Further radio pulsations from the direction of the NuSTAR 3.76-second X-ray pulsar, and a dispersion measure estimate.

ATel #5043; Ralph Eatough (Max-Planck-Institut fuer Radioastronomie: MPIfR), Ramesh Karuppusamy (MPIfR), Michael Kramer (MPIfR), Bernd Klein (MPIfR), David Champion (MPIfR), Evan Keane (Jodrell Bank Centre for Astrophysics; JBCA), Alex Kraus (MPIfR), Cees Bassa (JBCA), Andrew Lyne (JBCA), Patrick Lazarus (MPIfR), Joris Verbiest (MPIfR), Paulo Freire (MPIfR), Andreas Brunthaler (MPIfR), Heino Falcke (ASTRON, Nijmegen), Laura Spitler (MPIfR), Ben Stappers (JBCA).
on 4 May 2013; 08:27 UT
Credential Certification: Evan Keane (ekean@jb.man.ac.uk)

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During continued efforts to detect radio pulsations from the 3.76- second X-ray pulsar discovered with the NuSTAR telescope (ATels #5020, #5027, #5033, #5035, #5040), we have performed a further two observations with the Max Planck Institute for Radio Astronomy (MPIfR) Effelsberg radio telescope at frequencies of 4.85 GHz and 8.35 GHz (observations started 2013-05-04 at 01:11:20 UTC and lasted for ~ 1hr each). Observations were performed at these frequencies to mitigate the expected strong scattering of radio waves in the Galactic Centre region (scattering time ~ freq.^-4). The observational hardware used is as described in earlier Atels #5027, #5040.

At 8.35 GHz we re-detect a pulse as described in Atel #5040, with a lower S/N, as expected from the reduced integration length. At this frequency a further analysis using sub-banded data of the Atel #5040 observations has resulted in a DM estimate of ~1300 +- 200 pc cm^-3. We also note, small changes (larger than the measurement uncertainties) in the spin period between the NuStar detection and our observations, however further observations are required to constrain this further.

At 4.85 GHz a clear detection is now made (S/N ~ 52, duty cycle ~ 2%), unlike our earlier attempts at this frequency only one week ago (Atel #5027), suggesting large flux variations. There also appears to be a precursor pulse just before the main pulse, suggesting possible pulse shape evolution with frequency. At 4.85 GHz dispersive delays are non-negligible allowing a DM of ~1000 +- 600 pc cm^-3 to be measured with the psrchive software (see diagnostic plots in links provided), which is in good agreement with our estimate at 8.35 GHz.

Assuming typical contributions to the system temperature from spillover, we estimate the flux density at 8.35 GHz to be ~0.2 mJy, at 4.85 GHz ~0.1 mJy.

In order to rule out terrestrial RFI as the source of these signals, azimuth tracking was disabled for the last ~ 6 minutes of each observation; the marker of a true celestial source being a reduction in S/N as the object drifts out of the telescope beam. This is the case in both observations (see subintegration plots), strongly suggesting an astrophysical origin.

With these latest results we now believe the DM of this pulsar is large (>1000 pc cm^-3). The high DM suggests observations at lower frequencies could suffer from pulse scattering (and a reduction in S/N), which is consistent with the non-detections at lower frequencies (#Atel 5033). We note however, there is no measured increase in the pulse width between the two frequencies reported here, and therefore the non-detections (#Atel 5033) could also be related to the intrinsic spectrum.

Such a large dispersion measure would place this source in the Galactic Centre region, although it could also be not in the immediate vicinity of Sgr A*; PSR J1746-2849 ~10 arcmin from Sgr A* has a DM of 1456 pc cm^-3, Deneva et al., ApJ, 2009). We are continuing to work on constraining this further.

Diagnostic plots:

www.jb.man.ac.uk/~ekean/nustar/nustar_8GHz.pdf

www.jb.man.ac.uk/~ekean/nustar/nustar_aztrackoff.pdf

www.jb.man.ac.uk/~ekean/nustar/nustar_4.85GHz.pdf