It is often assumed that most, if not all, AGNs are variable at some level. For example, virtually all of the AGNs 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, (refered to as blazars) are among the most extremely 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 AGNs.
Variability Time Scales
Variability studies of AGNs 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. These time scales are not necessarily related in any meaningful way to physical process producing the variability. These time scales may be termed longterm, intraday, and microvariability.
|Time Scale||Typical time scale for viewing the data||Typical time resolution|
|Longterm||Several years or decades||Months or years|
|Intraday||Several days or weeks||Days|
Longterm Variability Studies
Historically, longterm studies have been based on estimates obtained from photographic plate collections. In some cases data can be 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 AGN Variability Longterm 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|
Longterm observational programs are always difficult to maintain. The difficulties are based on the commitments required in interest, in time, and in 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 homogeneous database for significant longterm studies of AGNs. Until the time that such large scale programs are approved and initiated, a small scale network of dedicated observers can make truly significant contributions to the study of the longterm variability of AGNs.
Intraday studies of the variability of AGNs essentially 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 AGN Variability Intraday data can be found for the objects linked in the table below.
Unfortunately, observing intraday 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 AGNs undergo active phases and quiescent phases. Intraday variability would be expected to 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 studies involve sitting on a single AGN for several hours and taking data as fast as possible with the available instrumentation. Using modern CCDs with medium and small telescopes, a large number of AGN are accessible with exposure times less than a few minutes. Thus, the brighter AGNs 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.
Examples of AGN Variability Microvariability data can be found for the objects linked in the table below.
Since microvariability has been confirmed for vitrually all classes of AGNs, such observing programs can be exciting. Flares can occur over a period of hours or less, and rapid qusi-periodic oscillations have also been observed. Variability over periods of hours has been well documented for numerous AGNs.
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.