[Lofarpwg] LOFAR LBA census draft -- ready for commenting

Fri Aug 30 15:38:15 UTC 2019

Hi Anya,

Thanks for sending the LBA paper around. It’s a nice summary of a lot of work, well done. Please find some minor comments below, which shouldn’t take too long to implement if you agree with them.

My main question is whether the ISM is also thought to possibly cause a low-frequency spectral turn-over? If so, is it worth plotting break frequencies against Galactic latitude, for example? Although this may be somewhat biased by the LBA detections…

Please just let me know if you have any questions.

Many thanks and best wishes,
Charlotte

--
Comments on Census_LBA_v_0.3

Introduction:

  *   However, because of instrumen- tal challenges, most pulsar observations ever since were con- ducted at higher frequencies of 300–2000MHz.  -> However, until recently most pulsar observations were conducted at higher frequencies of 300–3000MHz because of instrumental challenges.
{The Parkes 10/50 cm receiver for example observed at ~3100 MHz}

  *   Also, because of their profound impact on the received signal, ISM and ionosphere are best studied at lower frequencies. -> Also, because of their increased effects on the received signal, the ISM and the ionosphere can be more accurately studied at lower frequencies.
  *   The new generation of low-frequency radio telescopes al- ready started charting the meterwavelengh pulsar sky. Over the last years there have been conducted several surveys of the known pulsar population.  -> The new generation of low-frequency radio telescopes has already started charting the meterwavelengh pulsar sky. Several surveys of the known pulsar population have been conducted over the last few years.
  *   The first sta- tion of the Low Wavelength Array (LWA1) measured flux den- sity values of 44 pulsars at 30–88 MHz (Stovall et al. 2015) -> The first station of the Long Wavelength Array (LWA1) was used to measure the flux densities of 44 pulsars at 30–88 MHz (Stovall et al. 2015)
{Similar modification of sentences for the other telescopes}

  *   Overall, it appeared that the spectra of the majority of pulsars are not known very well and that there is a dire need of regular flux density measurements, as the fluxes can typically vary up to an order of magnitude. -> We demonstrated that the spectra of most pulsars are not well known. Also, regular flux density measurements are needed, as flux densities can typically vary up to an order of magnitude due to diffractive and refractive interstellar scintillation.

2. Source selection

  *   I think the details regarding the original plans could be shortened a bit, as it doesn’t describe the actual observations in this work.
  *   Perhaps reference Table B1 target list at the end too?
  *   made without proper RFI excision -> made without proper radio frequency interference (RFI) excision

3. Observations and data reduction

  *   Observations were pre-processed with the standard LO- FAR pulsar pipeline (Stappers et al. 2011), which uses the PSRCHIVE software package
 {and dspsr?}

  *   dedispersing and folding LBA data with these incorrect DMs was enough to bring a substantial smearing -> dedispersing and folding LBA data using incorrect DMs caused substantial pulse smearing
  *   These gaps appeared independently in two polarizations observed and led to significant decrease of overall signal-to-noise ratio
{It’s kind of interesting that the dropped packets didn’t result in both polarisations not being recorded. I think an early version of the raw data pipeline ensured that the same UDP packet number from X and Y was recorded, or if not it was not recorded? Also, were the packets for the same frequency/time not recorded for all 7 beams?  }

  *   is neces- sary to flatten the bandpass before cleaning RFIs  -> it is necessary to flatten the bandpass before cleaning RFI
  *   To remove RFIs from the

{unfinished sentence?, also RFIs -> RFI in several places}

  *   Cleaned archives had been visually inspected for residual RFI -> Archives that were automatically excised of RFI were also visually inspected for residual RFI
Fig. 2.

  *   dropped packed -> dropped packet
  *   statistic of the two polarizations -> statistics for the two polarizations (P1=X and P2=Y)

3.2. Flux calibration

  *   Our observing setup involved six beam in a circle surround- ing the central beam at pulsar coordinates. -> Our observing setup involved six beams surrounding the central beam formed towards the pulsars’ coordinates.
  *   beam, indicating possible refraction in Earth’s ionosphere due to difference in the electron column density (also known as “total electron content”, TEC) between line-of-sight of different antennas. -> beam. This is indicative of refraction in the Earth’s ionosphere due to differential total electron content (TEC) between the lines-of-sight of different LBA stations.

{Are the delta TEC numbers used reasonable? I also thought TEC was measured with m^-2? Perhaps Reinout’s Facet paper provides a good example of the TEC differences across the core stations:  https://ui.adsabs.harvard.edu/abs/2016ApJS..223....2V}

4.2. Results

  *   PSRs B0011+47, B0226+70, and B2022+50 did not exhibit any sign of low-frequency turnover or ostensible scatter- ing down to 40 MHz.
{Is there something interesting about these particular lines of sight, e.g., away from the Galactic plane and other small scale ISM structures?}

  *   For pulsars with smaller number of spectral points -> For pulsars with fewer spectral points
  *   For some of such sources -> For some sources,
  *   Reference Appendix C profiles and spectra somewhere in here too!

5. Discussion

  *   identifying more than a dozen of pulsars -> identifying more than a dozen pulsars
  *   where the spectral break is supposed to happen. -> where a spectral break is widely expected.
  *   Even more, the influence -> Furthermore, the influence
{Perhaps you could reference the LoTSS survey which could provide more accurate flux densities without scattering playing a role, e.g., https://ui.adsabs.harvard.edu/abs/2019A%26A...622A...1S/abstract }

  *   Future observations of pulsars below 100 MHz will allow ob- taining robust flux density measurements and constrain spectral break better. -> Future observations of pulsars below 100 MHz can provide more robust flux density measurements and better constrain spectral break(s).
  *   This will be reached, -> This will be achieved,

From: lofarpwg <lofarpwg-bounces at astron.nl> on behalf of Anna Bilous <hanna.bilous at gmail.com>
Date: Wednesday, 28 August 2019 at 10:32 pm
To: "lofarpwg at astron.nl" <lofarpwg at astron.nl>
Subject: Re: [Lofarpwg] LOFAR LBA census draft -- ready for commenting

Dear all,

if you have not read the draft yet (note that the deadline is August 30), here is the updated version. I have added a few sentences to the discussion following the input from Michael. However, I really do want to draw any physics-related conclusions (or do any theory matching) based on the sample of spectra in this paper, since most of the spectral breaks (or potential spectral breaks) there are subject to large, poorly known uncertainties. I think  it would be a very useful study to do (possibly with Nenufar, but LOFAR can be good here as well), but such things should be done carefully, or not done at all.

Below are some replies to individual comments.

Cheers,
Anya

Joeri:
>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."

I’m not sure about most sensitive, could be not the case for bright classical pulsars. Changed to
“While the data used here may not use LOFAR to its full capabilities, and future and ongoing low-frequency observations may reach higher signal-to-noise, the results we present here still provide useful infromation about low-frequency end of the pulsar spectra (see Section~\ref{subsec:results}).”

> *) 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.
It’s one of the two closest to Earth pulsars in our sample and the one with ATNF DMs being off by a factor of ~3 if compared to HBA census DMs. It has been independently detected at the latter DM with FR606, so ATNF DM is most probably wrong (or changed very rapidly in a few years), but still, it would have been nice to have LBA detection.

> *) 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.

I think there are several pulsars with LBA fluxes extending known spectrum while being on 1PL line, e.g. B0226+70 and others.

Patrick:

> Section 2: 2nd paragraph - Add a sentence at the end like "The consequence is that only 88 out of the 194 pulsars which are in the HBA census were observed and discussed here." At the moment the text doesn't quite explicitly states if only HBA pulsars were observed, or all pulsars HBA pointings are available for (without the pulsar necessarily being detected).

I modified the (now) last sentence in the paragraph to clarify that: “At present, with all HBA observations being processed and analysed (leading to substantial changes in some of S/N estimates), we can regard the LBA census source sample as being an arbitrary subsample of pulsars detected in HBA census, with some preference for closer/brighter sources. “

> We adjusted pulsar period P and DM with the PSRCHIVE pro-
gram pdmp, maximizing integrated S/N of the frequency- and
time-averaged profile over the set of trial values of P and DM. REMOVE INTEGRATED.
I’ll keep integrated (as opposed to peak) S/N.

> Section 3.2 - I agree with Joeri this should not be part of "Observations and data reduction"

I think it does belong to Section 3.1 “Detection and ephemerides update”

On Fri, Aug 16, 2019 at 8:53 AM Anna Bilous <hanna.bilous at gmail.com<mailto:hanna.bilous at gmail.com>> 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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