Seeking Markarian 421: The Brightest Blazar
March 2011 :
The largest telescope I own is 6” in aperture, but a number of objects I like to observe are considerably fainter than what a 6” scope is capable of showing. My usual answer to this shortcoming is to visit my good friend Tim Dube of East Douglas, Mass. Tim has owned a startling variety of telescopes since becoming involved in amateur astronomy back in the late 1990s, many of them larger than 6” and some 16” in aperture and larger. In early February, he sold a 16” Dobsonian he’d used for nearly two years to a friend and then acquired a pair of good refractors, one of 111mm aperture and a smaller scope of 72mm aperture. It didn’t take long for Tim to start missing the light-gathering power of the big scope and I was getting interested in trying to locate and finally observe a key object in Ursa Major, the magnitude 13.4 BL Lacertae-class quasar known as Markarian 421. Suffice it to say that another 16” Dobsonian was added to Tim’s arsenal in due course, one having a finer quality mirror than that of his prior 16”. The replacement Dob was picked up on a trip to Albany, NY on March 6 and we started testing it on the evening of the next day.
I think it must have been about a year ago that I became aware of a particular quasar in Ursa Major that intrigued me because it was classed as being a bright, possibly observable example of a “blazar”, a quasar oriented with respect to our position in a manner that permits observation nearly directly “down the barrel”, or straight into the axis of one of its relativistic jets. When I read that the variable magnitude averaged about 13.4, I resolved to check detailed star charts to plan a star-hop, knowing that I would be able to see it in a scope of sufficient size on a good night from Tim’s yard, provided the Moon would not be an interfering factor. Certain things didn’t develop during the winter-spring portion of last year and we didn’t end up seeing Markarian 421, for reasons I can’t recall. We succeeded, though, on March 7 of this year.
This object lies close to the magnitude 6.0 star 51A UMa, which in turn is positioned at the western tip of a distinctive, small 5-star asterism. Almost immediately upon locating this diamond-shaped star pattern in the 16” and verifying 51 UMa at one end, I could easily make out what had to be Mrk 421!
The accompanying table lists, progressively from north-to-south, 7 stars in Ursa Major which I used for my star-hop to a point very close to the quasar. You can access AAVSO star charts enabling an easy location of Mrk 421 by going online to a website called Washed-out Astronomy and clicking on a section entitled Quasars are Easy. (This is the same site I recommended in my article in the May 2010 issue of this newsletter on finding the quasar 3C 273 in Virgo. Although that object is brighter, on average, than Mrk 421 in Ursa Major, the latter is far easier to find and positively identify than 3C 273, owing to a lucky and coincidental placement on the sky close to a semi-bright star that is not so bright that it renders the quasar invisible!)
A simple star-hop begins by noting the 2 stars which form the lower corners of the Big Dipper. Beta UMa (Merak) marks the western end of this wide pair; Gamma UMa (Phecda, or Phad) lies at the eastern end. Visualize the mid-point between these stars and go just over 10° to the SSW to the 7th brightest star in the constellation, magnitude 3.0 52 (Psi) UMa. This star marks the S tip of a long, somewhat “spearhead”-shaped triangle having its shortest side marked by the line joining Beta and Gamma, which you might regard as being the base of the spearhead/spearpoint form. Next, go less than 3.2° WSW of 52 UMa to the magnitude 4.7 star 45 (Omega) UMa. This star is the first in a row of 4 semi-bright stars that form an almost straight line roughly 4.2° long that runs SSE from 45 UMa down to 49 UMa. These 4 stars will look approximately evenly-spaced apart from one another. The most important guide star nearest to Mrk 421, 51A UMa, lies about 1.2° SE of 49 UMa, an angular distance very close to that of the separation between 49 and 47 UMa. (47 is the 3rd star down the line of 4 from 45 UMa.)
51 UMa is a multiple star having 3 components, the faintest of which will not be confused with Mrk 421 because it is magnitude 12.6 and is only 8.5” away from the far brighter primary. I didn’t even notice it in the 16” scope, actually. 51C UMa is easily seen due E of 51A by a wide separation of 150” and is the closest star to 51A of the 5 stars comprising the diamond-shaped little asterism I mentioned earlier. Mrk 421 lies SSW of 51A a bit less than the distance from A to 51C, perhaps roughly 120” SSW. These 3 objects form a coathanger-shaped, small triangle, with 51A UMa marking the center “hook” point and the quasar marking the SSW tip, seen in a big enough scope as a bluish-gray, star-like point of light. The distance to Markarian 421 is estimated to be from 395 to 435 million light-years, meaning that to see this ancient light is to look back in time to the Devonian or Silurian Periods of the Paleozoic Era. This blazar is one of 7 or more types of Active Galactic Nuclei currently recognized. An AGN is the superluminous, immensely energetic core of a certain general class of galaxy and may only be about the size of our Solar System, yet is able to produce light visible at staggering distances of hundreds of millions–or more typically, a few billion light-years!
Mrk 421 has been observed at a maximum brightness of 11.6 magnitude down to 16 over the history of research on this object, but its average of 13.4 makes it a worthwhile target for those of you who own telescopes of sufficient size and are willing to try something different. Remember: It’s easy to find, easy to see!
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