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TGS: Help

TGS - EXOSAT TGS L and R Orders


Overview

Spectra obtained using the EXOSAT transmission grating spectrometer, TGS.

There are two databases TGS and TGS2 as follows:

 * TGS  - the grating spectra averaged over the positive and negative orders

 * TGS2 - the postive and negative orders kept separate
TGS provides a better overview of the spectrum, and is quicker to use with a spectral fitting program. This is the default that most users will want to use. Once a user has become more expert and wants to see, for example, if a subtle feature is present in both halves of the grating spectra, the user can access TGS2.

Description

The insertion of a gold bar transmission grating behind the focussing mirrors with a ruling of 1000 l/mm (LE1) or 500 l/mm (LE2) allowed the study of bright soft X-ray sources with medium resolution over a large dynamic range (LE1: 8-200 Angstrom with resolving power of 6 to 40 and LE2: 8-400 Angstrom with a resolving power of 3 to 40. The focal plane detector consisted of a microchannel plate (CMA) plus one of the five filters.

One of the CMAs failed on 28 October 1983, while the other proved more resilient and lasted until the end of operations. The mechanism for raising the grating on one of the telescopes failed on 15 September 1983 with the grating out of the telescope field of view. The other grating was unfortunately on the telescope where the CMA detector had failed and grating observations were no longer possible.

A total of 24 sources were observed with one or both transmission grating spectrometers, from which 19 gave a useful spectrum. These spectra were extracted from the CMA images. For some sources multiple spectra were extracted based on one of several criteria:

 * change in pointing position,
 * change in filter,
 * variability of the source (flaring),
 * simultaneous GSPC or ME observations,
 * level of signal-to-noise.
The final products database contains these spectra, corresponding background spectra and response matrices as input files to the spectral fitting program XSPEC.

Data Products

The products available for this database (spectra, lightcurves and response matrices) were created originally at the EXOSAT observatory in binary format. HEASARC converted all the products in FITS format. A plot of the spectra and lightcurves is also available in GIF format.

Extraction

The spectral image consists of the zero-order (which is almost identical to the telescope PSF) plus the dispersed spectra (positive and negative orders), which are aligned with the Y-axis. The spectra are extracted from the image by putting a mask over the spectrum of typically 40 to 50 pixels width and integrating perpendicular to the dispersion direction (along the X-axis). For weak sources a smaller mask width was taken, optimized towards maximum signal-to-noise and a reliable estimate of the flux normalization. The spectra generated in this way consist of a one-dimensional array of 2048 channels.

The contribution of (instrumental) background was determined from integrating over the same mask in the image of a long blank-field exposure. The normalization of this background spectrum came from comparing the number of counts in four boxes in the image. In addition the spectrum is corrected for dead time and sum-signal of the CMA. In the spectra of hard X-ray sources the zero order flux has a significant contribution to the (negative and positive) first order flux. For each spectrum this zero order contribution was determined and added to the background file.

For the TGS database the positive and negative orders were added, which resulted in spectra of 1024 channels, with channel 1 containing photons with the highest energy.


Parameters

Associated_File_Backgrou
Name of the Associated Background Files

Associated_File_Response
Name of the Associated Response Files

Associated_File_Spectrum
Name of the Associated Spectrum Files

BII
Source Galactic Latitude

Bintime
Accumulation interval for the lightcurve in seconds.

Comments
Comments Regarding Width of Mask Used to Extract Grating Spectra from LE Image

Count_Rate
The parameters count_rate and count_rate_error contain the overall ct/s and error for the source.

Count_Rate_Error
Count Rate Error

Creator
Creator (auto, aux)

Dec
The declination of the target.

Duration
Duration i.e. stop-start

Exposure_Time
The `exposure` is the total on-source observation time in seconds. This includes all dead time effects, interruptions in coverage etc.

File_Bgnd_Spectrum
Background spectral file

File_Lcurve
Lightcurve filename.

File_Response
Response filename

File_Spectrum
Source spectral file

Filter
The number of the filter used (see the CMA database documentation).

Grating
The number of lines per mm for the grating. It is either 1000 or 500.

ID_Number
Source Number

Instrument
Telescope/grating used:

   * `L1` - 1000 l/mm
   * `L2` - 500 l/mm
  

Last_Update
Creation Time of This Entry

LII
Source Galactic Longitude

Name
The `name` parameter gives the target name. This name usually corresponds to the target name as specified by the original observer. If a source has been observed by different observers it may be entered under two or more names. It is always recommended that any search by `name` be followed by a search on the found coordinates.

No_Of_Bins
Number of time bins in the lightcurve.

Pointdec
Pointing Declination

Pointra
Pointing Right Ascension

Process_Date
Creation Time of Data Files

Qflag
Quality Flag (ME uses 0=unusable 5=perfect)

RA
The RA of the target.

Roll_Angle
Roll Angle *10 degrees

Sequence_Number
Sequence Number of Observation

Time
The `time` of the observation refers to the start time. This is listed as yr.day, where yr is the last two digits of the year and day is the day of the year, e.g., day 300 of 1984 is 84.300. All times are stored internally as a short history file key (SHF) which is the number of seconds before or after 1 January 1980 at 00:00 hrs UT. Times are accurate to the nearest second.

X_Pixels
X pixel position of the zero order.

Y_Pixels
Y pixel position of the zero order.


Contact Person

Questions regarding the TGS database table can be addressed to the HEASARC User Hotline.
If you have any problems, please consult the help page or mail ledas-help@star.le.ac.uk
 
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