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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 electromagnatic spectrum (gamma rays and
X-rays). Participants in the GTN are encouraged to become invovled
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.
GTN
Program Object Catalog
Details about Observing
Program Objects
BLAZARS
GRBs
POLARS
Observing
Procedures
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.
BLAZARS
These are highly variable active galactic nuclei (AGNs) which have
some similiarities 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
blackholes in the cores of galaxies. Observing and discovering new
blazars will be one of the primary science objectives for the GLAST
mission.
GRBs
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
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.
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Observing
Procedures
Blazars
and Polars
GRBs
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.
- CCD images should be submitted to the GTN
Data Archive.
- Apparent outbursts or declines or other activity should be
immediately reported to the GTN. The GTN
Communication Systems should be used to report such changes
for the GTN Program Objects. In particular, such discoveres
should be posted to the GTN
Online Discussion Board.
- Individual GTN participants may wish to "adopt" a specific
GTN object to monitor expecially carefully. The GTN will be
available to coordinate such efforts. ( See Adopt
a Blazar for details. )
- Individual GTN participants may wish to commit to a "blazar
a month" or a "blazar a week" as observational goals. ( See
A Blazar a Month or
A Blazar a Week for
details. )
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 thransfer
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 occuring. 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 moasic 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.
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