[evla-sw-discuss] pulsar data

[Walter Brisken]

On Fri, 25 Aug 2006, Michael Rupen wrote:

> I thought we had one timer per each of *eight* 2 GHz basebands (four BB 
> pairs, 2 pol'ns each).

That could be.  It won't be a typical case where RR and LL will want 
different pulse phases though -- I'm inclined to think it shouldn't be 
supported.

> I'm confused as to gating vs. time bins -- I thought these were separate
> beasties.  Gating just means "correlate only at these times, and store one 
> output stream"; time binning means more output streams.  I'm not clear on the 
> "beating" between the gating and the phase bins.

Gating is time binning with only 1 bin.  No distinction is made.  Am I 
wrong?  In any case clocking is done by the pulsar timers.

> Am I right in thinking pulsar monitoring requires phasing up the array for 
> maximum sensitivity?  WIDAR will be delivered with 8 digitally phased 
> sub-bands for a total BW of 1 GHz.  WIDAR can phase up all 18 sub-bands of 
> every baseband, but we won't initially have that hardware.  Antennas may be 
> assigned to multiple phased sub-arrays; phased sub-arrays may be defined 
> differently on different sub-bands.  This implies for instance that one has 
> to have accurate positions which may be different for the different 
> sub-arrays, and for the different sub-bands.

The idea is that the correlator is the backend -- no phasing is needed. 
The entire FOV is available to the gated signal.  Pulsar "mode" is not a 
phased-array mode.

> Am I right in thinking this limit of 4 stems from the maximum of 4 phased
> sub-arrays per sub-band?  Does this increase if one is willing to spend
> fewer than the full number of sub-bands on each pulsar?

The four comes from the 4 timers (though you say 8 which could be the 
case -- the Project book is ambiguous if you ask me!).  In any case each 
BB can be locked to only 1 pulsar at a time, so you are BB limited.

>> 
>> "Monitoring" can mean either "imaging" or "timing", two distinct science
>> goals.  From the perspective of the correlator these two actions are the
>> same.  Typically timing will favor denser phase bins and imaging will
>> favor shorter integration times.
>
> Any thoughts on pulsar searches?

Pulsar searches are not done in pulsar mode -- they'd just fast dump mode. 
The pulsar specific hardware is only useful for synchronous (to a known 
period) observations.  If you know the period but not the phase (ie from 
X-ray timing) then you can use the pulsar bins with equal spacing across 
the pulse period.

> Anything special for higher frequency or astrometric observations? Though
> the latter may be boring without EVLA-II.

Not that I know of.

>> Specifying pulsar information:
>> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>> 
>> 2. Before the observation begins (but perhaps well after the scheduling
>> blocks have been submitted) the astronomer must provide the ephemeris
>> which contains a series of polynomials each valid for about 1 or 2 hours
>> and a dispersion measure used to calculate dispersive delays.  Each
>> submitted ephemeris should be labeled with a specifier that the observe
>> file references.  The pulsar ephemeris would be loaded into a database as
>> is done for the VLBA correlator.  The reason that the astronomer might
>> want to wait until the last moment to submit a polynomial is that many
>> pulsars are "unstable rotators" which have phases that can drift by 10s of
>> degrees relative to an ephemeris on 10s of days timescales.  Along similar
>> lines newly discovered pulsars often have very poorly known spin
>> parameters.  The use of the VLA to determine a position can be very useful
>> in refining the these.
>
> Are these likely to be refined during the observation? or is that too painful 
> to contemplate? I'm wondering whether one might start with an even 
> distribution of bins, then zoom in on the phases of interest.

Almost certainly not within a schedule block -- there are many reasons why 
you won't want to automate this!  If this is needed the astronomer should 
reduce the data from scheduling block 1, calculate a new ephemeris and use 
it in a second scheduling block to follow.

>> 
>> Note to the non-specialists:  Dispersion measure (DM) is the column
>> density of electrons along the line of sight to the pulsar.  Astronomers
>> use units of pc/cm^3 (parsecs per cubic centimeter) which is dimensionally
>> an inverse area.  In these units typical values range between 2 and 2000.
>> This number is never negative.  The delay incurred is frequency dependent
>> and is roughly equal to
>> 	dt ~= 4150 * DM / freq^2
>> with dt in seconds, DM in pc/cm^3 and freq in MHz.
>
> Is the ionospheric dispersion likely to be important at the lower 
> frequencies?

it is many orders of magnitude (perhaps 6 or 7?) less important.

>> Open questions:
>> ~~~~~~~~~~~~~~~
>> 
>> 1. Is fast-switching inconsistent with pulsar mode where a fairly
>>    substantial correlator mode change is required?
>
> I don't believe so -- we'd asked about this earlier.  Again I'll check to be 
> sure.  Assuming one can reconfigure the correlator on these timescales,
> we will want observe a calibrator with a wide-open bandwidth, then derive
> and apply appropriate phases/delays for each of multiple sub-bands and 
> phasing locations.
>
> Alternatively, given the initial hardware limitations, one could imagine
> simultaneously observing the pulsars in phased-array mode using 1 GHz, and an 
> in-beam calibrator with a broad bandwidth.  This calibrator might even be (a 
> gated version of?) the pulsar itself.

No phased array needed in pulsar mode.  As far as I know there is no 
bandwidth limits in pulsar mode as a result.

>> 2. How does pulsar mode interact with sub-arrays?
>
> See above for some aspects of this.  A related question:
>
> * Can one trade phased antennas for phased bandwidth?  I think not, but
>  this should be checked.

I'm almost certainly sure that you cannot.

The concerns I had were:

1.	Can one subarray apply the pulsar gate on all BBs while the other 
subarray does not?

2.	In the case that the same pulsar is being observed in 2 subarrays 
at different frequencies, can each get its own peculiar delay (in the 
pulsar sense)?

3.	Can the software keep track of all of this?

...

As an addendum to my original message I would like to put up for 
consideration that "pulsar mode" is not actually an observing mode.  In 
the current lingo modes are excusive (ie cannot do mosaicing and other 
mode simultaneously).  Rather I think the pulsar capability can be applied 
to any existing mode.  It is in Bryan's words "an intent".  If so I'd like 
to have finer-grained intents : "pulsar timing" and "pulsar imaging". 
This is mainly for the benefit of downstream software.