2MASS image of Markarian 501
|Observation data (J2000 epoch)|
|Right ascension||16h 53m 52.21s|
|Declination||39° 45′ 37.6″|
|Redshift||9915±25 km/s or 0.033640 Z|
|Distance||456 Mly (140 Mpc) 4.32x1024m|
|Group or cluster||zw1707.6+4045|
|Apparent dimensions (V)||94.86" × 71.1" |
|Notable features||brightest object in very-high-energy gamma rays|
Markarian 501 (or Mrk 501) is a galaxy with a spectrum extending to the highest energy gamma rays. It is a blazar or BL Lac object, which is an active galactic nucleus with a jet that is shooting towards the Earth.
The galaxy hosting the blazar was studied and catalogued by Benjamin Markarian in 1974. It was first determined to be a very high energy gamma ray emitter in 1996 by J. Quinn at the Whipple Observatory.
The gamma rays from Mrk 501 are extremely variable, undergoing violent outbursts. The gamma ray spectrum of Mrk 501 shows two humps. One is below 1 keV and can be considered to be X rays and the other is above 1 Tev. During flares and outbursts the peaks increase in power and frequency. Flares lasting 20 minutes long with rise times of 1 minute have been measured by MAGIC. In these flares the higher energy gamma rays (of 1.2 Tev) were delayed 4 minutes over the 0.25 TeV gamma rays. This delay has led to various theories, including that space is bigger at small dimensions with a foamy quantum texture. The foam would create a variation in the speed of light for higher-energy light gamma-rays and the lower-energy radio waves and visible light. Such a variation would contradict Lorentz invariance, but could provide a clue for unification theory. Observations of Dr. Floyd Stecker of NASA's Goddard Space Flight Center of Mrk 501 and Mrk 421 demonstrated that there is no violation of Lorentz invariance. The galaxy is also variable in visible light between magnitude 14.5 and 13.6.
During the discovery observations flashes at the average rate of one in seven minutes were observed. Cosmic rays (that is, fermionic or massive cosmic rays, as opposed to photons) were ruled out by the shape and size of the flashes which are small and elliptical for gamma rays. The flux for photons over 300 GeV at this point in time in 1995 was 8.1±1.5 x 10−12 cm−2s−1
Blazars are likely to originate from matter falling into a black hole and possibly a binary black hole. The velocity dispersion (which is the maximum difference in the velocity toward or away from Earth) observed in the galaxy is 372 km/s which predicts a black hole mass of (0.9 − 3.4) × 109 M⊙. However, dispersion of velocity was also measured as 291 and 270 km/s so the central mass may be less. A 23-day variability suggested that an object may be orbiting the central black hole with a 23-day period.
With Very Long Baseline Interferometry radio waves fine detail can be seen down to milliarcsecond (mas) resolution. A central very bright single point called the core is observed. From the core an extremely high speed blast of plasma comes out in a narrow cone shape as a one sided jet. After 30 milliarcseconds the jet does a 90° turn and fans out. The jet is 300 pc long. The inner jet before the kink shows bright edges or a limb brightened structure less than 10 mas wide. This is probably due to a fast moving central part to the jet, and slower edges. Normally there would be jets of gas shooting out in opposite directions. The observed jet is the one that faces the earth and project plasma towards the earth. There is also a jet heading away from the Earth called a counter jet. Close into the core this counter jet is so much dimmer than the main jet that it is invisible in radio waves. The brightness of the counter jet is less than the main jet by a factor of 1250. This implies that the jet is relativistic with Γ about 15 (that is the plasma is moving at 99.8% of the speed of light) and at an angle between 15° and 25° from the line of sight from the Earth. At 408 MHz the power level is 1.81 Jy, although this is variable. Beyond 10 kpc from the core the counter jet becomes visible, showing that the jets have become non-relativistic (plasma is no longer moving close to the speed of light). The symmetrical radio emission extends to 70" which corresponds to 120 to 200 kpc.
Other designations: B1652+39 or 1H1652+398 or TeV J1653+397.
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