Director BAA Deep Sky Section
(Published in "The Deep Sky Observer", the quarterly journal of the Webb Society, issue 116, April 1999)
Here we present an introduction to observing Active Galactic Nuclei by amateur astronomers. We include a brief historical perspective of professional observation, a basic explanation on the nature of AGN, a description of the methods employed by amateurs to monitor these objects, and a brief survey of some of the best examples for study using modest apertures.
The observation and study of Active Galactic Nuclei (AGN) is a fairly recent branch of astronomy. Carl Seyferts work on the class of galaxy that bears his name did not start until the 1940s. The discovery of Quasars in the 1960s led to an explosion in the number of observations of these objects, especially spectroscopically, which has had a profound influence on our understanding of cosmology. While AGN remain exotic objects and the majority are only detectable with the largest telescopes and most sophisticated equipment, a few of the brighter ones are accessible to the amateur observer, and the contribution of the dedicated amateur can be of a significant benefit to the professional astronomer.
Amateurs have taken an interest in observing the brighter AGN relatively early on in their history. Several British amateurs have fine observing runs from the late 1970s of a selection of objects, not least the relatively nearby BL Lac object Markarian 421. Many recreational observers have tracked down the faint speck 3C273 in Virgo and marvelled more at its nature than its telescopic appearance. And now the availability of sensitive CCD equipment to the amateur widens their potential to produce valuable results. The first co-ordinated monitoring began in the UK with the British Astronomical Association's Variable Star Section adding 3C273, NGC4151 and Markarian 421 to its programme. These AGN were monitored visually as if they were ordinary variable stars. As apertures of amateur telescopes increased during the 1980s, more AGN were added to the programme. The Astronomer organisation charted many AGN during the 1980s, and regularly published raw visual and CCD observations of AGN within its variable star pages.
The potential for photographic work was realised in the late 1980s when Denis Buczynski, then the BAA Deep Sky Section Director, taking advice from Professor Ian Robson of the University of Lancaster, attempted to develop a complementary observing programme of AGN, monitoring those on the programme and adding fainter examples . For a variety of reasons this did not take off as hoped, and it was not until the advent of CCDs became available to the amateur community that the next significant action was taken.
Currently, there is a small group of visual and CCD observers (see Table 1) making fairly regular observation of AGN; this represents a co-operative effort from observers in the Variable Star and Deep Sky sections of the British Astronomical Association, along with The Astronomer organisation. The project is co-ordinated by co-author Gary Poyner and results are fed by him to the professional community. Of professional interest in amateur observations there is no doubt. Much support and encouragement has been given by Professor Ian Robson, the late and much missed Dr Michael Penston (RGO), and Dr Mark Kidger (La Palma), amongst others .
Unusual activity in the nuclei of galaxies was first recognised by Minkowski and Humason (Mount Wilson Observatory), when in 1943 they asked graduate student Carl Seyfert to study a class of galaxy with an emission spectrum from the compact bright nucleus. Most normal galaxies show a continuum with absorption lines, but the emission in the Seyfert galaxies betrayed the presence of hot tenuous gas. In some cases the emission lines were broad (Type 1 Seyferts) indicating gas moving with high velocities; in other objects the emission lines were narrow (Type 2 Seyferts ) indicating that the gas was moving more slowly. In the 1950s, as radio astronomy became a rapidly developing science a whole new range of discoveries were made in astronomy. Amongst these were the Radio Galaxies; these objects appeared to be elliptical galaxies that were inconspicuous at optical wavelengths but were shown to have dramatically large, prominent lobes at radio frequencies, stretching for millions of light years from the main galaxy .
The next chapter in this intriguing story, and arguably the most important, deals with the detection of an optical counterpart for the radio-source 3C48 in Triangulum on the Palomar Sky Survey. This radiostar was thought to confirm the presence of real radio stars. The object was described as a Quasi-Stellar Radio Source which was contracted to the term "Quasar". Another optical counterpart to the radio source 3C273 was pinpointed with great accuracy during an occultation by the Moon as viewed by the radio telescope at Parkes Observatory in Australia in 1962. The spectrum obtained by Maarten Schmidt with Mount Palomar's 200 inch telescope in 1963 was extraordinary. He realised that the lines were dramatically red-shifted to a degree unheard of at that time. Subsequent spectroscopy of 3C48 then showed even greater red-shift, indicating that these seemingly faint stars were not only beyond our own galaxy, but more distant than most galaxies known at that time. This led to feverish activity in the professional world. In the words of Allan Sandage: Holy Ned broke loose! From 1963 to 1970 all the worlds spectroscopists ran into the field. It was a Barnum and Bailey's Circus, both at the telescope and in the Journals . A whole new field of study was created. The spectroscopists discovered many more Quasars, but many were found to be radio-quiet, rather ruining the original basis for the term which has nonetheless stuck.
Revision of old observations showed that some stars thought to be variable, were in fact AGN - most notably BW Tauri (3C120), W Comae (ON 231), AP Librae, and BL Lacertae. Spectroscopy of the so-called BL Lac objects show a featureless optical spectrum, but dramatic fluctuation in brightness in short time scales. Sub-classes were created in a rather confusing manner, including radio-loud and radio-quiet Quasars, BL Lac objects, Optically Violent Variables, radio-galaxies, and N-galaxies, as well as the two types of Seyferts . Blazars are categorised as AGN which display a high radio brightness, and/or dramatic optical variability over very short time scales - days or even hours in rare cases.
Thirty years on and the dust is settling! A reasonably manageable classification is emerging , and in addition a recent examination of AGN across the spectrum from Gamma-rays to Radio-waves have helped to form a more coherent view of these powerfully luminous and intriguing objects.
To explain the many characteristics which are observed in AGN, we have to look at Black Holes as providing the source of much of the energy detected at multi-wavelengths. A supermassive black hole, surrounded by a torus or accretion disk is generally regarded as the engine for these enigmatic objects. Also present are ionized gas clouds -probably the outer envelopes of stars - which appear to be in constant motion around the inner rim of the black hole. Recent work on OJ287 by the International Time Project Team, have revealed the possibility of a binary black hole system in an eccentric orbit driving this blazar. This model has been put forward in an attempt to explain the periodicity of OJ287  (periodicity is a term not usually associated with AGN), and indeed predictions relating to a deep, sudden fade in early 1998 have been justified, when such an event took place January of that year.
Aims and Methods
The broad aims of amateur AGN observations are threefold:-
1. To monitor apparent change in brightness of AGN.
2. To measure such changes and build up as accurate a light curve in visible wavelengths as possible.
3. To alert professional astronomers to unusual activity, and provide routine observations on a regular basis.
In order to offer as wide a choice in AGN to the amateur as possible, a co-ordinated programme has been set up. There are currently 2l AGN on the programme (Table 2), three Seyferts, eight Quasars and ten BL Lac objects. Although most are relatively faint, several are accessible to modest telescopes visually and all should be fairly straightforward targets for CCD users. Observers are encouraged to make observations as regularly as skies and conditions allow. Almost all AGN on the programme are plotted in Uranometria 2000 , although some have alternative designations from their more popular terms. Use of co-ordinates should allow identification without difficulty however. Uranometria only reaches stars of magnitude 9.5 and all AGN are fainter than this, so charts are needed to locate the objects in their field . These are available from the authors of this paper or The Astronomer organisation, which has provided many of the charts in use today. Most have sequences, of which some are accurate photoelectric data from professional sources, many however require verification. Checking the existing charts with accurate CCD photometry of field/comparison stars using filters would represent a useful project for the experienced CCD user. One or two AGN still have no accurate sequence. At such faint magnitudes, the field can be quite unfamiliar and problems can arise. Take for example 3C66A in Andromeda; the chart was taken from the Guide Star Catalogue and shows a prominent 8.3 magnitude star which is plotted in Uranometria. Slightly to its south on the chart is a 10.1 magnitude star. This latter star has been erroneously scanned and in fact represents a faint field galaxy flanked by two much fainter stars, giving an erroneous diameter and its confusing appearance. This chart error was first identified by visual observers, who noted that the allotted magnitude seemed incorrect. CCD images later showed the existence of the galaxy. In fact there is a scattering of faint field galaxies including UGC 1837, 1832 and PGC 9009.
Once located, the brightness of the AGN can be estimated or measured, visually, photographically or with a CCD. Each has its merits and problems. In favour of a visual approach is the ability to observe several objects in the session; the results are rapidly obtained and the equipment is relatively inexpensive, although realistically a larger aperture is needed for such faint objects. The telescope however need not be equatorially mounted. Set against these advantages are the problems of finding relatively faint objects from less than ideal sites, with problematic sky conditions due to light pollution, moonshine, high cirrus etc. Experience in variable star work is essential. Inevitably some observer bias occurs, although this can be compensated for as with other variable star observations. Even if the AGN poses no great problem, the fainter comparison stars may well be difficult. While the numbers of AGN available are limited, there are still over a dozen to be observed by the owners of a 40cm or larger telescope.
Photography would seem to reduce some of these barriers. Fainter magnitudes can be reached so more active galaxies become available and the comparisons more easily accessed. Personal bias is reduced and the results are easily displayed and reproduced. However, an equatorial or driven telescope becomes essential, raising the financial commitment. For the accurate results which the time and financial outlay deserve, standardisation and formal measurement become necessary. This would require the observer to use a standard film (Kodak 103aB was recommended in the past, but it is difficult to obtain and other emulsions need to be tested). Also filtered images are desirable, and for accurate magnitude estimates microdensitometric measures are ideal. Add to this the laborious process of photography itself and dark room work, it is perhaps understandable that the photographic programme of the late 1980s never really came to fruition. The chance of discovering a nova or supernova is often adequate incentive to be involved in patrol work, but monitoring of an existing object year-in, year-out, is perhaps less enticing. The CCD revolution might be the answer to this. Images made with CCDs can go very deep, even with modest apertures. All the objects on the programme, plus several others, are available to owners of 30cm telescopes equipped with a CCD. Finding objects is notoriously difficult with smaller CCD chips, but once found the AGN can be imaged rapidly and a measure made. Computer software is becoming more sophisticated month by month and accuracys of better than 0.1 magnitudes are now attainable to the discerning observer. The data is easily stored on disc and retrieved at leisure. Rapid comparison with a master image is possible to show significant changes in brightness rapidly. This happy state of affairs is tempered by the need to spend a fair amount of money on equipment! In addition to a driven telescope, there is the CCD camera and computer plus necessary software. So the first meaningful CCD image of an AGN may cost the observer several thousand pounds to achieve. Also, not all CCDs have the same sensitivities. All have a red bias which distorts the results, but different types have different profiles, so for photometry to be really useful to the professional astronomer it is increasingly clear that filtration is essential [10&11]. It may ultimately be acceptable to use just B & V filters, even one of them. However, simple monitoring without photometry does not need filtered images and this is still useful providing the user realises its limitations. At present the majority of observations currently being made are visual, with some complimentary CCD imaging. However with more CCD users looking for serious projects, the emphasis may change a little, although its unlikely that CCD monitoring on an amateur level will ever surpass visual work.
This bright spiral in Cetus is the best example of a Type 2 Seyfert for amateur telescopes, but it remains surprisingly under-observed. Easily located one degree south east of Ceti on the celestial equator, this autumnal target is of magnitude 9.5, is a respectable 7x6 arc minutes in dimensions, and has a relatively high surface brightness. It appears almost face-on, and the nucleus is strikingly bright visually. This effect is lost by long exposure imaging, whether photographically or with a CCD. Shorter exposures show its almost stellar nature. Colour photos reveal its excited hydrogen by a remarkably pink tinge on film. Despite its prominence, the variations in the galaxies magnitude is poorly known. An amplitude of around one magnitude is suspected.
The nearest Type 1 Seyfert is to be found in Canes Venatici and unlike its autumnal counterpart has been very well observed by amateurs and professionals, and has received extended telescope and satellite time from the late 1980s. Important data from the International Ultraviolet Explorer measured changes in brightness of both the central nucleus and the gas rotating around it. The ability to measure the distance between the central core and the gas, as well as the radial velocity of the gas, has enabled the central mass to be calculated. The value obtained is a staggering ten billion solar masses. The host galaxy is a round 6' diffuse glow which is much brighter in the centre, where the nucleus varies between magnitudes 11 and 13.0. This AGN may seem like an ideal object for the newcomer to the field, but making estimates by both visual and electronic means is extremely difficult. The problem lies with the nebulous appearance of the galaxy, which by means of estimating its brightness, must be compared to a point source of light - a star! The light curve taken from BAAVSS data shows visual scatter well. Careful CCD imaging can - to some extent - alleviate this problem . Two "outbursts" of note have been reported in the last few years - 1993 & 1995. Deciding whether NGC 4151 is in outburst or not is not at all straightforward, but it is generally regarded that a magnitude of 11 or higher is classified as an outburst. Since 1995, NGC 4151 has been in a fairly high state, with an average magnitude of ~11.5.
One of the easiest AGN to locate and observe visually is the BL Lac Object Markarian 421. Located 2' south-preceding 51 Ursa Majoris (mag.6), Mark 421 is easy visually with a 20cm or larger telescope at around magnitude 13. It is stellar in appearance, which is not surprising at an estimated 400 million light years distant! CCD users are at a disadvantage with this object, because suitable comparison stars lie out of the field of most current chips at normal focal lengths. Variations in magnitude between 12.3 and 14 have been recorded, and short term variations over short time-scales have also been detected. This is one AGN which is kept under regular scrutiny by professional astronomers, but usually in wavelengths other than those available to amateurs. Part of long term work is to understand how variations at various wavelengths corresponds to those detected in the visual. This is a splendid example of an area ideal for Pro-Am cooperation. Markarian 421 has been monitored by the BAAVSS for many years. The light curve shows activity during the period 1981-1999. It can be seen immediately that the AGN nucleus is in a constant state of change, with several outbursts being detected. The two brightest in this period (1982 & 1992) both reaching magnitude 12. The 1992 outburst was prolonged and saw a slow decline following maximum. It is also evident from the light curve that Mark 421 experienced a 'quieter' period from 1983-88. This was followed by several short lived outbursts during the period 1988-91. Further monitoring by both variable star and deep sky sections will hopefully lead to more outbursts being detected, which will in turn play a small part in future professional studies of this object.
This most historically important quasar is the brightest of its class and amongst the nearest, but is far from easy to locate in a star-depleted region in Virgo. 3C 273 is quite blue on colour film but is merely a faint 13th magnitude star to the visual observer. The quasar is radio-loud and the optical jet responsible has now been imaged by Canadian amateur Jack Newton using a CCD on his home-built 25" reflector--a remarkable feat! The magnitude range is normally between 12.5 and 13.3, which shows quite well in the light curve. There are one or two estimates which would seem to indicate outburst activity, but these do not fit in well with other results obtained at the same time. Quite large seasonal gaps are also evident in the plot, which are unavoidable for an object lying on the ecliptic.
This blazar, which lies in Cancer is a short hop from Praesepe (M.44). It is stellar in appearance in a field of useful asterisms, and the sequence is one of the most accurate on the programme. OJ287 became the centre of attention in 1993, when an international group of professional astronomers drew up a rigorous observing programme to monitor it using land base and satellite telescopes, and asked for the participation of amateurs to help with the long term monitoring. The quiescent magnitude of OJ287 is usually around 16, but short duration flares can take it to mag 14, with full outbursts reaching magnitude 12. One of the prime reasons for the aforementioned observing campaign, was to monitor for an expected outburst in 1994. Both visual and CCD observers played an equally important role in this project. The Visual/CCD/Photographic light curve of OJ287 from amateur data during the campaign can be seen by clicking here (or alternatively a light curve by the co-author Poyner can be seen by clicking here). Negative observations have been omitted for clarity. It can be seen that OJ287 was recorded both visually and by unfiltered CCD at minimum brightness of 15.5-16.0. A short duration flare to magnitude 14.8 was observed on December 11th 1993, which had faded to 15.6 by the 21st. This was followed by another on March 3rd 1994 at 14.5, but by March 5th OJ287 had faded to magnitude 15.5. The predicted outburst occurred in October and November of 1994 (within three months of the expected date) but at a fainter level than was expected. Interestingly both flares and outburst were detected by visual observers. The brightness peaked on Oct 10th at mag 14.0. Further high states were recorded visually in 1995/6. Following this active period, a low state was predicted for 1997 . This indeed occurred towards the end of that year, where OJ287 was recorded visually below magnitude 16. As the field approached conjunction with the sun in 1998, indications are that OJ287 was again increasing in brightness.
This BL Lac object is the most distant of this small number being introduced here, estimated to be an astonishing 4 billion light years distant. 3C66A has been monitored as a comparison source during the international OJ287 project. By sheer coincidence 3C66A entered a high state of activity during the project (quiescent magnitude <15.0), and was recorded at the brightest level that it has ever been seen at magnitude 13.8 during this period. Long term light curves show that 3C66A has brightened steadily for several years. This blazar remained in outburst until mid 1997, when a sharp decline occurred to magnitude 15.3. This was then followed by a short lived rise to magnitude 14.5, after which 3C66A returned to magnitude 15.5v. A light curve taken from observations by Poyner can be seen by clicking here.
W Com (ON 231)
This blazar was discovered in 1911 by Wolf as a 12th magnitude star on a photographic plate. It was at this time thought to be a variable star, and was given the variable designation of W some time later. In 1971 it became the first radio source to be classified as a BL Lac object. A look at the long term light curve of W Com shows an active history . Energetic outbursts were observed in 1940, 1952, 1968 & 1980. The object has been monitored extensively by CGRO since 1991, and has been seen to show the hardest Gamma ray spectrum of all the 60 known Gamma ray blazars. W Com has been under close monitoring by the TA/BAAVSS recurrent objects programme since 1992. A major outburst was detected by Poyner in 1995, which reached magnitude 13.6. Bright levels (14.0-14.5v) were maintained until mid 1997, when W Com returned to a faintish state of below magnitude 15.0. During early 1998, a major outburst was detected by Poyner, with W Com reaching magnitude 13.2 visually -the brightest level reached since discovery. Further, short duration variations were also detected, giving rise to 0.5 magnitude fluctuations over a period of several hours. During this current outburst, it has been suggested that W Com is now the brightest blazar in the entire sky . Light Curves here.
Active galactic nuclei offer a wonderful challenge to the serious amateur observer who wishes to contribute in his or her small way to an area of science in which the professionals welcome help, if carefully provided. While not giving much in the way of spectacular views, AGNs feed the imagination in a way that is equally satisfying to the enquiring mind . The techniques are improving, the quality of results will become more refined, and the study of the deepest of deep-sky objects is about to take off.
1. Meeting report: J. Br.Astron.Assoc.,98, 60(1987).
2. Penston, M: Lovers of Active Galaxies. (Mtg). J.Br. Astron.Assoc.,100, 203-4(1990).
3. Moore, P.:The Astronomy Encyclopaedia,81-83; 342-344; 375. Mitchell Beazley (1987).
4. Finkbeiner, A.,: Active Galactive Nuclei: sorting out the mess, Sky and Telescope Vol.84:138-144,(1992).
5. Schorn Ronald, A.: The Extragalactic Zoo,Parts1-3: Sky and telescope,Vol. 75:23-27, 376-378; Vol. 76: 36-37, (1988 ).
6. Longair, M.: Active Galaxies and Quasars: Images of the Universe, CUP(1991)
7. Kidger, M. Private communication (1997)
8. Tirion, Rappaport, and Lovi : Uranometria 2000, Vol.1.,Willmann-Bell (1987).
9. Abrams, B: Observing Active Galactic Nuclei : Deep Sky, 32, 32-33 Kalmbach Publishing Co.(1990).
10. Mobberley, M.; Arbour R.: Letters: J.Br.Astron.Assoc. 104(4), 202-3.
11. Naylor, T.; Richmond M.W.: Letters: J.Br.Astron.Assoc. 104(6), 312.
12. Tosti, G et al: Light curve of ON 231 during 1994. Available from the www at http://wwwospg.pg.infn.it/wcom.htm
13. Tosti, G. Private communication (1998)
E-Mail - Nick Hewitt
E-Mail - Gary Poyner
|Ron Arbour||South Wonston||Photography||40cm|
|L. Jensen **||Denmark||Visual||35cm|
|M. Lekhy*||Czech Republic||Various||Various|
*coordinator of group **TA observers
Visual observations were received from the following in the 1980s:
A.T.Bueno, J.Ells, N.S.Kiernan, I.Middlemist, R.W.Middleton, R.A.H.Patterson, D.Stott.
|W Com/ON 231||BLLAC||12.21||+28.13||11.5-<16.0v||Com|
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