Blazar, blazar, burning bright: these exotic galaxies are some of the most distant objects visible through backyard telescopes.
Galaxies all shine with a constant luminosity, right? That assumption was shown to be false in 1965 when California Institute of Technology astronomer Fritz Zwicky discovered an unusual, variable object on plates from Palomar Mountain's 48-inch Schmidt telescope. On dozens of plates taken over a 30-year period, this slightly fuzzy "star" varied from magnitude 16 to 18. At first, astronomers thought it might be a variable star superimposed on a distant galaxy. But a spectrum revealed that Zwicky had stumbled on a Seyfert galaxy more than 1 billion light-years distant with the first known variable nucleus.
By 1971 astronomers were scrutinizing the General Catalogue of Variable Stars in search of additional variable galaxies, and several entries were found to coincide with extragalactic radio sources--including BW Tauri (3C 120), BL Lacertae (VRO 42.22.01), AP Librae (PKS 1514-24), and W Comae (ON 231).
Astronomers now classify these objects as blazars, a term coined in 1978 by Columbia University astronomer Edward Spiegel to encompass two classes of objects: Optically Violently Variable quasars and BL Lacertae objects. Blazars are members of a larger group, galaxies with active galactic nuclei (AGNs) powered by actively feeding black holes. As heated material spirals down the accretion disk surrounding an AGN, an intense magnetic field helps produce high-energy, relativistic plasma jets (see page 20).
What makes a blazar special is that one of its beams happens to point directly toward us. From our head-on perspective looking down the throat of its jet, a blazar appears much brighter than it normally would. Changes in the jet result in variability in radio, infrared, optical, X-ray, and gamma-ray wavelengths. In some cases, short-term outbursts of several magnitudes have been recorded over just a few days.
I have observed a number of blazars through my 18-inch Dobsonian telescope at dark sites in northern California. I will describe here four that are visible in the Northern Hemisphere's spring sky, and four more for the rest of the year.
Markarian 421 Spring RA 11h 04.46m, Dec. +38[degrees] 12.5' Vmag 12.0-14.4, Size 0.8' x 0.6'
Markarian 421 is a relatively nearby BL Lacertae object (400 million light-years) and is one of the brightest gamma-ray sources in the sky. It has displayed rapid, violent variations over a period of hours to days along with longer periodic fluctuations of roughly 23 years.
Using 220x in my 18-inch scope, Markarian 421 appeared similar to a 12.7-magnitude star, though I glimpsed a small, fuzzy halo at 300x. This blazar is very easy to locate just 2' south-southwest of 6.0-magnitude 51 Ursa Majoris. Use high power, and keep the bright star off the edge of the field so its glare doesn't overpower the faint blazar.
W Comae Beren ICeS Spring RA 12h 21.53m, Dec. +28[degrees] 14.0' Bmag 13-17.5
German astronomer Max Wolf discovered variable star W Comae Berenices based on observations at Heidelberg-Konigstuhl State Observatory from 1892 to 1916. In 1971 the radio source ON+231 was found to coincide with W Com. This distant blazar (1.3 billion light-years) has varied by 4 magnitudes over the past century.
In June of 2009, I saw W Com as a 14.7-magnitude stellar object at 325x. To find it, locate the 14th-magnitude galaxy NGC 4295 on the northwest side of the Coma Berenices Star Cluster. Shift your position 6' northeast to a 12th-magnitude star and look for the blazar 1' north-northwest of that.
AP Librae Spring RA 15h 17.70m, Dec. -24[degrees] 22.3' Bmag 14-16.7, Size 0.5' x 0.4'
In 1942 Harvard College Observatory astronomer Martha Ashbrook listed AP Librae as one of 74 new variable stars she found on plates taken since 1935. Nearly 30 years later, Howard Bond noticed its equivalence with the extragalactic radio source PKS 1514-24. AP Librae displays a nearly three-magnitude optical variation with occasional short-term oscillations of over magnitude in just 5 hours.
I picked up this compact galaxy in my 18-inch at 225x, though the view improved after upping the magnification to 375x. At this power it appeared as an extremely faint patch just 6" across punctuated by a dim stellar nucleus. AP Lib resides just north of a neat semicircular asterism.
Markarian 501 Spring
RA 16h 53.87m, Dec. +39[degrees] 45.6' Vmag 13.5-14.0, Size 1.1' x 0.9'
Markarian 501 is one of the brightest and closest blazars in the sky, at a distance of 460 million light-years. The NASA-IPAC Extragalactic Database lists 689 references to this object in journal articles and 63 aliases based on its inclusion in numerous radio, X-ray, ultraviolet, infrared, and quasar catalogs.
Viewing Markarian 501 through my 18-inch telescope at 283x, the galaxy appeared as a 14th-magnitude knot just 10" in diameter, though with averted vision the halo doubled in size. With direct vision a sharp, stellar nucleus appeared and the halo nearly disappeared. See if you can detect this blinking effect.
3C 371 Late Spring to Early Autumn RA 18h 06.84m, Dec +69[degrees] 49.5' Vmag 13.5-15.0, Size 0.15'
The radio source 3C 371 was first identified as a galaxy in 1966. A year later Allan Sandage measured a redshift of z = 0.05, corresponding to a distance of nearly 700 million light-years. Various studies have revealed long-term optical variations of 1.5 magnitudes along with spikes of 0.1 or 0.2 magnitude over several hours. The Hubble Space Telescope revealed an optical jet in 1999.
I picked up 3C 371 easily at 175x as a faint, quasi-stellar object that appeared softer than similar field stars. Upping the magnification to 300x, I resolved a compact 10" knot containing a 15th-magnitude stellar nucleus.
3C 66A Autumn / Early Winter RA 2h 22.66m, Dec. +43[degrees] 02.1' Vmag 13.5-15.6
In 1974 Texas astronomers Beverley and Derek Wills identified the radio source 3C 66A as a high-redshift quasar (z = 0.44, distance 4.6 billion light-years). This blazar shows rapid fluctuations as well as long-term changes from magnitude 13.5 to 15.6.
3C 66A appeared as a 14th-magnitude star when I observed it in July 2007, so it was probably near its maximum brightness. Coincidentally, 3C 66A lies behind the rich galaxy cluster AGC 347 in Andromeda and shares a field with three foreground cluster galaxies (circled above).
BL Lacertae Late Summer / Autumn RA 22h 02.72m, Dec. +42[degrees] 16.7' Vmag 12.7-16.0
German astronomer Cuno Hoffmeister discovered BL Lacertae in 1929 on photographic plates. In 1968 John Schmitt of David Dunlap Observatory noted its equivalence with the radio source VRO 42.22.01. BL Lac is the prototype of a class of AGNs with a nearly featureless spectrum displaying very weak absorption and emission lines. It exhibits a slow, irregular variation from magnitude 12.7 to 16.0 with brief one-magnitude flares. It is 900 million light-years away.
At 280x I was able to identify a 15.5-magnitude stellar object just 25" west of a 13th-magnitude star. At moments BL Lac seemed slightly fuzzy, with a tiny 2" or 3" envelope.
BW Tauri Late Autumn / Winter RA 4h 33.19m, Dec. +5[degrees] 21.3' Vmag 13.5-15.5, Size 0.8'x0.6'
Harvard astronomers Harlow Shapley and Catherine Hanley discovered BW Tauri in 1940 during a photographic search for new Cepheid variable stars. In 1968 Michael Penston (Royal Greenwich Observatory) discovered that BW Tauri is a compact galaxy coinciding with the variable radio source 3C 120. This blazar varies from magnitude 13.5 to 15.5 and lies at a distance of 450 million light-years.
Through my 18-inch scope, BW Tauri appears as a fuzzy 14th-magnitude star (the nucleus of the galaxy) encased in a low-surface-brightness halo about 20" across.
Steve Gottlieb has observed almost all of the NGC and much of the IC through his Dobsonian telescopes.
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|Title Annotation:||Observing Variable Galaxies|
|Publication:||Sky & Telescope|
|Date:||Apr 1, 2010|
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