ~ 0.5
Crab. TeV gamma-ray emission from MGRO J1909+06 and MGRO J1854+01 are
consistent with that seen from the Crab Nebula for a -2.62 spectrum, as shown in Table
2.
TABLE 2. Galactic Sources and Source Candidates
Object
Location1 (deg)
Significance2
(l, b)
(std. deviations)
Crab Nebula
-175.4 ± 0.1, -5.7 ± 0.1
15.2
MGRO J2019+37
75.1 ± 0.1, 0.3 ± 0.1
11.3
MGRO J1909+06
40.5 ± 0.1, -1.0 ± 0.1
8.2
MGRO J2033+42
80.4 ± 0.1, 1.0 ± 0.3
7.1
MGRO J2032+37
76.3 ± 0.1, -1.9 ± 0.2
5.8
MGRO J2043+36
77.2 ± 0.2, -4.0 ± 0.2
5.6
MGRO J1854+01
34.1 ± 0.3, 0.0 ± 0.2
5.1
MGRO J2233+60
106.4 ± 0.5, 1.7 ± 0.8
4.5
Flux3 (x10-14)
(TeV-1cm-2s-1)
*
2.3 ± 0.3
4.8 ± 0.7
1.6 ± 0.3
0.8 ± 0.2
1.0 ± 0.2
5.1 ± 1.1
1.1 ± 0.3
Items in bold have >5 post trials significance.
* fluxes are normalized to the HEGRA Crab flux at 12 TeV
1 add 0.3° error for pointing systematics and unknown source region morphologies
2 pre-trials significance
3 add 20% error due to systematic uncertainty in the HEGRA measured Crab flux
The number of trials is based on the large regions of the Galactic plane searched and
results in ~ 100,000 trials factor that should be applied to the pre-trials significances of
Table 2. The post-trials significance is greater than 5 standard deviations for MGRO
J2019+37 MGRO J1909+06, MGRO J2033+42. Each of these detections is the
discovery of a new VHE gamma-ray source.
MGRO J2019+37 is coincident in location with EGRET unidentified sources 3EG
J2016+3657 and 3EG J2021+3716. The former is possibly associated with AGN of
unknown redshift and the latter with latter with pulsar wind nebula G75.2+0.1. An open
star cluster, Berkeley 87, is also nearby and could be a site of particle acceleration.
MGRO J2033+42 is in agreement with the location of EGRET source 3EG
J2033+4118 as well as HEGRA TeV 2032+4130 (recently also reported by Veritas).
The HEGRA source has a differential photon flux at 12 TeV of
(0.5±0.2)x10-14TeV-1cm-2s-1. This is 12% of the HEGRA measured Crab flux
extrapolated to 12 TeV. This discrepancy between Milagro and HEGRA may be
understood as due to both uncertainty in the spectrum or a possible diffuse component
underling the HEGRA identified point source that makes a greater contribution to the
Milagro detection due to Milagro’s poorer angular resolution.
Crab Nebula
MGRO
MGRO
MGRO
Figure 5. The Crab plus the three newly discovered Milagro sources.
Energy Spectra with
Milagro
Since the 2005 review, we
have developed and refined
our
event
energy
reconstruction.
Figure 4
shows preliminary result from
this effort. Note that the
spectrum measured by Milagro
is consistent with the ACT
measurements at low energies
and at the highest energies
(~50TeV) the error in the
Milagro measurement is the
smallest.
This is achieved
because
of
Milagro’s
Figure 6
enormous exposure, which can only be achieved by a wide-field continuously
operational instrument.
We anticipate that further forthcoming improvement in the high energy reconstruction
will further improve the sensitivity for Milagro at energies >50 TeV, an energy regime
critical to the understanding of Galactic sources, but difficult to access from ACTs.
HAWC
Our simulations which accurately reproduce Milagro data, including flux measurements,
rates, gamma-hadron separation, etc. predict HAWC will be 15 times more sensitive
than Milagro. (This factor of 15 is in signal to root background meaning our time to
detect a signal will be ~200 times shorter than with Milagro. HAWC will not only be
competitive with IACTs, it will be much more sensitive for diffuse sources.
Figure 7 - HAWC survey vs. IACT sensitivity as a function of the source extent. The units
are given in the integral Crab flux with the IACT threshold of 200 GeV and the HAWC
(>200 multiplicity trigger) median energy of 3.8 TeV. The HESS sources are plotted with
the >200 GeV integral fluxes. The Milagro source in the Cygnus region is shown both for a
median energy of 15 TeV and for 200 GeV by extrapolating using a differential spectrum of
index -2.3, which is the average of the HESS galactic sources. Note that the HESS sources
cluster just above their sensitivity suggesting that more sources are just below this level.
SUMMARY
What we have done.
What we can do with another year of running – at least 3 more sources (need count of
total northern hemisphere sources and what fraction we have found + plus say
something about diffuse emission and cosmic ray sources)
Promise of the technique (first generation) – no upgrades
Development for HAWC
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