G301: The MALT90 Characterization source

Introduction

As part of MALT90, we perform a short position-switched (PSW) observation of a known ultra-compact HII region at the start of every observing session. The source we use is G300.968+01.145 (or G301). The purpose of these observations is primarly to verify system performance; they serve as a sanity check to make sure that we see lines and that the system is performing as expected.

These observations form a unique dataset. Becuase we have observed the same source during hundreds of different observing sessions over multiple 3+ years we can use these observations to characterize aspects of the MALT90 survey and the Mopra Telescope's performance at 90 GHz. This page provides additional access to the data for the first three years of observations which have been published (Foster et al. 2013 -- ADS|astro-ph|local).

Characterized Properties of MALT90 Survey

Observations of G301 as part of MALT90 are ongoing. This page provides our current best estimates for the characterized properties of the MALT90 survey.

We find that the gain of the Mopra telescope changes as a function of elevation. In addition, there is a roughly linear decline in the efficiency of the Mopra telescope seen throughout each observing season. We expect that this is due to thermal deformation of the telescope. The best proxy found for this deformation is time-of-day (this explains the trend better than current ambient air temperature or our efforts to quantify the change in ambient temperature over a period of time preceeding our observations). We present a gain relation with time-of-day, but this is only a rough proxy for the expected physical cause of the gain variation.

The above relationships show our estimates of the relative gain of the Mopra telescope as a function of elevation and time-of-day. These gains are relative to a baseline observation taken at 44 degrees of elevation and 5.5 hours past sunrise. We present a different relation for each of our four strongest transitions, but it is likely that the spread among these relations is indicative of our uncertainty in determing the relations, rather than some physical reason.

We expect that the pointing uncertainty and gain relations given here will be appropriate for other users of the Mopra telescope at 90 GHz as well, but users should read the reference paper and carefully consider whether this is the case for their individual datasets. The gain relations are characterized over a narrow range of elevation and time-of-day and should not be extrapolated outside of these ranges.

Pointing Uncertainty

RMS Pointing Uncertainty (Elevation < 45 degrees) 10 arcsec
RMS Pointing Uncertainty (Elevation > 45 degrees) 6 arcsec
RMS Pointing Uncertainty (overall) 8 arcsec

Gain Variation

The following gain factors can be used to scale measured line amplitudes relative to a baseline obsevation taken at z0 = 44 degrees of elevation and t0 = 5.5 hours past sunrise.

Variables N2H+ HNC HCO+ HCN
Absolute flux uncertainty [K] 0.38±0.02 0.34±0.01 043±0.02 0.29±0.02
Elevation Gain Factor (δ) [deg-1] 0.016±0.002 0.010±0.001 0.010±0.001 0.012±0.002
Time-of-day Gain Factor (β) [Hr-1] 0.024±0.009 0.028±0.006 0.28±0.006 0.040±0.007

Access to G301 Data

We provide access to the G301 data through this website as well as through the characterization paper (Foster et al. 2013 -- ADS|astro-ph|local). If additional information is required, please contact Jonathan Foster.

The above figure shows the time series of transition amplitudes for all observations of G301. This file is automatically generated during the data reduction and includes all observations, even observations taken under very poor weather conditions. The colored ranges represent the 1 and 2-sigma ranges in transition amplitudes for the four strongest lines in our survey.

Currently, Tables 4 and 5 from Foster et al. (2013) are available for download in IPAC table format.

Table 4 lists the information about each PSW observation of G301. Table 5 lists the fit parameters for PSW observation of G301 (where the fit meets certain criteria for reasonableness). Both tables have the same convention for source number and same, so combining them is fairly easy.