Notes from MCP Meeting - 23-24 April 2012
In attendance: Jim Braatz, Mark Reid, Jim Condon, Fred Lo, Feng Gao, Wei Zhao
- Mark reviewed Bayesian fitting formalism
- How does VLBI sensitivity affect H0 measurement?
- One good VLBI track (GBT+VLBA+EB) gives reliable maser spots to ~ 10 mJy ( This is a cutoff of ~ 7-sigma detections)
- Most of our previously observed galaxies have essentially ~4 good tracks: get maser spots to ~ 5 mJy
- With JVLA we expect ~ 50% improvement in VLBI sensitivity so with 4 good tracks we get maser spots to ~ 3 mJy
- Critical plot for Aug 1 proposal: Show a spectrum with a horizontal line at ~ 10 mJy and ~ 3 mJy to demonstrate how many more lines we can get, esp, at systemic velocities, with the improved sensitivity.
- Need multiple tracks closely-spaced in time to avoid ambiguities in averaging widely-spaced (in time) data.
- Note 3 mJy noise improves position accuracy of strong lines and also allows us to detect more lines, and in an ideal case this extends the velocity range mapped (more important)
- However, it may be ok to average data widely spaced in time if the disk geometry is appropriate, i.e. masers at a given velocity always have approx the same position on the sky and same accel, even when observed over many years. Some evidence that this is the case for U3789 (all 3 VLBI maps look the same)
- How does 3 mJy sensitivity affect Bayes H0 result?
- Next, we need to show how this improved sensitivity and more velocity coverage of VLBI-mapped systemic lines improves the H0 measurement. Best ways to approach this question are:
- Starting with UGC 3789 results, degrade the data to simulate the same galaxy ~ 2X farther away. Refit the degraded data. How much worse is it?
- Generate fake data for a simulated disk, complete with accelerating systemic features, and fit with Bayes program. Critical to make fake spectra modeled on a real galaxy, and it must have systemic maser features that extend the mappable velocity range, with 10 mJy < flux < 3 mJy
- Can we use HI recession velocity as a prior on Bayes fitting?
- Jim B has GBT data on a few of the maser disk galaxies -- he will circulate sample spectra
- Need JVLA data to get recession velocities accurate enough. Is this feasible?
- Can we detect the continuum, and if so, can we use the position of the continuum to constrain the position of the dynamical center?
- How much difference in H0 would it make to provide a more accurate v0 prior?
- Try the disk fitting program with artificially tight prior on v0.
- How do we assess the need for better acceleration data or different cadence?
- Average 3 epochs into 1 using a rolling boxcar average
- NGC 6323 and J0437 are good candidates for this experiment; Feng and Jim will work on this.
- Test importance of cadence by dropping every other epoch from one of our good cases, and refit the accels.
- Need to write software that gives us a better look at results of the accel fitting program, along the lines of Jim B's tools for plotting fitted accels over "old style" plots
- Modifications requested to the new acceleration fitting program:
- print residuals for each solution
- try to fit a single acceleration for a given velocity range
- try to fit a parametrized acceleration a(v) = a0+k(v-v0)
Aug 1 proposal planning
- Do we understand the physics of the disk and mechanism for making masers?
- Fred will investigate some of these questions during his sabbatical
- Why are systemic and h.v. lines comparable in intensity?
- Alignment of maser clouds leads to visible maser spots?
- What happens to maser line width during a flare?
- Several good cases to investigate: NGC 1194; Mrk 1419. Good project for Eugenia.
- Discussion of priorities for VLBI
- Three periods to consider
- Fall 2012 (GBT+EB+VLBA)
- Winter 2013 (JVLA+GBT+EB+VLBA)
- Fall 2013 (GBT+EB+VLBA)
- We mainly discussed period 2 (with JVLA) and we decided on priorities:
- (1a) NGC 5765 : 8 tracks x 6 hours
- (1b) J0437+2456 : 8 tracks x 6 hours
- (2) ESO558-G009 : 8 tracks x 6 hours
- We discussed other galaxies but decided they were lower priority.
- Mrk 1419
- NGC 6323
- NGC 6264
- Need status update from Cheng-Yu
- UGC 3789
- IC 2560
- NGC 1194
- NGC 2273
- UGC 6093
Improving our VLBI methods
- What about Fall 2012 and Fall 2013?
- Get lots of tracks on N5765.
- Get more BH candidates?
- Dependence of VLBI imaging on absolute maser position for self-cal tracks
- This is calculable from first principles - we think the position offset of a maser spot relates either to absolute freq difference or "video" offset
- Easy way to test this is to insert a position offset using CLCOR on a maser with good position, then reimage
- VLA positions may not be sufficient for galaxies that we intend to observe for distance measurement
- Discussion of the need for geodetic blocks in our VLBI observations
- Mark says we don't need these for self-cal tracks; We do still need them for phase-ref tracks
- Saves ~ 2h per track for self-cal tracks. Can get clock error and residual delay from hourly observations of the delay calibrator (needs to be <~ 10 degrees from target and >~ 500 mJy continuum source). We already observe these in our tracks.
- Note, required structure of self-cal VLBI tracks is then:
- Fringe finder/bandpass calibrator (very strong, northern target, need not be close to target)
- Loop (1 hr) : Delay cal (~ 5 mins) then Target (~ 55 mins broken into two ~ 30 min segments)
- Fringe Finder/bandpass cal
- Delay cal