Pulsation searches

Note

For a general introduction to HENDRICS’s workflow, please read the Introductory concepts and example analysis tutorial

We have a pulsar observation with, e.g., NuSTAR and we want to find pulsations on it. The general procedure is looking for pulsations using a power density spectrum (see Introductory concepts and example analysis) or similar methods, and if we do find a promising candidate frequency, investigate more with the Epoch Folding or the Z search.

Let’s say we have found a peak in the power density spectrum at about 0.101 seconds, or 9.9 Hz, and we want to investigate more.

We start from the _event_ file. If we have run HENreadevents on the original mission-specific event file, we have a HENRICS-format event file (ending with _ev.nc or _ev.p), e.g.

$ ls
002A.evt 002A_ev.nc

To look for pulsations with the epoch folding around the candidate frequency 9.9 Hz, we can run HENefsearch as such:

$ HENefsearch -f 9.85 -F 9.95 -n 64 --fit-candidates

where the options -f and -F give the minimum and maximum frequencies to search, -n the number of bins in the pulsed profile and with --fit-candidates we specify to not limit the search to the epoch folding, but also look for peaks and fit them to find their centroids.

The output of the search is in a file ending with _EF.nc or _EF.p, while the best-fit model is recorded in pickle files

$ ls
    002A.evt 002A_ev.nc 002A_EF.nc 002A_EF__mod0__.p

To use the Z search, we can use the HENzsearch script with very similar options:

$ HENzsearch -f 9.85 -F 9.95 -N 2 --fit-candidates

where the -N option specifies the number of harmonics to use for the search.

The output of the search and the fit is recorded in similar files as Epoch folding

$ ls
    002A.evt 002A_ev.nc 002A_Z2n.nc 002A_Z2n__mod0__.p

We can plot the results of this search with HENplot, as such:

$ HENplot 002A_Z2n.nc

zn_search.png

NEW: Accelerated searches

HENDRICS now implements the accelerated search à la Ransom+2002: starting from a single FFT, we convolve it with responses corresponding to different values of “acceleration”, or spin-up/down. It is an extremely fast technique to find binary pulsars, albeit less sensitive than a focused Z search if the pulsations are known. The method is experimental, and detections should be assessed through simulations. Use with care.

Example with a NuSTAR observation of a famous X-ray binary pulsar:

$ HENaccelsearch mistery_psrA_nustar_fpma_ev.nc --fmin 0.01 --fmax 5
WARNING: The accelsearch functionality is experimental. Use with care, and feel free to report any issues. [hendrics.efsearch]
100%|██████████████████████████████████████████████████████████████████████████████████████████████████| 200/200 [00:00<00:00, 7416.52it/s]
100%|████████████████████████████████████████████████████████████████████████████████████████████████████| 200/200 [00:20<00:00,  9.63it/s]

Best candidates:

       time           frequency                fdot                power             pepoch
----------------- ------------------ ----------------------- ----------------- ------------------
81916106.03609267 2.8520477603166845  -5.024586630979715e-08 8771.116170366908 56145.103845139965
81916106.03609267  2.852015406199768  -4.815228854688893e-08 8194.657124664562 56145.103845139965
81916106.03609267  2.852080114433601  -5.233944407270536e-08 6016.665557776852 56145.103845139965
81916106.03609267  1.426007703099884 -2.4076144273444466e-08 5460.323423255257 56145.103845139965
81916106.03609267 2.8520477603166845 -4.9199077428343043e-08 5329.925664622505 56145.103845139965
81916106.03609267 1.4260400572168006  -2.616972203635268e-08 5142.912459419216 56145.103845139965
81916106.03609267 2.8519830520828515  -4.605871078398072e-08 4846.570816537878 56145.103845139965
81916106.03609267  2.852015406199768  -4.710549966543483e-08 4618.153190476991 56145.103845139965
81916106.03609267  2.852080114433601  -5.338623295415947e-08  4228.74089043784 56145.103845139965
81916106.03609267  2.852080114433601  -5.129265519125125e-08 3847.880659073288 56145.103845139965
See all 24740 candidates in mistery_psrA_nustar_fpma_accelsearch.csv

See HENaccelsearch -h for more details.

NEW: Fast Z2n searches

HENDRICS now implements a much faster, experimental algorithm for pulsation searches with the Z^2_n statistics. Select this algorithm with the --fast option on the command line of HENzsearch. Instead of calculating the phase of all photons at each trial value of frequency and derivative, we pre-bin the data in small chunks and shift the different chunks to the amount required by different trial values.

fast_zsearch.jpeg

Each pre-folding leads to a large number of trial values to be evaluated. This only works if we assume that the trial frequency is sufficiently close to the initial one that no signal leaks into nearby bins inside the sub-profiles. This requires that we choose a sufficiently large number of sub-profiles, and limit the total shift to reasonable values to limit this leak.

Given the wanted range of frequencies to search, the program chooses automatically the number of trial frequencies and fdots to derive from each given pre-folding, and when to perform a new pre-folding.

At the moment, the trial fdots are chosen automatically and cannot be defined by the user. The only actions the user can do are the selection of the mean fdot and the parameter --npfact that increases the number of trial values to obtain from a single central frequency/fdot combination (npfact=2 means that the number of trial values will be double for both the frequency and the fdot, so four times the trials in the end).

The results of this Z search can be plotted with HENplot. There is at the moment no automatic fitting being performed as in the slow option.

Measuring frequency derivatives interactively

HENphaseogram is an interactive phaseogram to adjust the values of the frequency and frequency derivatives of pulsars.

phaseogram.jpeg