Upper limits on the pulsed fraction of the ULX pulsar NGC 7793 P13 during its recent recovery

ATel #13996; Felix Fuerst (ESAC/ESA), Dominic Walton (IoA Cambridge), Matteo Bachetti (INAF/Cagliari; Caltech)
on 8 Sep 2020; 12:00 UT
Credential Certification: Felix Fuerst (felix.fuerst@sciops.esa.int)

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NGC 7793 P13 is one of the best studied ultra-luminous X-ray pulsars (ULXPs), thanks to its high flux and typically strong pulsations. It recently showed renewed activity after having been in an off-state for most of 2020 (see ATels #13791, #13753, #13751). Following up on our Chandra observation as reported in ATel #13791 we obtained an XMM-Newton observation on 2020-06-27 (ObsID 0861600101) and a NuSTAR observation on 2020-08-22 (ObsID 90601327002). This renewed activity seems to mark the end of the off-state which lasted only a few months, much shorter than the previous observed off-state in 2011-2012, which had a duration for ~2 years. Currently the source is clearly detected in ongoing Swift/XRT monitoring, albeit at flux levels much lower than during the bright state. With XMM-Newton we obtained a good exposure of 56 ks with the EPIC-pn camera, detecting a total of 2373 counts, implying a flux of (9.5+/-1.3)e-14 erg/s/cm^2 in the 3-10keV energy bandpass and (1.57+/-0.13)e-13 erg/s/cm^2 in the 0.3-10keV band. We could not identify any pulsations around the expected period of 412ms, searching in a range between P=410-415ms and \dot P = -5e-9--5e-9 s/s, using an accelerated epochfolding search. We performed simulations using HENDRICS (Bachetti, 2018, ascl:1805.019) by faking pulsations with varying pulsed fraction on the same sampling as the real data and can place an upper limit of a pulsed fraction of 40% on any pulsations, at the 95% confidence level. The NuSTAR observation yielded a good exposure of 51 ks with a detection of 1067 photons in both modules combined, which implies a flux of (3.6+/-0.5)e-13 erg/s/cm^2. Also in these observations we could not find significant pulsations over the same search range as used for the XMM-Newton data, and can place an upper limit on the pulsed fraction at around 45%, based on simulations. Previous NuSTAR observations at similar flux levels taken in late 2019 showed pulsed fractions of >60%. The new observations rule out pulsations at this strength, indicating a change in the emission geometry of the source (Fuerst et al., in prep.). A study of the spectral hardness shows the source seems to have considerably softened while recovering from the off-state, when compared to data taken during the high state between 2016--2019. This might be in agreement with a change of accretion geometry, however, detailed spectral modeling is necessary to investigate this possibility further (Walton et al. in prep.). Swift/XRT monitoring of the source is ongoing and we encourage further observations, in particular in the optical/UV to investigate the large scale structure of the system and search for any change in the companion. We thank the XMM-Newton team and project scientist, as well as the NuSTAR team and PI for approving and executing our DDT observations so quickly and flawlessly.