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Last modified: 28 October 2009

URL: http://cxc.harvard.edu/ciao4.2/data_products_guide/pha_descrip.html

PHA (Spectrum) Files

Data Products Guide

Level Instrum ICD Data product Content Pipeline Filename template
L2 ACIS v1.4 ACIS Grating Spectrum SPECTRUM TG2 pha2.fits
L2 HRC v1.4 HRC Grating Spectrum SPECTRUM TG2 pha2.fits

ACIS Grating Spectrum

Level Instrum ICD Data product Content Pipeline Filename template
L2 ACIS v1.4 ACIS Grating Spectrum SPECTRUM TG2 pha2.fits

description:

Transmission grating spectra for a Chandra observation are provided in the form of PHA files: 1-D counts histograms. (The name "PHA" is historical, referring to "Pulse Height Analyzer" or "Pulse Height Amplitude".) PHA are actually available in two formats, referred to as "Type I" and "Type II". Type I PHA are the traditional format of PHA, with sequential channels stored in sequential rows. Type II PHA files may contain many Type I PHA, with each Type I PHA stored as a row of the overall file. Thus, for any given row of a Type II PHA, the counts histogram is stored as an array. Since Chandra grating observations have at least two orders (for LETGS; 12 orders for HETGS), the Type II PHA is the standard method of storing grating observation data.

Among other things, the pipeline processing will find the grating arms using tgdetect, and assign each event to an order with tg_resolve_events. Finally tgextract is used to create the final pha2 file.

Certain combinations of Chandra's gratings and instruments may give leveraged utility in an observation. One particular combination that is used with the ACIS-S array and the HETG is called the HETGS or High Energy Transmission Grating Spectrometer. The Proposers' Observatory Guide has examples of HETGS Observations.

To begin analysis on the pha2 file, see the Examining PHA2 Files (S-Lang or Python) thread.

file name template:

acis*_pha2.fits

creator pipeline:

TG2

creator tool:

tgextract - tgextract is the final step of a series of tools designed to process chandra grating data. See the Grating Spectroscopy threads page for more info on reprocessing this data.

useful links:

SPECTRUM-specific columns:

Column Name Units Description
SPEC_NUM - Spectrum Number (each row indicates a different PHA Type I)
TG_M - Diffraction order (m)
TG_PART - Spectral component (0=zero order, 1=HEG, 2=MEG, 3=LEG, 99=background)
TG_SRCID - Source ID, output by detect
X pixel X sky coord of source
Y pixel Y sky coord of source
CHANNEL[8192] - Vector of spectral bin numbers
COUNTS[8192] count Counts array (a spectrum)
STAT_ERR[8192] count Statistical uncertainty (error) on counts column
BACKGROUND_UP[8192] count Background count vector
BACKGROUND_DOWN[8192] count Background count vector
BIN_LO[8192] angstrom Bin boundry, left edge
BIN_HI[8192] angstrom Bin boundry, right edge

The Type II PHA files will also have a second block called "REGION" attached. This block spatially defines the regions used to extract the source and background spectra for the observation. For an HETG/ACIS observation (a.k.a "HETGS") there is a source, upper background, and lower background defined for each of the six orders of the HEG and MEG spectra. This creates a total of 36 regions.

REGION-specific columns:

Column Name Units Description
SPEC_NUM - Spectrum Number (each row indicates a different PHA Type I)
ROWID - Source or a background region?
SHAPE - Shape of region
TG_LAM angstrom Dispersion coordinate vector for SHAPE
TG_D degrees Cross-dispersion coordinate vector for SHAPE
R[2] (angstrom , degrees) Radius vector for SHAPE
ROTANG degrees Rotation angle for SHAPE
TG_PART - Grating part index (HEG=1, MEG=2, LEG=3)
TG_SRCID - Source identification number
TG_M - Diffraction order

HRC Grating Spectrum

Level Instrum ICD Data product Content Pipeline Filename template
L2 HRC v1.4 HRC Grating Spectrum SPECTRUM TG2 pha2.fits

The creation of a PHA file is from HRC data is fairly similar to the ACIS method. The discussion of this method may be seen in the ACIS Grating Spectrum section.

The Proposers' Observatory Guide has an example of an LETGS observation. The Low Energy Transmission Grating Spectrometer is the name given for the combination of the LETG with HRC-S.


Last modified: 28 October 2009