[Lofarpwg] [DKIM: Failed] Re: LOFAR LBA census draft -- ready for commenting

Tue Aug 20 10:53:37 UTC 2019

Dear All,

as promised in the telecon, here are some thoughts to perhaps include in the discussion of the low-frequency turnover.

There is the ‘Beskin-school’ which relates the turn-over, or ‘low-frequency cut-off’ (as they call it), is due to
mode propagation in the magnetosphere. Here the ‘ordinary wave has a refractive index which prevents
it to propagate below a certain frequency. I attach a recent review of Vasilii for your convenience. Have a
look at Eq. 118, which he compares with existing data in Figure 24. One could easily produce a new
figure with the new LOFAR data. This is certainly meat for discussion as it would be interesting to see how
this hold up.

There is work by Petrova (2002), which is along similar lines, but she concentrates more on the power-law
behaviour above the low-frequency turn-over. Nevertheless she summarised that the is indeed attributed
to propagation effects  e.g. to free-free absorption (Malov 1979) or induced scattering (Lyubarskii & Petrova 1996).

The synchrotron self-absorption that I mentioned is probably the oldest model, as I find the first reference to it
by Sturrock (1971).  But perhaps the Beskin explanation is the most commonly cited these days.

There is also a paper that I have been involved in (Loehmer et al. 2008), which concentrates on the
high-frequency part of the spectrum, but where we model the radio emission in general as a superposition
of short pulses. We derive the spectrum for the superposition of such pulses (Eq 3). This produces a 
flattening of the spectrum at low frequency, but not a real turn-over. Having said this, the spectrum’s 
formula has only three (physical) parameters (see end of section 5.2) and it may be interesting to
see if you can get a fit done. Perhaps, it is a combination of this plus the propagation effects mentioned
above.

I hope this is useful.

Cheers, Michael

> On 19 Aug 2019, at 17:14, Joeri van Leeuwen <leeuwen at astron.nl> wrote:
> 
> 
> Hi Anya, all,
> 
> Thanks for sending this! Some first comments: 
> 
> *) When an average astronomer reads
>  "Overall, the fraction of band that has been deleted due to
> dropped packages or RFIs is quite substantial (Fig. 3), ranging
> from few percent to almost the entire band. Deleted fraction
> varies considerably from beam to beam and is present in most
> observing runs, not showing a clear dependence on the observing
> date." 
> 
> They may wonder -- why not re-do it? So right there I would mention why this data is still interesting. Maybe add:
>  "
> While the data used here may not use LOFAR to its full capabilities, 
> and future and ongoing low-frequency observations (cf. $5.2) may reach higher signal-to-noise, 
> the results we present here are still the most sensitive obtained to date.
> "
> 
> *) It's not clear to me why is $3.2 is where it is. Is that pulsar so special? There are many other non detections. Perhaps give it some intro or move to "results" section.
> 
> *) I think the results section could be stronger if we derive some conclusions from the Fig. C plots. It seems to me that for all pulsars that previously had no 100 MHz flux, this flux is
>  1) below a single PL
>  2) to far below that is requires a multiple PL.
> I.e., evidence for a turn over around X MHz (~200?)
> Perhaps you can create an "ensemble" spectrum that makes this statistically stronger? 
> Because on a per pulsar basis the evidence is reasonably strong but not solid (as you write) while as a group this is clearly the behaviour.
> 
> Some, other, smaller, points:
> 
> *) "The last decade faced" 
>   "faced" to me suggests a hurdle, not a boon.
> -> 
> "The last decade brought" 
> 
> Very nice work, this is clearly a very large data undertaking. 
> 
> Cheers,
> Joeri
> 
> ==
> 
> 
> 
> On 16-8-2019 08:53, Anna Bilous wrote:
>> Dear all,
>> 
>> please find attached the draft of LBA census. Please let us (myself,
>> Louis, JM & Vlad) know if you have any comments on it. We would like
>> to submit it on Aug 30 due to Louis'es graduation constraints. Sorry
>> for the quite short notice, but the paper is short and very simple, so
>> it should not take long to read it.
>> 
>> Cheers,
>> Anya
>> 
>> 
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> 
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----------------------------------------------------------------------------------------------------------
Prof. Dr. Michael Kramer
Director - Max-Planck-Institut für Radioastronomie, Bonn, Germany
Professor for Astrophysics - University of Manchester, UK
Professor (Hon-Prof.) - University of Bonn, Germany

Address:   MPI fuer Radioastronomie
                 Auf dem Huegel 69
                 53121 Bonn, Germany

Phone: +49-228-525-278 (direct)
           +49-228-525-299 (secretary)

EMAIL:  mkramer at mpifr-bonn.mpg.de / michael.kramer at manchester.ac.uk
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