Mopra 115GHz The MAGMA Perspective Annie Hughes & Tony Wong

Why this talk? Flux monitoring undertaken as part of the MAGMA survey may be valuable for: users who want to combine data from MPCOR and MOPS (2006 to present) managing observer expectations of system performance at 115GHz quantitative examples for why users should follow good millimetre observing habits at 115GHz practical feedback from Large Programme to user community (not just their science results)

MAGMA Overview Observations: OTF mapping of LMC at 12 CO, 13 CO + … LMC v rad ~ 200 to 300 km/s (LSR) 12 CO Sky Freq ~ MHz For MAGMA, consistent calibration over duration of survey is more important than absolute flux scale. Flux Monitoring at 115GHz: Regular Orion KL position switched spectra Ori KL v rad ~ 9 km/s, 12 CO Sky Freq ~ MHz Bootstrap to SEST T mb (FWHM = 115GHz)

Results Assuming T xb ( 12 CO) = 100K, an average bootstrap flux scaling factor for different observing epochs was derived using measurements of Orion KL’s peak CO brightness. “flux scaling factor” includes efficiency + other system effects Flux monitoring suggests efficiency consistent with Ladd ea (2004), but with “glitches” in 2006 (?) and early 2008 (focus?). Peak Temperature [K] Integrated Intensity [K km/s]

Elevation After excluding OriKL data in poor weather & “glitches”, we find elevation dependence ~ 0.8 to 1.2 K T A * per 5 degrees for el>35 degrees (steeper at lower elevation). This is within 20-30% absolute flux calibration uncertainty, but is reproducible (in good weather). Origin: beam distortion & pick-up (?). Atmospheric model more uncertain at low elevations Moral #1: stay above 35 degrees if absolute/consistent flux matters Moral #2: monitor calibrator and source at similar elevations Blue/green dots – different MOPS centre frequency setting Blue = MHz Green = MHz Peak TA* [K]

The choice we faced: MOPS configuration for LMC: 12 CO sky freq ~ MHz V cen = MHz 12 CO Zoom Window = [54] Configuration for Ori KL: 12 CO sky freq ~ MHz Need to choose between: V cen = MHz 12 CO Zoom Window = [54] V cen = MHz 12 CO Zoom Window = [55] Does it matter which of these configurations we choose? In either case, it is nearly inevitable that sky frequency of 12 CO line will be different for extragalactic source and Galactic standard. MAGMA tried both configurations … Peak TA* [K]

Vcen scans empirical approach: repeat Ori KL spectra with different MOPS v cen green/red (2007); grey (2006) v cen dependence of 0.8 to 1 K T A * per 100 MHz between and GHz net effect may lie within 30% guarantee, but reproducible – should consider during obs. setup? Origin: consequence of T sys rising across the 2.2 GHz block at highest MOPS operating frequencies? Moral #3: Keep v cen the same for source and standard Moral #4: Stay as far from the danger zone as practical for all measurements (dependence probably flattens at lower frequencies)

Summary I MAGMA flux monitoring of Orion KL suggests system has been reasonably stable since 2005, with small glitches in 2006 and early 2008 Useful to quantify v cen effect across the 3mm band MAGMA is happy to provide feedback about our flux monitoring to Mopra user community. Could other Large Programmes do the same? How useful would it be? Useful for Mopra website to explain the rationale for millimetre observing guidelines to interested – but potentially renegade! – observers… (pages like this are currently under construction?)

Summary II Observing at 115GHz with Mopra is pushing the envelope of the site and system. If a consistent flux scale matters: 1. stay above 35 degrees 2. be religious about taking standard spectra 3. take standards with an observing set-up that mimics target observations (e.g. elevation, v cen ) 4.use lowest frequency setting for MOPS that works Most of 1-4 is not new, but MAGMA flux monitoring analysis could be used to show why the advice is sensible.