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AGN Variability

Variability Timescales
Long Term Variability
Interday Variability
Microvariability

One of the general characteristics of Active Galactic Nuclei (AGN) such as quasars, blazars, and Seyfert galaxies is that their brightness tends to vary. In fact, it is generally assumed that most, if not all, AGN are variable at some level. For example, virtually all of the AGN observed in the Hubble Deep Field were determined to exhibit detectable variability over a period of two years. Furthermore, some categories of AGN known as BL Lacs, (also referred to as blazars) are among the most active variable sources known in the universe. Based on the limitations of the existing data and analysis, some systematic observational material would substantially improve our understanding of the nature of the variability of AGN.

   
Variability Time Scales

Variability studies of AGN typically focus on one of three time scales. These time scales are related to the nature of the data and to the logistics and practical matters associated with obtaining astronomical observations; they are not necessarily related in any meaningful way to the physical processes producing the variability. When discussing AGN variability we typically use long term, interday, and microvariability to differentiate the three regimes.

 
Time Scale Typical time scale for viewing the data Typical time resolution
Long Term Several years or decades Months or years
Interday Several days or weeks Days
Microvariability Several hours Minutes
   
Long Term Variability Studies

Historically, long term studies have been based on estimates obtained from photographic plate collections. In some cases data is available for nearly a century, but the coverage is random and can have significant gaps due to economic and political circumstances. Data will normally be based on different photographic materials and processes plus different instrumentation (cameras and telescopes). At best the precision of the data can be a few tenths of a magnitude, but measurement uncertainties of a half magnitude or more are certainly possible. Any detected variability can be expected to be decidedly undersampled.

Examples of long term AGN variability data can be found for the objects linked in the table below.

 
OJ 287 AO 0235+164 B2 1215+303
B2 1308+326 PG 0804+762 PG 1001+054
PG 1626+554 PG 1704+608 PKS 0754+100
 
Long term observational programs are always difficult to maintain because of the commitments required in interest, time, and funding. Several of the large scale survey programs that have been proposed by major observatories and institutions, in conjunction with programs such as the National Virtual Observatory could ultimately provide a uniform database for significant long term studies of AGN. Until such large scale programs are undertaken, a small scale network of dedicated observers can make significant contributions to the study of the long term variability of AGN.
   
Interday Variability

Interday studies of AGN variability compare the brightness of an AGN from one night to the next for a period of days. Such programs typically involve the intensive observation of a selection of a dozen or so objects for five or ten days. This can be the extent of a typical allocation of observing time on a medium-sized telescope. It is not uncommon for such telescope allocation to be repeated in six months or a year. This can provide a target sample distributed around the sky and can provide follow up observations for targets previously observed.

Examples of interday AGN Variability data can be found for the objects linked in the table below.

 

Unfortunately, observing interday variability for only a few days at a time will grossly undersample the true extent of such variability. For example, it is well known that AGN undergo active phases and quiescent phases. Interday variability would probably be different during these different phases, and a few isolated observations may not be sufficient to indicate the true activity level. The effective average time resolution for such limited programs is more likely near 5 or 10 days.
   
Microvariability

Microvariability studies involve observing on a single AGN for several hours and taking data as fast as possible with the available instrumentation. Using modern CCD with medium and small telescopes, a large number of AGN are accessible with exposure times less than a few minutes. Thus, the brighter AGN may be observed with a time resolution of about one minute. Of course, such a program requires the exclusive use of a telescope for a single object: another reason that small telescopes are able to make a unique contribution to this field.

Examples of microvariability AGN data can be found for the objects linked in the table below.

Since microvariability has been confirmed for virtually all classes of AGN, such observing programs can be exciting. Flares can occur over a period of hours or less, and rapid quasi-periodic oscillations have also been observed.

Microvariability studies can be attractive to professionals since, for a limited commitment of time, if one is lucky, one is virtually guaranteed a publication. Do you feel lucky?All published descriptions of microvariability typically report that several objects were observed without positive results, and that even objects exhibiting variability on one night may have been monitored on several other nights with no indications of variability.


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This page was last modified on Thursday 05th February 2009 @ 12:58pm

Science Mission Directorate Universe Division

Responsible SSU Personnel:

Dr. Kevin McLin (mclin at universe dot sonoma dot edu)

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