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Hand-on Project GBT-2

Jim Braatz

Overview

We will observe OH maser systems in sources near and far using the GBT L-band receiver and the spectrometer. A position-switching technique will be employed. The observations are rather straightforward, so we will take the opportunity to inspect the calibration in some detail. For example, we will calculate the gain, system temperature and antenna temperature "by hand" and then compare these results with the automated calibration procedures available in the software. We will observe Galactic masers, where OH emission is bright and easily detected. We will then observe an ultraluminous infrared galaxy. These systems, which produce bright IR emission associated with the collision of gas-rich galaxies, also produce OH "megamasers". We will determine the isotropic luminosities of the Galactic and extragalactic OH maser systems, and learn why the extragalactic maser sources are prefixed by "mega."

Science Background

Using your web resources and the library, it would be beneficial to review a little background material on OH maser systems.

• What is a maser?
• What types of objects show OH maser emission?
• Why does the OH molecule exhibit four maser frequencies between 1612 - 1720 MHz?
• What physical conditions are required for the production of OH masers?

Moshe Elitzur's book "Astronomical Masers" is an excellent source for background information. Elitzur also has an article in the 1992 Annual Reviews that covers some of the same material. There are many good resources available on the web, as well.

Here's one example:

http://www.ub.rug.nl/eldoc/dis/science/h.r.kloekner/c1.pdf

Observations

The GBT Observations will be carried out using the observing software known as Astrid. Documentation for astrid can be found here:

http://wiki.gb.nrao.edu/bin/view/Data/AstronomersIntegratedDesktop

http://wiki.gb.nrao.edu/bin/view/Software/ObservingTools

http://wiki.gb.nrao.edu/bin/view/Observing/ScanTypes#OnOff

Here is an astrid script similar to what we will use for our observations:

#Configuration
lband_acs_tp = """
receiver  = 'Rcvr1_2'
obstype   = 'Spectroscopy'
backend   = 'Spectrometer'
nwin = 1
restfreq  = 1667.359
deltafreq = 0
bandwidth = 50.0
swmode    = "tp"
swtype    = "none"
vlow      = 0
vhigh     = 0
vframe    = "lsrk"
vdef      = "Optical"
noisecal  =  "lo"
pol       = "Linear"
nchan  =  "high"
spect.levels = 9
tint = 30
"""

Comment("Configuring the system for L band,  ACS with total power")     
Configure(lband_acs_tp)  
SetSourceVelocity(-44.0)
mySource = Location("J2000","2:27:04.1","61:52:22")
Slew(mySource)
Balance()
Break("Check IFRack power levels and Spectrometer duty cycles")
SetValues("ScanCoordinator",{"source":"W3OH"})
OnOff(mySource, Offset("J2000", 0.0, 0.3, cosv=True), 60)

Data Analysis

Data reduction and analysis will be accomplished in GBTIDL. Documentation can be found here:

http://gbtidl.sourceforge.net

An example reduction session that closely matches what we will do for our project can be found here:

http://wiki.gb.nrao.edu/bin/view/Data/GBTIDLPosSwitchSingleBeamRecipe

Observing plan

We will review the observing plan in detail after I arrive on Monday evening. But you may wish to get a jump on things using the following notes.

We would like to observe (at least) a couple of OH masers in stars, a couple of OH masers in star forming clouds, and a couple of OH masers in extragalactic sources.

We have two observing sessions:

Date         EDT Time    LST Time
----------  ----------- -----------
Tue 12 Jul  1730-2000  11:34-14:04
Wed 13 Jul  1730-2000  11:38-14:08

OH masers in stars:

Refer to Engels' catalog of OH sources:

http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1979A%26AS...36..337E&db_key=AST&data_type=HTML&format=&high=42b33a52e904524

Browse the sources and select 5 - 10 candidates at appropriate coordinates for the LST ranges available to us. Choose stars that have known OH masers and known distances. You might also pick a few stars that do NOT have known OH masers, in case we have time to try to discover a new source.

Another catalog of OH masers in stars is Benson's catalog:

http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1990ApJS...74..911B&db_key=AST&data_type=HTML&format=&high=42b33a52e904524

Note: DO NOT PRINT these papers ... they're very long! Just look at the catalog online and choose a few appropriate sources.

OH masers in star forming regions:

W3OH is available for observation. This is one source we will certainly observe. The disk of our Galaxy is not well positioned during our observation slots so most HII regions are not available, but we can still perhaps find a couple of candidates. Use the Paladini et al. catalog to look for candidate HII regions:

http://cdsweb.u-strasbg.fr/viz-bin/VizieR?-source=J/A+A/397/213

For example, 166.2+04.4 might be a good candidate.

OH masers in Ultraluminous Infrared Galaxies (ULIRGs):

There are many good extragalactic maser candidates that we could observe. See the following paper for a sample catalog:

http://www.ub.rug.nl/eldoc/dis/science/h.r.kloekner/c3.pdf

A couple of the more famous sources include Mrk231, Mrk273 and Arp220.

Exercises

1. What is the signal to noise ratio (per channel) attained on a 50 mJy maser if we observe with total power position switching for 10 minutes (5 minutes on source, and 5 minutes on the reference position) using 8192 channels in a 50 MHz bandwidth? Assume a system temperature of 17K, 9 level sampling in the spectrometer, and hanning smoothing turned on.

Be sure to understand how the noise is derived, based on the Radiometer equation. But feel free to use the GBT sensitivity calculator to estimate the noise:

http://wwwlocal.gb.nrao.edu/GBT/setups/senscalc.html

2. We will measure total maser flux in units of Janskies * km/s. Determine the formula to convert total observed maser flux in these units to luminosity in solar luminosities, for a source at a distance D measured in Mpc.

A galaxy at a distance of 100 Mpc has an observed maser flux of 5 Jy km/s. What is its luminosity in solar luminosities?

-- JimBraatz - 07 Jul 2005