During the period of this contract, June 1, 1991 to November 14, 1992, the major results of our research effort have come from the Whipple air shower experiment in Tucson, AZ. The most notable development has been the discovery of TeV photons from the BL Lac object, Markarian 421. This result depended critically on the identification of Mrk 421 by the EGRET team as a source of GeV gamma rays

Akerlof, Carl W.

English

NASA Technical Reports Server

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NASA-CR-192781
Correlative Studies of Astrophysical Sources
of Very High and Ultra High
Energy Gamma-Rays
NASA Grant: NAG5-1591
Final Report
Carl W. Akerlof
January 20, 1993
Overview
During the period of this contract, June 1, 1991 to November 14, 1992,
the major results of our research effort have come from the Whipple air
shower experiment in Tucson, AZ. The most notable development has been
the discovery of TeV photons from the BL Lac object, Markarian 421. This
result depended critically on the identification of Mrk 421 by the EGRET
team as a source of GeV ")'-rays.
Very High Energy Astronomy
Following the announcements of numerous extragalactic sources of GeV
radiation by the EGRET experiment onboard the CGRO, the Whipple Col-
laboration undertook searches at higher energies for all those objects visible
from the latitude of Tucson, AZ. Six AGN's were targets for extensive obser-
vation, but only one of these yielded a detectable signal. The BL Lac object,
Markarian 421 was monitored for 7.5 hours on-source and an identical time
off-source. The data was analyzed using techniques developed in the course
of numerous measurements of the Crab Nebula. The signal from Mrk 421
corresponded to a statistical significance of 6.3 a. Numerous checks were
made to ensure that this result was not some artifact of the measurement
technique. We were particularly concerned that the two bright field stars
within a few arc-minutes of Mrk 421 might bias the signal. Absence of such
effects was demonstrated by removal of the central photomultipliers from the
offiine data analysis and by observing a bright field star similar in spectral
class to the ones close to Mrk 421. The energy spectrum is shown in figure 1.
(NASA-CR-192781) CORRELATIVE
STUDIES OF ASTROPHYSICAL SOURCES OF
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Markarian 421
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Figure 1. The integral energy spectrum for Markarian 421.
The fact that Markarian 421 is the nearest AGN detected by EGRET
(z = 0.031) is probably not coincidental. Shortly before our discovery, Stecker
et al. had published a paper demonstrating that TeV radiation Would be diffi-
cult to detect from more distant objects such as 3C279 due to inverse annihi-
lation with infrared photons in intergalactic space. If the detection threshold
of our ground-based Cerenkov air shower telescopes could be reduced to 100
GeV, these absorption effects would diminish significantly, making it possi-
ble to view many more of the EGRET-identified AGN's. Such a detector
development program has been a major aspect of our work at the University
of Michigan during the period of this contract.
The discovery of TeV radiation from Mrk 421 raises many new questions.
To the level of our measurement accuracy, the spectral index does not change
from the GeV to the TeV energy range. This implies that this object radiates
as much energy in the TeV band as at all lower energies. The physical
processes that can produce such efficient generation of high energy quanta
are not understood, particularly given the propensity of such photons to
degrade via the inverse annihilation processes mentioned above.
Observations of time variability give some hint to the physical size of the
emitting region of the AGN. Our data was insufficient to resolve whether or
not there was a significant variation of flux during the four month period
when Mrk 421 was observed. We expect to improve the sensitivity of these
observations in the next few months.
Some additional observations of the Crab Nebula were taken during the
winter of '91-'92 with results similar to those reported in the past. Our
group did attempt to search for time correlations in the TeV gamma-ray
flux using an autocorrelation technique. No deviation from Poisson statistics
was observed. We also began development of a method to optically detect
the 30 Hz periodicity of the Crab pulsar using only the l 1-meter diameter
Cerenkov telescope. Normally, this would be impossible because Of the poor
focal properties (the point spread function is about 0.25 ° FWHM). How-
ever by integrating the photomultiplier current from the counter tube of the
Cerenkov air shower imaging camera, it appears to be possible to obtain the
requisite sensitivity using the FFT algorithm to filter the data. We have not
yet detected the Crab optical modulation with this technique but hope to
obtain better results in the next two months.
One of the major interests of our VHE program has been observations
of Geminga. With the recent results reported by ROSAT, EGRET, and the
reanalysis of older data by COS-B, we had hoped to extract a periodic signal
from our TeV data. Although there are some questions regarding the phase-
linking of observations that span from COS-B to EGRET, the bottom line
is that we found no evidence for a signal in our data. The upper limit to the
flux is about 10% of the Crab Nebula for energies above 400 GeV. A paper
describing this search is now being completed.
Ultra-High Energy Astronomy
During the period covered by this contract, the UMC experiment contin-
ued to amass extensive air shower data. This experiment has as a principle
goal the observation of astrophysical sources of gamma-rays by selecting those
showers with significantly fewer penetrating muons. No convincing evidence
for gamma rays at these energies (> 100 TeV) has yet been found but the
upper limits for the Crab Nebula are now beginning to approach the extrap-
olations of the flux from lower energies. Detection of Markarian 421 will
probably be difficult due to the interaction of PeV photons with the cosmic
microwavebackground radiation in intergalactic space. The UMC experi-
ment has also searchedfor diffusesourcesof radiation causedby cosmicray
collisionswith molecular clouds within our galaxy. No signal has yet been
detected but current upper limits approachvaluessuggestedby theoretical
estimates.
Publications
"Locating very high energygamma-ray sources with arcminute accuracy",
C. Akerlof, M. Cawley, M. ChanteU, D. Fegan, K. Harris, A. HiUas,
D. Jennings, R. Lamb, M. Lawrence, M. Lang, D. Lewis, D. Meyer,
G. Mohanty, K. O'Flaherty, M. Punch, P. Reynolds, M. Schubnell, T.
Weekes, and T. Whitaker, The Astrophysical Journal 377, L97-L100
(1991).
"Mirrors for an imaging gamma ray telescope", Curtis Weaverdyck, Donald
Meyer, and Carl Akerlof, Nuclear Instruments eJ Methods in Physics
Research A310, 690-692 (1991).
"Search for diffuse cosmic gamma rays above 200 TeV", D. Ciampa, et at.,
Astrophys. J375, 202 (1991).
"An inexpensive circuit for monitoring photomultiplier anode current", Dov
Frishman and Carl Akerlof, Nuclear Instruments 8J Methods in Physics
Research A311, 306-308 (1992).
"CAMAC modules for recording arbitrarily long time series", Steve Freeman
and Carl Akerlof, Nuclear Instruments (J Methods in Physics Research
A320, 305-309 (1992).
"Markarian 421", Whipple Observatory Collaboration, IAU Circular No.
5522_ May 16, 1992.
"Detection of TeV photons from the active galaxy Markarian 421", M.
Punch, C. Akerlof, M. Cawley, M. Chantell, D. Fegan, S. FenneU,
J. Gaidos, H. Hagan, A. Hillas, Y. Jian, A. Kerrick, R. Lamb, M.
Lawrence, D. Lewis, D. Meyer, G. Mohanty, K. O'Flaherty, P. Reynolds,
A. Rovero, M. Schubnell, G. Sembroski, T. Weekes, T. Whitaker, and
C. Wilson, Nature 358, 477-478 (1992).
"Search for discrete sources of 100 TeV gamma radiation", J.W. Cronin, et
al., Phys. Rev. D45, 4385 (1992).
"Search for compact sources of cosmic photons above 200 TeV", D.P. Ciampa,
et al, Phys. Rev. D46, 3248 (1992).
"A northern sky survey for astrophysical point sources of 100 TeV gamma
radiation", T.A. McKay, et al., submitted to Astrophys. J (1993).
"A large air shower array to search for astrophysical sources emitting 7-rays
with energies > 1014 eV", C.E. Covault, et al., submitted to Nucl. Instr.
Meth. (1993).
"Observations of the shadows of the Sun and the Moon using 100 TeV
cosmic rays", A. Borione, et al., submitted to Phys. Rev. D (1993).
Capital Expenditures
No equipment was purchased from funds supplied by this NASA contract.


Correlative studies of astrophysical sources of very high and ultra high energy gamma-rays

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