HEASARC Staff Scientist Position - Applications are now being accepted for a Staff Scientist with significant experience and interest in the technical aspects of astrophysics research, to work in the High Energy Astrophysics Science Archive Research Center (HEASARC) at NASA Goddard Space Flight Center (GSFC) in Greenbelt, MD. Refer to the AAS Job register for full details.

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This Legacy journal article was published in Volume 1, May 1992, and has not been updated since publication. Please use the search facility above to find regularly-updated information about this topic elsewhere on the HEASARC site.

The HEASARC On-line Service

An Overview

K.L. Rhode

HEASARC

One of the primary objectives of the HEASARC is to provide a user-friendly, multi-mission, multi-wavelength environment in which to access the data that it archives. An On-line Service has been established to help accomplish that objective. Users who log in to the Service can access data from orbiting observatories (the list of which currently includes Cos-B, HEAO-1, Einstein, EXOSAT, and Ginga) along with many well-known ground-based catalogs. The database system and software within the On-line Service allow users to search and locate data, and then to either analyze it remotely, or to export it to their home computers for analysis there.

Introduction to the XOBSERVER environment

The HEASARC On-line Service resides on the NSSDC Data Archive and Distribution Service, NDADS, a VAX cluster at GSFC. (For more information about the NSSDC, see the article by Jim Green and Jeanne Behnke in this issue.) A captive account called XRAY has been set up on the NDADSA VAX to provide secure remote access to the database system and software. A program called XOBSERVER is run when users log in to the XRAY account. XOBSERVER provides:

  • automatic registration of users,
  • a user directory where files can be stored and manipulated,
  • TCP/IP, DECnet and PSI access (plus a gateway option),
  • runtime execution of database access and analysis tasks, and
  • tasks to transfer files to the user's computer.

A user logs in remotely to XOBSERVER and then runs database and analysis tasks to search through astronomical catalogs, locate data files, and analyze or transfer data. The XOBSERVER program originated from the EXOSAT Database System, which was written by the European Space Agency's (ESA's) Space Science Department between 1986 and 1990 to provide remote access to archival data obtained by EXOSAT. Recently the HEASARC, the ESA, and the University of Leicester, UK, have been jointly developing the system to provide a generic environment for on-line astrophysics archival access and analysis. The system has since been renamed XOBSERVER, in recognition of its multi-mission nature and to distinguish the controlling environment from the analysis and database tasks.

Logging in

Users can log in to the HEASARC On-line Service 24 hours a day via one of the following methods (user entries are indicated in courier type):

NSI-DECnet:
NODE NAME: NDADSA
NODE NUMBER: 15761
DEC Example:
$ set host ndadsa
$ set host 15761

NSI-TCP/IP*:
NODE NAME: NDADSA.GSFC.NASA.GOV
NODE NUMBER: 128.183.36.17
DEC Example:
$ telnet ndadsa.gsfc.nasa.gov
$ telnet 128.183.36.17

GTE TELENET:
dial local TELENET phone number
@C NASA
*LOGON
ENTER USERID>**
ENTER PASSWORD>**
ENTER Service>**

DIRECT DIAL:
Dial autobaud phone number (301)286-9000
CALL, DISPLAY, MODIFY OR ENTER NUMBER?
CALL SISC
LOCAL> CONNECT NDADSA

*INTERnet users will be prompted for their local userid.
**Users must register a PSI address. Send a message to NDADSA::REQUEST or 15761::REQUEST.

Once the user has connected, NDADSA will signal its presence and ask for a username. The user enters XRAY at the USERNAME prompt, and is logged in to the On-line Service captive account. A new user, or one who is logging in from a different location than usual, will be asked to type a HEASARC username. This should be the user's first initial and last name, (e.g., INEWTON).

The HEASARC username forms part of the name of the top-level directory (e.g., user INEWTON's directory would be called OBSERVER:[INEWTON]) where the files created during each session are written. The username ensures that the user regains access to this directory during subsequent sessions. User directories are not secure, and should be treated as scratch areas where data and files are temporarily kept. The directories are periodically cleaned up, with scratch files deleted and multiple versions purged.

Once the user has successfully logged in to the captive account, the HEASARC> prompt appears; this indicates that HEASARC XOBSERVER is ready to receive input. At this point there are a number of options available to the user, including accessing the bulletin board or the databases, running data analysis programs, or transferring files. On-line help can be accessed by simply typing help at the HEASARC> prompt; this will call up the XOBSERVER User's Guide, which contains information about all of the programs in the XOBSERVER package.

The bulletin board

The HEASARC bulletin board contains the latest news concerning the HEASARC and the On-line Service. Announcements are posted there on topics such as new or revised databases, new software, procedural or structural changes to the Service itself, and occasionally even job announcements. In addition, ROSAT and CGRO status reports are posted on the bulletin board as soon as they are released. To access the bulletin board, the user simply types

HEASARC> bulletin

(Note that all commands throughout the XOBSERVER system can be abbreviated to the least number of characters which make them unique; e.g., bull for "bulletin".)

Commands within the bulletin board program are logical: DIR displays a bulletin directory, and READ followed by a bulletin number displays a particular bulletin. Other commands are BACK, NEXT, and EXIT. HELP will invoke the bulletin board help facility.

The BROWSE databases

Typically, a user will begin an XOBSERVER session by running the database access program, BROWSE, to search through and display the available data and catalogs. BROWSE is a command-driven program that uses a database management system to select data from tables of information and results, and access any associated data product files. The analysis tasks available within the XOBSERVER environment can be executed from within BROWSE, or outside of it, whichever the user prefers.

BROWSE is started by typing

HEASARC> browse [dbname]

The optional parameter dbname specifies the name of the database to be accessed; if no dbname is given, BROWSE will prompt for a database name. Typing a ? or carriage return will display a list of all available HEASARC databases. Once a user chooses a database to "browse", that database is loaded as the current database, and the user enters commands to manipulate the database entries. Users can sort, search, display and plot entries in the database, and can also cross-correlate the data with entries in other databases. In addition, the data can be passed to other programs for spectral, timing, and image analysis, and interactive plotting and fitting.

Note that here the word "database" simply means a collection of information or results which has been organized into a table. The rows in the table are called "entries", and the columns are "parameters". HEASARC databases typically contain observation logs, astronomical catalogs, bibliographies, file names and locations, and/or data analysis results. In the case of an observation log, the database may contain one entry for each observation, with the associated parameters being the observation date, time, and duration, pointing coordinates, and instrument used. Catalog databases often are organized with one entry (or row) for each source, and parameters (or columns) such as source names, coordinates, count rates, magnitudes, and classifications.

At present, there are more than 50 HEASARC databases on line. A complete listing of the available databases appears in Table 1. Some databases contain data from missions like Einstein, EXOSAT, Ginga, and ROSAT, while others contain well-known ground-based catalogs, such as the HST Guide Star, SAO, and HD catalogs. The databases can be accessed separately, or cross-correlated with one another to find matches in common parameter fields. For example, celestial coordinates can be used to find all matching sources in two databases, based on positional coincidences within a specified radius.

Table 1:  HEASARC On-line Service Available Databases

  4UHURU      	Catalog: 4th Uhuru X-ray
  IRAS        	Catalog: IRAS
  VSTARS      	Catalog: Variable Stars 4th ed
  VSTARSUSP   	Catalog: Suspected Variables
  3ARIEL      	Catalog: 3A (Ariel-V)
  PULSAR      	Catalog: Lyne Pulsar
  GSC         	Catalog: HST guide stars
  VERON       	Catalog: AGN Veron and Veron
  RITTER      	Catalog: Ritter CVs & LMXRBs
  HD          	Catalog: Henry Draper
  SAO         	Catalog: SAO stars
  TD1         	Catalog: Stellar UV Fluxes
  MCKSION     	Catalog: McCook & Sion WD's
  QSO         	Catalog: Hewitt & Burbidge QSO
  WOOLLEY     	Catalog: Stars <25 pc from Sun
  MRC         	Catalog: Molonglo Radio Srcs
  6CMNORTH    	Catalog: 6cm radio
  ABELL       	Catalog: Abell Clusters
  20CMNORTH   	Catalog: 20cm radio
  DOCUMENTS   	Documents: HEASARC
  LE          	EXOSAT CMA (central 6 arc min)
  CMA         	EXOSAT CMA Database
  CMAIMAGE    	EXOSAT CMA Images
  GS          	EXOSAT GS database
  ME          	EXOSAT ME database
  TGS         	EXOSAT TGS L and R orders
  TGS2        	EXOSAT Transmission Grating
  EXOPUBS     	EXOSAT bibliography
  EXOLOG      	EXOSAT observation log
  IPC         	Einstein IPC Source List
  SSS         	Einstein SSS database
  EINLOG      	Einstein observation log
  SSSHME      	Einstein SSS dbase-IBM listing
  HRIIMAGE    	Einstein HRI Image Database
  HRICFA      	Einstein HRI CFA Source List
  HRIEXO      	Einstein HRI EXO source list
  IPCIMAGE    	Einstein IPC Image Database
  EMSS        	Einstein EMSS catalog
  GINGALOG    	Ginga LAC Log Catalog
  GINGAMODE   	Ginga LAC Mode Catalog
  A4          	HEAO 1 A4 X-ray catalog
  A2PIC       	HEAO 1 A2 Piccinotti Catalog
  A1          	HEAO 1 A1 X-ray Catalog
  A2POINT     	HEAO 1 A2 Point Catalog
  A2LED       	HEAO 1 A2 LED Sky Catalog
  A3          	HEAO 1 MC LASS Catalog
  IUE         	IUE ULDA database
  X-RAY       	Master Catalog: X-ray
  XCOLL       	Master Catalog: X-ray Collim.
  RADIO       	Master Catalog: radio
  ZZDB        	Metabase
  ROSTL2      	ROSAT AO2 long-term timeline
  ROSTL1      	ROSAT AO1 long-term timeline
  ROSAO       	ROSAT accepted AO proposals
  ROSATLOG    	ROSAT log of observations
  ROSOBS      	ROSAT observations status
  ROSSTL      	ROSAT short-term timeline
  SMMGRS      	SMM GRS [[gamma]]-Ray Burst D'base
The databases called X-RAY and OPTICAL contain master catalogs, which are created by merging the common parameters from several individual catalogs. X-RAY combines data from six separate X-ray source catalogs (A2PIC, A3, EMSS, HRICFA, IPC and IPCSLEW), while OPTICAL combines information from about a dozen optical catalogs (including VSTARS, QSO, RITTER, SAO and ABELL). The master catalogs allow users to obtain general information about astronomical objects without having to access separate databases one by one.

Some of the BROWSE databases contain data products. The IPCIMAGE and HRIIMAGE databases provide access to the Einstein IPC and HRI FITS images which have been distributed by SAO. The SSS database contains spectral and lightcurve data products from the Einstein SSS, plus the associated MPC spectra. These spectra are currently in XSPEC format, and will be available soon in FITS format as well.

The section of this article called XOBSERVER examples illustrates some commonly-used BROWSE commands, including commands to search the databases and extract data products. Also, examples and on-line help documentation can be displayed from within BROWSE by typing the help or dbhelp commands. Help will display general BROWSE information, while dbhelp displays information about the currently-loaded database, including a description of each parameter, the available data products, and the name of a contact person.

File transfer options

Files that are created during an XOBSERVER session are stored in the user's directory, including any extracted data products, and log and history files created by the XOBSERVER programs. (A complete record of all output from a user's BROWSE session or XOBSERVER session is written to log files called BROWSE.LOG or XOBSERVER.LOG.) These files can be transferred from the user's directory to his or her home computer. The transfer can be made either from the XRAY account using the mail, send or FTP commands, or from the user's computer using the VMS copy command. The file transfer method used depends on whether the file is ASCII or binary, and whether it is a VMS to VMS or VMS to UNIX transfer.

Help documentation, which describes what each file transfer command is suitable for, is contained in the XOBSERVER User's Guide. Type help at the HEASARC> prompt to access the guide.

Spectral analysis

The spectral fitting package available within the XOBSERVER environment is XSPEC, which is part of the XANADU analysis package that originated at the Institute of Astronomy, Cambridge, UK. XSPEC was written by R.A. Shafer and is currently maintained jointly by the EXOSAT Observatory, the HEASARC, and MSFC. XSPEC can be run either stand-alone from the main HEASARC> prompt, or from within BROWSE. In the latter case, BROWSE will provide the required spectral files, while in the former the user must extract them or create them manually.

To run XSPEC from within BROWSE, the user must first ensure that the database being "browsed" contains spectral products (the xp/summary command displays a summary of available data products). Next, the user searches the database for the entries containing the spectra to be analyzed. Then the user types xspec on the BROWSE command line; this invokes an interface to XSPEC which locates the required files and creates the necessary response matrices and background files. Thus most of the complexity of the program is hidden from the user.

To run XSPEC as a stand-alone procedure, independent of BROWSE, the user should first extract the chosen spectral file using BROWSE, and then exit BROWSE. If necessary, the user next runs the VIMAT program to create a response matrix and background files. Finally, the user types xspec at the HEASARC> prompt.

On-line help documentation on XSPEC can be accessed from a number of places. Users can type help when inside XSPEC for descriptions of commands. Also, both the BROWSE and the XOBSERVER on-line documentation contain information about XSPEC and examples of how to use it; the former can be accessed by typing help within BROWSE, and the latter by typing help at the HEASARC> prompt.

For users who wish to copy spectral files back to their home computers to analyze them, XSPEC and its associated programs are available for local installation. Both the VAX/VMS and Sun-UNIX versions can be obtained by sending e-mail to ndadsa@request.gsfc.nasa.gov, or Karen Smale at ksmale@heasarc.gsfc.nasa.gov.

Timing analysis

The timing analysis package available in the XOBSERVER environment is called XRONOS. XRONOS is made up of a collection of programs, each dedicated to one task, which can be run from within BROWSE, as stand-alone routines, or from a XRONOS command-driven environment. There are XRONOS tasks for:

  • plotting light curves,
  • plotting hardness ratio and color-color diagrams,
  • epoch folding,
  • power spectrum analysis,
  • auto-correlation, cross-correlation and time skewness analysis, and
  • statistical analysis.

If the XRONOS tasks are run from within BROWSE, then the command-driven interface is used; that is, applications are run by entering a command string followed by qualifiers and arguments. For example, a user selects data with BROWSE, and then plots the data by simply typing the name of the XRONOS task, e.g., lc1. On the other hand, to run XRONOS in stand-alone mode, the user should first type the BROWSE command xp/li to extract the data product file. After exiting BROWSE, the user would then run individual XRONOS tasks on the extracted files.

Each of the XRONOS tasks is explained in the on-line help documentation within the program itself; the documentation is accessed by typing help at the XRONOS> prompt, or replying help to any question asked by a XRONOS application. Information about XRONOS can be found in the on-line help for BROWSE and XOBSERVER as well.

At present, XRONOS is available for remote installation on VAX/VMS machines, and will soon be available in Sun-UNIX and HP-UX versions as well. Information can be obtained from Lorella Angelini at angelini@heasrc.gsfc.nasa.gov.

Image analysis

XIMAGE is the program used in the XOBSERVER environment to provide an on-line image display and analysis capability. Like the other analysis packages, XIMAGE can be run either from within the BROWSE program, or in stand-alone mode starting from the HEASARC> prompt.

Before running XIMAGE, users should first access a database with BROWSE, and then type the command xp/summary or pp/summary to see whether images are available for that database. Next the user searches the database and selects the data to be analyzed.

At this point, users can run XIMAGE from within BROWSE by simply typing a command that looks like

> ximage[/qualifiers] entry_number

where entry_number is the database entry number of the image the user wishes to analyze. Qualifiers, such as /smooth, /threshold, and /rebin, can be used to control features of the image display.

In order to run XIMAGE as a stand-alone procedure, a user must first use BROWSE to select and extract the images. The BROWSE xp command will extract the images in their original format, while xfits extracts them in FITS format. After exiting BROWSE, the user types XIMAGE at the HEASARC> prompt to start the program. The extracted files can be read into XIMAGE using one of the read commands in conjunction with the necessary qualifiers.

Detailed help documentation for XIMAGE can be accessed from within the program by typing help. Information on XIMAGE and associated image analysis routines is also contained in the XOBSERVER help and in the BROWSE help.

XIMAGE can be installed locally on the user's computer. Currently, only a VAX-VMS version is available; Sun-UNIX and HP-UX versions are under development. Contact Lorella Angelini at angelini@heasarc.gsfc.nasa.gov for more information.

QDP/PLT

The plotting and function-fitting package that is used within the XOBSERVER environment is called QDP. The program operates on ASCII files which contain both the data and the commands that control the display. QDP can also be used in an interactive mode to customize the plot and perform simple analysis tasks on the data.

The QDP package is used by the BROWSE, XSPEC and XRONOS programs to plot light curves and spectra. From each of these programs, a QDP file can be generated, and later input to QDP for plotting and fitting. Users can also generate QDP files in the XRAY account, and then transfer them to their home computer, where they can replot and manipulate them locally.

Like XSPEC, QDP originated as part of the XANADU EXOSAT analysis package at the Institute of Astronomy, Cambridge, UK. It was written by Allyn Tennant, and is now maintained by him at MSFC. Users who wish to install a copy of QDP on their home computers can contact the HEASARC. QDP is available in VMS, UNIX and MS-DOS versions.

A full help facility is available within QDP. There is also information about QDP in the on-line documentation for XOBSERVER and BROWSE.

Obtaining user's guides and information

This article describes only a fraction of the tasks that can be performed and the information that can be accessed within the HEASARC On-line Service. In addition to the extensive on-line help in the XRAY account, a variety of documentation is available from the HEASARC. User's guides for XOBSERVER, the BROWSE databases, XSPEC, XRONOS and XIMAGE are among the documents available; a complete list is contained in the BROWSE database called DOCUMENTS. Requests for documentation should be sent to request@ndadsa.gsfc.nasa.gov.

User questions or problems with any of the XOBSERVER software or the database system should also be reported to requests@ndadsa.gsfc.nasa.gov. Comments and suggestions are welcome, and will be implemented when possible.

	XOBSERVER Examples


Below are examples of the database access and analysis tasks which can
be run within the XOBSERVER environment.

HEASARC> help                            ! Display the XOBSERVER help documentation
HEASARC> bulletin                        ! Access the HEASARC bulletin board

Search the ROSAT and EXOSAT observation logs, using the "sc", "sn",
and "sp" commands:

HEASARC> browse ROSATLOG                 ! Browse the ROSAT pointing log

ROSATLOG_TOTAL_DEC> keywords             ! Display a list of BROWSE command keywords
ROSATLOG_TOTAL_DEC> search               ! List all of the search commands
ROSATLOG_TOTAL_DEC> dbhelp               ! Display a description of the contents
                                         ! of the ROSATLOG database

ROSATLOG_TOTAL_DEC > sn cygx-1           ! Search for entries with target name "CYGX-1"
ROSATLOG_TOTAL_NAM 3> dall 1-**          ! Display all of the information in
                                         ! the three found entries

ROSATLOG_TOTAL_NAM 3> sp pi mushotzky    ! Search for all observations with
                                         ! Mushotzky as PI
ROSATLOG_TOTAL_NAM 20> dall 1-**         ! Display all the parameters in the
					 ! twenty found entries

ROSATLOG_TOTAL_NAM 20> cdb exolog        ! Change to the EXOSAT observation log

EXOLOG_TOTAL_DEC > sc                    ! Search by coordinates
R.A. (1950  d/f= 20  h 47 m 58.20sec): 18 20! RA = 18 20
Dec  (1950 d/f=-52 deg 31 m  4.42sec): -30  ! Dec = -30
Radius in arcmin. (d/f=  60.00):         ! Cone radius = 60 arc min
EXOLOG_TOTAL_DEC 15> dsam                ! Redisplay the found entries

Search ROSOBS, the ROSAT observations status database, for all entries
beginning "LMC".  Then use the filtering and sorting commands:
EXOLOG_TOTAL_DEC 15> cdb rosobs          ! Change to the ROSOBS database
 
ROSOBS_TOTAL_DEC > sn lmc*               ! Search for names starting "LMC"
ROSOBS_TOTAL_NAM 13> sif country US      ! Set an inclusive filter on the
                                         ! COUNTRY parameter.  Include only 
                                         ! the entries where the COUNTRY 
                                         ! parameter is "US"
ROSOBS_TOTAL_NAM 13:11> dsam		 ! Display the remaining entries
ROSOBS_TOTAL_NAM 13:11> sort done	 ! Sort on the DONE TIME parameter
ROSOBS_TOTAL_NAM 13:11> dsam		 ! Display the sorted entries

ROSOBS_TOTAL_NAM 13:11> sef done 	 ! Set an exclusive filter.
Exclude
Enter minimum number: -100		 ! entries with DONE TIME between 
Enter maximum number: 0.01		 ! -100 and 0.01 ksecs

ROSOBS_TOTAL_NAM 13:11:2> dsam           ! Display the remaining 2 entries
ROSOBS_TOTAL_NAM 13:11:2> rlf            ! Remove the last filter that was set

ROSOBS_TOTAL_NAM 13:11> cdb cma          ! Change to the EXOSAT CMA catalog

Search the EXOSAT CMA catalog for all entries of 3C382, and then plot
the count rates from different filters against each other:
CMA_SOURCES_DEC >sn 3c382		 ! Search for name 3c382
CMA_SOURCES_NAM 16> sc                   ! Search by coordinates, to be sure 
R.A. (1950  d/f= 18  h 33 m 11.75sec):	 ! all entries are found
Dec  (1950  d/f= 32deg 39 m 17.93sec):	
Radius in arcmin. (d/f=  30.00):	
CMA_SOURCES_DEC 47> sif filter 7	 ! Keep only filter 7 data (thin lexan)
CMA_SOURCES_DEC 47:26> dsam              ! Display the remaining entries

CMA_SOURCES_DEC 47:26> cpd /te           ! Change plot device to Tektronix
CMA_SOURCES_DEC 47:26> psam/plt time count  ! Plot TIME vs. COUNT RATE
                                         ! The "/plt" qualifier leaves the user
                                         ! in QDP/PLT for interactive changes to
                                         ! the plot



CMA (time vs. count rate)


PLT> hard 3c382_lex.ps/ps                ! To have a hard copy of the plot,
                                         ! write it to a postscript file
PLT> exit                                ! Exit from PLT mode

CMA_SOURCES_DEC 47:26> rsam              ! Now, reset the sample to include
                                         ! all entries

Plot an image of Cas A and extract it in FITS format.  Also plot,
extract and fit Cas A spectra using XSPEC:
CMA_SOURCES_DEC > sn casa                ! Search for all Cas A entries
CMA_SOURCES_DEC 11> cpd /gterm           ! Change plot device to GTERM
CMA_SOURCES_DEC 11> ximage 1             ! Plot the entry #1 image


CasA Image

CMA_SOURCES_DEC 11> xfits/im 1           ! Extract the image in FITS format

CMA_SOURCES_DEC 11> cdb me               ! Change to the EXOSAT ME database

ME_TOTAL_DEC > sc/default                ! Search using the default coordinates
                                         ! from the previous search
ME_TOTAL_DEC 5> pp/sp 1                  ! Plot the spectral product for
                                         ! the first entry


Spectrum 1

ME_TOTAL_DEC 5> xp/sp 1-**               ! Extract all the spectral products
ME_TOTAL_DEC 5> xspec/int 1              ! Run XSPEC interactively on the
                                         ! spectrum from entry #1
ME_TOTAL_DEC 5> exit

HEASARC> mail 3c382_lex.ps               ! Mail the 3C382 postscript plot 
                                           file and the
HEASARC> mail browse.log                 ! BROWSE log file to the user's 
                                           home computer
HEASARC> lo                              ! End the XOBSERVER session

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Last modified: Monday, 19-Jun-2006 11:40:53 EDT

HEASARC Staff Scientist Position - Applications are now being accepted for a Staff Scientist with significant experience and interest in the technical aspects of astrophysics research, to work in the High Energy Astrophysics Science Archive Research Center (HEASARC) at NASA Goddard Space Flight Center (GSFC) in Greenbelt, MD. Refer to the AAS Job register for full details.