TeV Data

Whipple 10m Telescope

Gamma-Ray astronomy

Markarian 421

Markarian 501

1ES 1959+650

Kirsten Larson's REU Home Page

Gamma-Ray Astronomy

Much of our information about the universe comes from electromagnetic radiation. Scientists observe radiation at all wavelengths, from radio waves to gamma rays to learn about the cosmos.

The Electromagnetic Spectrum

The universe has been observed with optical light for centuries. However, visible light gets can’t tell us everything. By observing objects at different wavelength we can learn about the different physical properties and how they work. Gamma-rays are the most energetic form of light and can tell us about high energy astrophysical objects like supernovas, Active Galactic Nuclei, and blazars.

Mrk 421 with an optical telescope

Mrk 421 with the Whipple gamma-ray telescope

The first gamma ray satellite, Explorer-XI, was launched in 1961 and detected fewer then 100 gamma-ray photons. Since then, dozens of gamma ray satellites have been created including the more resent GLAST, and INTEGRAL. Gamma rays are studied by satellites because they get absorbed by our atmosphere, which is only transparent to visible, infrared, and long radio waves. However, these satellites are limited in their size and weight and therefore have low thresholds of about 10 GeV.

Fortunately,physicists discovered that high energy gamma-rays could be detected from ground base telescopes indirectly. When high energy gamma-rays enter our atmosphere and are absorbed, they create a shower of high energy particles.

Cherenkov Radiation produced by the gamma-rays.

These secondary particles emit a radiation, called Cherenkov radiation, at a characteristic angle. This Cherenkov radiation passes though our atmosphere and can be detected in the visible and UV range by Imaging Atmospheric Cherenkov Telescopes.

Imaging Atmospheric Cherenkov Telescopes (IACT)

These IACT’s are sensitive to energies of 100 Gev’s and greater. They require a large collection mirror to detect the faint light produced by the Chenenkov effect. These mirrors focus the light on a photomultiplier that detects the radiation. Because the Chenkov radiation is so faint, IACT’s are usually located on mountains away from light pollution and where there are less clouds. The moon also hinders the detection of the light and therefore clean detections can only be made on moonless nights. Cosmic ray protons are also absorbed by the atmosphere and produce showers and Cherenkov radiation. The IACT’s detect these cosmic rays and the numerous events of this cosmic ray background must be sorted out from the desired gamma ray showers. Gamma ray photons rather then cosmic rays produce less then 1% of the events detected.


Veritas Gamma-Ray telescope

MAGIC Gamma-Ray telescope

Air Cherenkov Detectors

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Markarian 421

Markarian 421 is a blazar that was detected by Whipple in 1992 as the first extragalactic VHE gamma-ray. Even though Mrk 421 had been observed by Whipple in the past, this was the first detection of TeV energies measuring a flux of approximately .3 crabs. The first flare was observed in 1994 when the flux increased from .15 crabs to 1.5 crabs. Flaring was also observed in 1995.

Schubnell Mrk 421

Shubnell et al. (1996)

Variability on timescales shorter then a day was first observed in 1996. On May 7 1996 Mrk 421 flared to about 10 Crab, doubling its intensity in less then an hour. By May 8 its flux was back down to 0.3 Crabs. Then, a Flare on May 15 showed variability on a 15 min. time scale.

Gaidos Mrk 421

Gaidos et al. (1996)

Throughout 1997 and 1998 Mrk 421 had an average flux of 0.5 Crabs and flared only 3 times to about 1 Crab. Then in 2000/2001 Mrk 421 had high flaring ranging from 0.4 to 13 Crabs and lasted for about 5 months.

Aharonian Mrk 421

Aharonian Mrk 421

Aharonian et al.

Aharonian et al.

Piron Mrk 421

It was relatively quiet in 2002/2003 except for a flare around March 30, 2003 (MJD 52,728) where it reached a peak of about 4 gamma/min and lasted for a week.

Steele Mrk 421

Blazejowski Mrk 421

David Steele

Blasejowski et al (2005)

Mrk 421 was much more active in 2003/2004. It had a large flare that lasted for more then 2 weeks (from about MJD 53,104 to MJD 53,120) with a flux reaching peaks of about 3 crab in the TeV and ~135 mcrab in the X-ray.

Blazejowski Mrk 421

Blasejowski et al (2005)


Kerrick et al.,"Outburst of TeV photons from Markarian 421"

M.S. Schubnell et al., "Very High Energy Gamma-Ray Emission from the Blazar Markarian 421"

J.H. Buckley et al., "Gamma-Ray Variability of the BL Lacertae Object Markarian 421"

Gaidos et al. “Extremely rapid bursts of TeV photons from the active galaxy Markarian 421” 1996.

Frank Krennrich, “TeV Gamma-ray Astronomy in the new Millennium” 2001.

L. Maraschi et al.,"Simultaneous X-Ray and TEV Observations of a Rapid Flare from Markarian 421"

X-ray Observations of TeV Blazars and Multi-Frequency analysis, Tadayuki Takahashi et al

A Multiwavelenth View of the TeV Blazar Markarian 421: Correlated Variability , Flaring, and Spectral Evolution, Blazejowski et al

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Markarian 501

Markarian 501 was first detected by Whipple in 1995 and had an average flux of about .1 crab. In 1997 Markarian 501 became highly variable with hour-scale variations. Markarian 501 had a large flares on April 16, May 9th, and June 8th with hour scale variability measured in May and June.The average flux above 350 GeV during this period was 1.6 crabs. Previously, the gamma-ray flux from Mrk 501 was never recorded to be higher than 0.5 crabs.

Krennrich Mrk 501

Petry Mrk 501

Quin et al. (1999)

Petry et al.

Piron Mrk 501

Piron et al.

The flux during April ranged from a low of 4.9 +/- 1.8 * 10^11 photons cm^2 /s on April 19 to a high of 40.5 +/- 9.6 * 10^11 photons cm^2 /s on April 16.

Gliozzi Mrk 501

Gliozzi et al. (2006)

After 1997 Mrk 501 has been fairly quiet and only flared for 2 weeks in June of 1999.


Detection of Gamma Rays with E > 300 GeV from Markarian 501, J. Quinn et al

Multiwavelength Observations of a Flare From Markarian 501, Catanese et al

Long-Term X-ray and TeV Variability of Mrk 501, M. Gliozzi et al

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1ES 1959+650

1ES 1959+650 was first observed flaring in 2002 and has variability on timescales as short as several hours. It was most active between MJD 52428 and 52429.The flux between May and July 2002 was highly variable, with an average flux of .64 +/- .03 crabs and reached up to 5 crabs.

Furthermore, an orphan flare was observed on June 4, 2002. The TeV flux increased from .26 +/- .21 Crabs to about 4 Crabs in only 5 hours while the X-ray flux, X-ray photon index, and optical brightness stayed the same.

Holder 1ES 1959+650

Aharonian 1ES 1959+650

1ES 1959+650 during an orphan flare. (Holder 2002)

Aharonian et al.

Throughout the 2003, no major TeV gamma-ray flares were observed and no significant flux variability was measured in the X-ray or optical data.

Steele 1ES 1959+650

Gutierrez 1ES 1959+650

David Steele

Gutierrez et al.


Multiwavelength Observations of 1ES 1959+650, One Year After the Strong Outburst of 2002. Gutierrez et al

Detection of TeV Gamma Rays from the BL Lacertae Object 1ES 1959+650 with the Whipple 10 Meter Telescope,J. Holder, et al

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Whipple Gamma ray telescope