[evla-sw-discuss] pulsar data

[Walter Brisken]

This message describes my best guess as to the astronomers interface to 
the EVLA pulsar modes with the WIDAR correlator.  At the moment it is 
mainly for the benefit of David Harland who is working on the obs prep 
tool.  Any comments are welcome.

The pulsar gating hardware:  (EVLA project book 8.2.14)
~~~~~~~~~~~~~~~~~~~~~~~~~~~

WIDAR will support one pulsar timer per 2 GHz baseband (4 total).  Each 
timer is driven by a polynomial which returns pulse phase as a function of 
the current time (UTC).  The pulse phase is in the range [0,1).  Each of 
the 16 subbands per IF can operate with a different fixed delay relative 
to the IF timer.  Up to 1000 time bins can be accumulated.  Each time bin 
has a start time and a stop time.  Start and stop times are specified by a 
"pulse phase" ranging between 0 and 1.  I believe that just as for 
frequency channels the number of pulsar bins can be different for each 
subband.  Note: pulsars' spin periods range from 1.5 ms to 8.5 sec.

Common use cases:
~~~~~~~~~~~~~~~~~

1. Monitoring a single pulsar

The same polynomial will drive all pulsar timers.  A different delay for 
each subband will be added to account for pulsar dispersion delay (a 
frequency dependence in pulse arrival time due to propagation through 
ionized intersteller medium).  This delay is parameterized by a single 
parameter called the dispersion measure.

The user will choose a number of phase bins <= 1000 and will specify start 
and stop times for each.  In the case of > 100 channels, it is likely that 
the bins will be uniformly distributed across [0,1) as would be used in 
"phase-ignorant" observations where the observer does not know the pulse 
phase prior to observation.

If the pulse phase is known in advance the astronomer may place a far 
smaller number of phase bins at pulse phases of interest to him/her.

2. Monitoring up to 4 pulsars

Same as above, but in cases where multiple pulsars are in the same primary 
beam of the telescope it will be possible to observe up to 4 pulsars 
simuntaneously, each with its own polynomial.


"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.

Specifying pulsar information:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The exact mechanics of this are still to be discussed, but I imagine the 
following is nearly accurate:

1. Specify "Pulsar Observing Mode" which will require the following
    information:
   A. For each 2 GHz IF:
       1. pulsar ephemeris (polynomial table) specifier (see below)
       2. pulse dispersion measure (DM) (note -- the purpose here is only
          to warn the user if the dispersion smearing within one channels
          is detrimental.  The DM used in driving the phase bins will come
          from the ephemeris.
       3. pulse period (for same purpose as DM above)
   B. For each sub-band :
       1. gate enable (default enabled)
       2. peculiar gate delay (default zero.  This is in addition to
 	 dispersion delay)
       3. number of phase bins required (default: 64???)
       4. table of start/stop phases  OR  one or two start/stop times for
 	 the requested bins to be evenly divided into. (default: [0,1) )

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.

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.

At observe time if no ephemeris has been loaded into the database the 
pulsar mode should be disabled and "standard interferometry" mode should 
be used.  An alert should be triggered.


Open questions:
~~~~~~~~~~~~~~~

1. Is fast-switching inconsistent with pulsar mode where a fairly
    substantial correlator mode change is required?

2. How does pulsar mode interact with sub-arrays?


-Walter