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Lower Frequency Alternative Survey Plans

Posted: Thu Mar 27, 2014 8:45 pm
by gsivakoff
Hi All,

I'm sorry for joining this discussion late. I've tried to read all of your previous posts, but please ask forbearance if I missed something that was already brought up.

With all due respect to the existing Working Group plan, I think we might want to consider lower frequency alternative plans that would have strong synergy with the extragalactic and galactic transient crowd. I realize that there is a strong desire to cover thermal emitters, so let me put up a strawman alternative that may achieve both goals. I wish I had more time to list science goals, but with the telecon tomorrow (and a strong possibility I can't make it because I am at a conference), I wanted to get this up sooner and leave time to fill in more details.

I think the key thing a Galactic survey needs to do is go deeper than the plans for the extragalactic (or even the transient) proposals. My nominal idea is to push to a 25 muJy RMS in either S or C band. First, this would be a huge improvement over CORNISH sensitivity and the THOR resolutions. Second, this would correspond to 625 muJy / 157 muJy equivalence at Ku band for f_nu proportional to nu^2 dependence, and 125 muJy / 62.5 muJy equivalence at nu^1 dependence. (I realize we are going to lose some spectral lines).

Using Steve's white paper numbers, this corresponds to a survey speed of 1.04/0.46 hours / square degree for S/C band. To achieve this I would suggest at least 6 epochs, each deeper than 100 muJy, or possibly as many as 16 epochs 100 muJy epochs.

In terms of where to do this, I would argue for a 30x30 square degree box on the Galactic Centre (I apologize, but I have not had time to double check that the VLA would fully cover this; if it does not this plan would of course need adjustment). I would then also argue for at least a 250x5 square degree strip covering the remaining bit of the Galactic plane. This corresponds to 2150 square degree. If we were to argue for S and C band, then this would take ~2100+4700 hours, which I think is a bit much (although given that this subsumes large parts of the transient proposal, perhaps not too bad). Alternatively, we could increase the Galactic plane strip to cover 250x10 (3400 square degrees) and concentrate on S Band. This would take ~3330 hours. Note that concentrating on C band would take ~7400 hours.

As I said above, this would essentially cover Galactic transients better than the transient working group plan, by excluding X Band. This would also cover ~ 2150-3400 of the all sky plan of the extragalactic working group if done in S Band.

If we want better coverage of thermal sources, we might want to go to C Band. The increased sensitivity of the above plan makes up for the higher frequency for pulsars, also given a ~50 muJy equivalent S Band RMS for pulsars (assuming f_nu propto nu^-1).

As one final note, I would also love to add the globular clusters to our plan. This would be a low cost, potentially high science gain addition.

Re: Lower Frequency Alternative Survey Plans

Posted: Fri Mar 28, 2014 1:30 am
by claw
Hi Greg,
An idea not unlike this came up in the transients working group this week. There is transients science at Ku band, but the case is much clearer at lower frequencies. One exciting possibility is that a fast transient detection system will be commensally searching by the time VLASS is going. If so, then Galactic coherent transients like pulsars and stellar flares are accessible at L and S band.
Of course, all of this hinges on the science case. The extragalactic case is strong and well demonstrated; as such it is driving much of the design of the VLASS. Personally, I haven't the science case crystallize as clearly within the transients and Galactic communities.
FYI: my sense from the last Transients WG telecon is that the X band survey will probably get dropped. There is more interest in deeper and surveys at S and C bands.

casey

Re: Lower Frequency Alternative Survey Plans

Posted: Fri Mar 28, 2014 2:45 am
by gsivakoff
I think it is good that there seems to be some strong synergy with the transient case. I fully agree that the science case needs stronger development. Here's some rough, albeit not uberstrong ideas, in no particular order.

What: Quantifying the compact object population of the Milky Way
Why: Major observational input to population synthesis and stellar evolution models; potential identification of critical exotic objects like BH-NS pulsars and other highly accelerated pulsars

How:
a) Imaging Pulsar Search using spectral index
b) Quiescent Black Hole X-ray Binaries, where sufficient X-ray data exists
c) Unbiased search for radio emission from Cataclymic variables (probably an SDSS footprint idea)
d) Probe of the X-ray Radio relations with known objects, and exploration of radio variability
e) MSP search, probably best done in entire sky
f) Unbiased Galactic SNR search

What: Multi-wavelength exploration of the thermal properties of Galactic sources
Why: (Lorant & Elizabeth should comment here)
How: GUTS like sensitivity, but at lower frequency

What: The radio flare properties of "typical" stars
Why: (Rachel should comment here)