Galactic counter proposal

claw
Posts: 26
Joined: Mon Mar 03, 2014 7:18 pm

Galactic counter proposal

Hi All,
The current process of defining the VLASS has a baseline plan of a S-band, B-config all-sky survey. We are currently waiting for proposals to modify that baseline, while still fitting in the same ~8000 hour survey duration.
I've found myself wondering if there are relatively small modifications that can be proposed to include a bit more Galactic science than the baseline. For example, there was a concept for an A-config survey in the Galactic plane (I believe related to Shami's WP). I can also imagine that larger spatial scales (a la C config) would be interesting. And, in light of the fact that much of the Galaxy is observed at low declination, where Tsys can rise by a factor or 2, it is worth arguing for a bit more time in the plane.
I wonder if this WG could come up with a compelling case for a pass with A and C config in the plane to complement the all-sky B-config baseline survey. Is the science case strong enough to argue for a trim to the depth of the all-sky survey?

casey

shami
Posts: 7
Joined: Wed Feb 19, 2014 2:24 pm

Re: Galactic counter proposal

Hi Casey and others,

For compact objects, the resolution requirement is driven primarily by the need to get unambiguous counterpart IDs. This is more stringent (on average) in the Galactic plane than outside it due to the density of stars etc. In our whitepaper, we'd argued for 1-2 GHz / A-array, but 2-4 GHz / B-array is not *that* much worse. It would certainly help to cover the inner Galactic plane (|b|<2 degrees, say) at A-array resolution, but that wasn't a minimum requirement for us.

I continue to be concerned by the constant nominal sensitivity suggestion - we will get significantly higher Tsys in the Galactic plane, especially towards the Galactic center which is both brighter (more complex structure) and lower in Dec. I would strongly recommend 2-4 GHz BnA observations for the innermost Galactic plane / GC region, possibly *in addition* to the standard B array survey observations, as a way to get the actual sensitivity up to the nominal value and to add resolution in the most confused region of the sky. That may be a fairly self-contained and well-justified add-on to the primary survey.

-Shami

gsivakoff
Posts: 17
Joined: Tue Mar 25, 2014 11:28 am

Re: Galactic counter proposal

Hi All,

After the telecon, it became clear the simplicity in message was going to win out. So I think there are only two possibilities we could push for; higher resolution and higher sensitivity.

1) If resolution is the truly unique thing the VLASS will add, then why are we not pushing to A configuration for all science? We have already given up on extended objects. As far as I can tell there are two arguments against that: the desire for a uniform-ish circular beam and the more important reason may be the higher proposal pressure in A array. The nearly uniform beam argument would mean that the southern part of the survey would get done in BnA. I think that's an easy push. But frankly, I'd rather have the smallest possible beam at any given declination rather than a uniform less good beam.

2) The sensitvity in the Galactic regions do not make a significant dent in uncovering the pulsar population. While I would be happy to post a bunch of smaller science areas that would gain from higher sensitivity, I think one major one will be more effective in the simpler is better regime. I would love to see us pushing down farther in the plane and bulge to reveal the pulsars. And in doing so, we should push to include the effect of T_sys. One other possible science area to push this over the top would be the thermal science. This is not my area, so I can't talk about numbers, but between work we did here and Rick's analysis, I still think that there will be a strong component of thermal (or quasi-thermal) sources. It'd be great if one of the GUTS proponents could give a stronger sell for this than I can.

I'd love for us to push back a little against the base plan as follows, probably in the order I would fight.
Add in a BnA requirement for southern sources; this would come at no cost to the rest of the survey.
Add in the effect of higher T_sys; this comes at a minimal cost to the rest of the survey. Even if we assume roughly 3000 of the 40000 square degrees covered need 4 times the observing time, there is an 11% larger RMS goal for the whole survey. I think this should be strongly pushed.
The more difficult push would be to ask for more sensitivity in the Galactic plane. If we assume roughly 3000 of the 40000 square degrees covered need 8/16 times the observing time (to get two to four times the real sensitivity), there is an 23%/46% larger RMS goal for the non Galactic survey.
The most difficult push may be to do the entire survey in A array to get the smallest possible beam.

lsjouwerman
Posts: 21
Joined: Mon Jul 18, 2011 3:49 pm

Re: Galactic counter proposal

I think with going to S (or L) band A or B array the VLASS will loose all of the Galactic interest except for the pulsar people - we have two other main areas: 1) nonthermal extended continuum stuff - much of this done in the past - and 2) thermal stuff (incl lines, extended and compact) - something only the VLA would be good at. Note that Ku and C array gives you almost the angular resolution of S/A (and beter than S/B) for identification purposes but S/A won't detect anything with some structure nor thermal emission. I can't imagine that (m)any Galactic people get excited for S/A or S/B as pushed for by the very vocal pulsar (and transient, if covered more than once) community - and that is why it is so extremely quiet here with this baseline plan (i.e almost nobody cares for this direction) - so maybe it is best to avoid the plane at all. I think I speak for many of the Galactic community when I suggest to completely forget about a Galactic contribution and save significant hours that otherwise will waste everybodys (observing) time. There are many good PI programs that could use this LST range for much more interesting science than a half-hearted S band A/B array not supported by the Galactic community.

o&o

gsivakoff
Posts: 17
Joined: Tue Mar 25, 2014 11:28 am

Re: Galactic counter proposal

lsjouwerman wrote:I think I speak for many of the Galactic community when I suggest to completely forget about a Galactic contribution and save significant hours that otherwise will waste everybodys (observing) time. There are many good PI programs that could use this LST range for much more interesting science than a half-hearted S band A/B array not supported by the Galactic community.

I think part of the silence here is the lack of email notification for posts; I know I have missed many threads due to this.

But more importantly, if we look at the history of this group, we very rapidly divided into a thermal and non-thermal perspective, because that is what is in the Galaxy. The extragalactic community never needed to worry about such a divide. It's not surprising that we are still split along these lines.

I don't think there will be a question of saving observing hours given Tony Beasley's comments at the end of the day long telecon. I think the group will put in a (simple) proposal of 6000-8000 hours and that the counter proposals by groups will largely sacrifice some science in another group to achieve their major goals without a large change of time. I don't see a difference in LST coverage occurring because I think there is likely to be an emphasis on the all-sky aspect.

As members of a diverse Galactic community, is there important science (not necessarily our bread and butter) to be done here? Does it outweight what the extragalactic community would do if we completely fold? If so, I think it behooves us to push a little more for this science, as well as to determine what could be done with the baseline plan. While I'm not a pulsar person, I can see that there's really good science to be done here. While I'm not a thermal person, I think there still is good science to be done in the thermal and quasi-thermal regime. For instance, I think that a stand alone GUTS proposal will benefit from the spectral index constraints enabled by the large wavelength baseline to the S band observations. But without the experts in those fields speaking up, I think we could see minimal to no Galactic science getting done, when there is still good science that can get done.

There is also one other regime that Tony (in the telecon) and Lourant (above) mentioned that we have largely ignored, extended emission. Is there anyone in the group that would champion that cause?

chomiuk
Posts: 12
Joined: Fri Feb 28, 2014 4:16 pm

Re: Galactic counter proposal

First things first: I am a thermal person. I care about novae, symbiotic stars, and red giant winds. *and*-----S band is fine for thermal sources. Great, in fact! Whether the spectrum is nu^0.6 or nu^2, the much larger beam at S band still enables us to go deeper in a given unit of time than Ku band.

I somewhat fell off the forum because I, too, couldn't figure out how to get timely updates. Also, because I was happy with the S band B config proposal that was the default, and so felt that the survey was headed in the right direction. My silence was the silence of happiness!

It's not fair to say that the extragalactic people are partial to nu^-0.7 sources. AGN actually have all sorts of crazy spectral indices, including quite steeply inverted. But they have settled on S band, because S band is the optimal band for the largest range of spectral indices, and it has a nice large primary beam!

For myself, my desire would be going deeper in the Galactic plane, ideally using multiple epochs so we could investigate the population of Galactic radio transients. Multiple epochs would enable science goals like finally parameterizing radio variability of symbiotic stars, and carrying out the first complete search for classical novae in the Galaxy.

I like the ideas put forward by Casey/Greg very much, to try and even out Tsys---if not at all low decs, then at least at low decs in particular regions of interest.
I am happy with either B config or BnA. BnA would be great but i don't think it's critical.

The 'default' survey proposed will do wonderful science for both thermal and non-thermal sources. It will do even better science the deeper we can go.

claw
Posts: 26
Joined: Mon Mar 03, 2014 7:18 pm

Re: Galactic counter proposal

Hi Everyone,
Thanks for putting your thoughts down again. I wanted to update you on the track the discussion has headed lately.
This coming Monday, we are expected to come together on a consensus and start doling out writing assignments. The plan now is for a pure S-band survey (in something close to B-config) like this:

-- All-sky (Dec > -40) to 100 microJy,
-- Wide (~1e4 sq deg aligned with DESI) to 50 microJy, and
-- Deep (10 sq deg) to 1.5 microJy.

This totals up to around 9000 hours, give or take. There is a Galactic option that requires extra justification. If the chairs are convinced, that time will be taken out of the wide tier. The sketch for a Galactic enhancement is a bulge survey in A config (2800 sq deg) to 50 microJy.
The sense I get is that some would rather have more Wide than to enhance the Galactic plane sensitivity. We need to make the case on Monday that there are concrete benefits from a 50microJy S band Galactic survey. Specificity is really what drives the extragalactic cases, so it would help if we could identify Galactic science lost if only covered to 100 microJy (or ~150 microJy considering spillover noise).
This consensus plan has a lot of momentum, so a decision is very likely on Monday. Any new thoughts here would help us make the pitch!

casey

rosten
Posts: 19
Joined: Fri Feb 14, 2014 3:40 pm

Re: Galactic counter proposal

Thanks, Casey, for the notice that you had started another thread. I wanted to come back to a couple of comments that surfaced on this thread:

o we investigated whether we would need to increase integration times to account for any increase in Tsys due to low galactic latitude. Casey did the calculation just now, and to retain uniform sensitivity will require of order 1.5 times more time (final number still t.b.d). This is a strong driver to retain homogeneity and not compromise Galactic science that happens to be at low dec.

o we've (Cornelia really) has contacted Karl Menten to ask for more details about the GLOSTAR survey parameters, but we haven't heard back from him. So whatever information we have about GLOSTAR that is publicly available is what we have.

o I am philosophically opposed to the partitioning up of the Sky Survey into the tiers that are being proposed: to me the fragmentation is going to serve some communities very well but it's not going to serve as a unifying project for radio astronomy, or even have as much impact with other wavelengths. That said, we need to argue for improvements that would add to the Galactic science case, and higher spatial resolution in the plane is important to deal with crowding issues. I'm not sure I buy the argument that going for a narrow part of the bulge with deep, high resolution is the best answer, as it is going to favor only a few source populations.

o Cornelia and I had come up with a list of Galactic source types, their spectra, and are in the process of filling out the expected numbers for the sensitivity of the survey (100 muJy in the plane).
1. stellar winds/massive stars $\nu^{0.6}$
2. colliding wind NT emission $\nu^{-0.5}$
3. Planetary Nebulae flat
4. Evolved HII regions flat
5. Active Stars/incoherent flat
6. Active Stars/coherent steep - $\nu$,t structure
7. pulsars steep
8. Supernova remnants steep
9. Pulsar Wind Nebulae flat
10. YSOs/thermal $\nu^{0.6}$
11. YSOs/nonthermal flat
12. black hole X-ray binaries
13. neutron star X-ray binaries
14. Cataclysmic variables

Laura added a couple of different types in her e-mail, which I copy here for completness:

**The population of quiescent accreting Black Hole binaries** could be searched for in the Galaxy, and identified by comparison with X-ray and optical data sets. An rms sensitivity of 20--30 muJy/beam allows us to observe V404 Cyg (quiescent stellar-mass accreting black hole) out to 3--4 kpc. Recent work by our team implies that there is likely a large population of detectable quiescent black holes that can be identified by co-spatial radio and X-ray coverage. For this science case, the priority is to go as deep as possible and at least cover the Galactic Bulge, where there have been some nice X-ray sources. Multiple epochs wouldn't hurt, but aren't crucial.

**A complete survey of symbiotic stars** It remains an open question how many symbiotic stars (red giant + accreting white dwarf) are out there, because they often 'hide' at optical bands, not showing strong emission lines. We'd really like to have a complete census of symbiotics, as symbiotics are enjoying something of a resurgence as SN Ia progenitor candidates (and the relative rates are an important discriminant). Radio is a nice way to detect them, as we're simply sensitive to the ionized red giant wind. Reaching ~100/150 muJy limits should uncover a nice sample of these objects---we should be able to detect a typical S-type to the Galactic bulge (Seaquist & Taylor 1990). Again, more areal coverage would be nice, but focusing on the bulge is a fine thing to do. Multiple epochs would be awesome to---for the first time---constrain radio variability of symbiotics (which would help constrain their energy source: accretion or nuclear fusion).

**An unbiased search for Classical Novae** There are expected to be ~35 Galactic novae each year, with about half of them exploding in the Galactic bulge. However, due to the effects of dust and optical incompleteness, we usually only detect about a quarter of these, dramatically biasing our understanding of these most common explosions. With a radio survey of the bulge, we will carry out the first well-defined and complete sample for classical novae, constraining their distribution of masses, energetics, and mass loss mechanisms. With a sensitivity limit of 100/150 muJy/beam, we should be able to detect classical novae to the Galactic center. For this one, we want multiple epochs.

chomiuk
Posts: 12
Joined: Fri Feb 28, 2014 4:16 pm

Re: Galactic counter proposal

I got a few more specifics about GLOSTAR from Nirupam Roy: the key one is the coverage: whole Galactic plane visible from VLA with b = +/- 1 deg, B- and D-configuration (BnA and DnC for the very southern part).

His whole paragraph:
For GLOSTAR, we have the C band continuum - 2 GHz BW and standard spectral set up centred at 4.7 and 6.9 GHz, with full polarization - about 15 sec per pointing, and ~ 40 uJy noise. Then we have CH_03 maser, HCHO absorption, and 7 H-alpha RRLs - so a total of 9 more spectral window to cover these lines (with velocity resolution of ~0.18, 0.25 and 4 km/s respectively). The plan is to cover the whole Galactic plane visible from VLA with b = +/- 1 deg, B- and D-configuration (BnA and DnC for the very southern part).

So either our unique contribution is to go broader b> +/-1, and/or it's to go deeper on a particular portion of the plane.

chomiuk
Posts: 12
Joined: Fri Feb 28, 2014 4:16 pm

Re: Galactic counter proposal

rosten wrote:o we investigated whether we would need to increase integration times to account for any increase in Tsys due to low galactic latitude. Casey did the calculation just now, and to retain uniform sensitivity will require of order 1.5 times more time (final number still t.b.d). This is a strong driver to retain homogeneity and not compromise Galactic science that happens to be at low dec.

To be clear---when we say 'uniform sensitivity', we are talking about matching the 100 muJy/beam of all sky VLASS, not the 50 muJy/beam of 'wide' VLASS---right?

rosten wrote:o we've (Cornelia really) has contacted Karl Menten to ask for more details about the GLOSTAR survey parameters, but we haven't heard back from him. So whatever information we have about GLOSTAR that is publicly available is what we have.

See my other post---GLOSTAR is |b|=1 deg along the whole plane visible to VLA.

One thing we should consider doing is covering the GLIMPSE footprint (that's a Spitzer survey). They've gradually expanded their coverage of the Galaxy over several Spitzer cycles. It now reaches:
|l| = 5--65 deg, |b| < 1 deg
l = -5 to 8 deg, |b| < 3 deg
|l| = 0--2 deg, |b| < 4.2 deg
other spurs at a variety of Galactic latitudes that reach |b| < 3 deg.
Here is an illustration of GLIMPSE coverage.

It would then be a point for discussion if we should actively exclude the GLOSTAR area or not, in an effort to cover the GLIMPSE plane.