GTN banner
Goddard Space Flight
													      Center Sonoma State University
													  Education and Public
GTN Observing Program

Basic Photometry and Astrometry
GTN Program Object Catalog
Details about Observing Program Objects
Observing Procedures
Observing Strategies

One of the science objectives of the Global Telescope Network (GTN) is to obtain observations from the surface of the Earth of objects and classes of objects that will be observed from space in the high energy regions of the electromagnetic spectrum (gamma rays and X-rays). Participants in the GTN are encouraged to become involved either by obtaining observations of these objects or by reducing and analyzing observations obtained by other GTN participants. Participants can also monitor observations to search for changes that can indicate changes in the activity level of the program objects. Such changes can warrant further observations by ground-based observers and targeted observations from spacecraft.

Program Objects

The primary observing programs include Blazars, GRBs, and Polars. For each of these categories of objects, what follows is a brief description of the nature of the objects plus lists which include coordinates, finding charts, and photometric sequences of standard comparison stars in each field.
The complete list of our program objects can be found in the "Program Object Catalog" link above or here.
These are highly variable active galactic nuclei (AGNs) which have some similarities to the less variable quasars (QSOs). Quasars can slowly vary by up to a magnitude, while blazars can vary up to 4 magnitudes or more. Blazars can also vary by several tenths of a magnitude in the course of a single evening. Blazars are also known as BL Lac objects (BLLs). The GTN blazars have been detected as gamma-ray sources, and, indeed, the blazars are the only known extragalactic point sources of Gamma-rays. The Gamma-rays are presumably produced in jets which are pointed directly at the Earth. The jets are associated with accretion disks surrounding supermassive black holes in the cores of galaxies. Observing and discovering new blazars will be one of the primary science objectives for the GLAST mission.
Gamma-ray bursts (GRBs) are intense but short lived bursts of gamma rays which occur unpredictably across the sky. The bursts rarely last longer than a few seconds, but during that time they can be the strongest gamma-ray sources in the sky. While gamma rays appear to dominate the bursts, the bursts have also been detected in X-rays, in visible light, and in the radio region. Sometimes, after the burst, sources can be detected at the burst location on the sky in visible light, in X-rays, and in the radio region. These afterglows can last for hours or even for days and will slowly decrease in intensity. It is now believed that at least some GRBs are associated with some form of supernova phenomenon. Detecting GRBs is the primary science objective of the Swift mission. GRBs will also be detected by the GLAST mission.
Polars are a special form of magnetic cataclysmic variable star (CV). CVs are interacting binary star systems in which one component is a white dwarf and the other component is a cool main sequence dwarf star. The main sequence star is losing mass to the white dwarf. The mass transfer is not continuous and mass transfer events produce outbursts when the transferring matter approaches or strikes the surface of the white dwarf. The outbursts can produce dramatic increases in the visible-light brightness (up to 4 magnitudes or more) and can also produce X-rays. During these outbursts the streams of matter being transferred and the regions where this matter strikes the white dwarf can become far brighter than the combined light of the two stars. Polars are a special form of magnetic CV or AM Her star. In these objects the magnetic field of the collapsed component is so intense that the transferring matter is constrained to follow along the magnetic field lines and strike the white dwarf only at its magnetic poles. Polars are one of the categories of CV that are being observed by the XMM-Newton mission.
[ top of page ]

Observing Procedures for Blazars and Polars

The details of the variability for individual blazars and polars are generally poorly known. Thus, careful systematic observations of brightness over all time scales can document important characteristics of these objects. Surveillance of these objects once or twice a month or once or twice a week would be most useful. Rapid reduction of the data to determine magnitudes and precise times of observation is essential. Magnitudes should be determined and calibrated using one of the standard photometric filters such as BVRI with reference to stars of known magnitudes. Sequences of stars with known magnitudes are available from the charts provided for all GTN Program Objects. To be most useful, preliminary magnitudes should be determined within 24 hours of obtaining the data.

  • Magnitude determinations should be submitted to the AAVSO international database maintained by the American Association of Variable Star Observers(AAVSO). User IDs for the submission process are available at no charge from the AAVSO. Membership in the AAVSO is not required. GTN participants should request special GTN IDs.

Blazars generally exhibit slow, irregular variation over periods of decades.But this slow variation can be interrupted at irregular intervals with outbursts or declines that can amount to several magnitudes. Blazars can also vary significantly by several tenths of a magnitude during a single night of observing. There are no well documented instances of periodic behavior among the blazars (except possibly for an 11 year quasi-period for OJ 287).
All the bright blazars will be intensively observed in gamma rays during the first year all sky survey of the GLAST mission. Continuous observation of all the GTN blazars from the ground will be extremely important during this period. In addition, the GLAST survey is expected to discover and monitor several thousand new blazars. Some of these new discoveries will surely be bright enough to be observed optically by the GTN. Subsequent to the all sky survey, optically detected outbursts will serve as triggers for intense pointed gamma-ray observations by GLAST. Furthermore, gamma-ray detected outbursts will need to be followed optically by the GTN.
The polars have well established orbital periods of less than a few hours. However the outbursts associated with mass transfer events occur on a much longer irregular basis extending over many months or years. While some CVs have dramatic outbursts amounting to several (or many) magnitudes that recur in a quasiperiodic fashion, the polars tend to have smaller "outbursts" that occur on an irregular basis. When the system is in outburst the light is dominated by the spot produced by the channeled mass being exchanged as it strikes the surface of the white dwarf component near the magnetic poles. Copious quantities of X-rays are produced. When the system is quiescent the light is generally dominated by the surface of the white dwarf and virtually no X-rays are detected. Determining when a system is in outburst can be used to coordinate X-ray observation by spacecraft such as XMM-Newton.
For both the blazars and the polars, magnitude determinations need to be compared with other recent magnitude determinations to determine if variability is occurring. Magnitude determinations can also be compared with long term averages or trends to access activity levels.

Observing Procedures for GRBs

No one knows where or when a GRB will go off. Initial detections may have positional uncertainties of many arcminutes. To catch an actual burst requires a telescope that is listening on the internet and is able to move accurately to a set of coordinates in a matter of minutes or seconds and begin taking images. Since initial coordinates can be highly uncertain and telescope fields of view are relatively small, it may be necessary to take a mosaic of images centered on the nominally reported coordinates. Then these images must be compared with deep field images such as the POSS (Palomar Observatory Sky Survey) or with deep catalogs such as the Naval Observatory A2 or A3 to seek objects that do not appear in the archived data. To be scientifically valuable a telescope must be capable of responding to a burst alert in a matter of minutes (or even seconds). The images obtained must then be analyzed immediately in real time to attempt to detect and quantify the burst in terms of precise position and brightness. It appears that not all GRBs actually produce optical bursts. Attempting to catch an optical burst associated with a GRB is a high stakes, high stress endeavor.
After the GRB has been detected optically and precise coordinates at the arcsecond level have been posted to the internet, several hours (or days) have usually elapsed. The burst is over, but the afterglow may be observable. For this activity (to observe an afterglow) one needs a telescope that can point accurately to the arcsecond level and a CCD imaging system that can reliably record objects as faint as18th magnitude and fainter. An afterglow may stay in the range of 18th to 20th magnitude for a few days, or possibly only for a few hours. It appears that not all GRBs actually produce optical afterglows. Chasing after GRB afterglows is not for the faint of heart!
If an afterglow is detected, it is important to determine magnitudes for the detected source and the precise times of the observations. Magnitudes should be determined and calibrated using one of the standard photometric filters such as BVRI with reference to stars of known magnitudes. Magnitude determinations should be reported using a GRB Observation Report form provided by the AAVSO.
For more information about observing GRB afterglows, consult the Gamma Ray Burst Afterglows material provided by the AAVSO. For an example of observations of a GRB afterglow obtained with small telescopes, consult the work on GRB030329 compiled by the AAVSO.

If you have a question about the GTN, please contact one of the "Responsible SSU Personnel" below.

This page was last modified on Monday 02nd February 2009 @ 10:50am

Science Mission Directorate Universe Division

Responsible SSU Personnel:

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

Privacy, Security, Notices