Chandra Constraints on the Current Low-Flux State of NGC7793 P13

ATel #13791; Dominic Walton (IoA, Cambridge), Felix Furst (ESAC), Marianne Heida (ESO), Matteo Bachetti (INAF-OA Cagliari), Murray Brightman (Caltech), Hannah Earnshaw (Caltech), on behalf of a larger team
on 8 Jun 2020; 22:01 UT
Credential Certification: Dominic Walton (dwalton@ast.cam.ac.uk)

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The ultraluminous X-ray pulsar (ULXp) NGC7793 P13 has recently entered a low-flux regime, as noted by ATEL #13751 (see also ATEL #13753); P13 has been largely undetected by the ongoing monitoring with the XRT on board the Neil Gehrels Swift Observatory (Gehrels et al. 2004, ApJ, 611, 1005) since it emerged from sun-constraint in late April (note that the latest XRT observation on 2020-06-08 now shows a weak detection with ~6 counts). As noted in those previous postings, the last time P13 exhibited a low-state was in 2011/12 (Motch et al. 2014, Nat., 514, 198).

We recently obtained a DDT observation of P13 with Chandra (Weisskopf et al. 2002, PASP, 114, 1) in order to determine its current flux and test the point-like nature of its emission in this regime (extended X-ray emission has recently been seen around NGC5907 ULX1, another ULXp, during one of its low-flux states; Belfiore et al. 2020, Nat. Ast., 4, 147). The observation was performed with the ACIS-S detector from 2020-06-04T16:40:40 to 2020-06-05T01:33:24 (UTC), with ~30ks of good exposure. P13 is clearly detected, with 401 counts registered within a 2.5" aperture. A preliminary analysis of these data is described below.

The 0.3-8 keV spectrum, binned to 1 count per bin and fit by reducing the Cash statistic (Cash 1979, ApJ, 228, 939), is well described with a simple absorbed powerlaw model (Cstat = 198 for 228 degrees of freedom). We include neutral absorption from both our own Galaxy (Nh,Gal = 1e20 at/cm^2; HI4PI collaboration, 2016, A&A, 594, A116) and local to P13 (for which we assume Nh = 8e20 at/cm^2 based on our previous spectral analysis; Walton et al. 2018, MNRAS, 473, 360), assuming that both have solar abundances. The best-fit photon index in the 0.3-8 keV Chandra band is Gamma = 1.17 +/- 0.15, and the observed 0.3-10 keV flux is Fx = (2.7 +/- 0.3)e-13 erg/cm^2/s (including a slight extrapolation outside of the Chandra band for comparison consistency with previous work). The 0.3-10 keV luminosity implied is Lx = (4.1 +/- 0.5)e38 erg/s (assuming isotropic emission; the distance to NGC7793 is 3.5 Mpc, Pietrzynski et al. 2010, AJ, 140, 1475).

The photon index is consistent with that seen from P13 in its 2011/12 low state (Gamma = 0.7 +0.4 -0.5), but the source was found to be significantly brighter in this latest Chandra observation; the 2011/12 data show luminosities of Lx ~ 4-8e37 erg/s, up to an order of magnitude fainter. We do not find any evidence for an additional T ~ 1 keV thermal plasma in these data, as seen in the 2011/12 low-state data (Walton et al. 2018, MNRAS, 473, 4360), and also reported during its decline to the current low-flux state (ATEL #13751). However, the limits on such emission are consistent with the plasma flux seen in the 2011/12 data.

Investigating the extent of the source flux, we find that a basic evaluation of the encircled energy as a function of radius is consistent with P13 being a point source at the Chandra resolution, even at these fairly low luminosities. Any extended emission is therefore either too compact or too faint to be detected above the PSF of the central point source. However, since P13 was significantly brighter in this latest observation than the 2011/12 data, it may be that such emission could be detectable with on-axis Chandra observations targeting even lower fluxes in the future (unfortunately the 2011 Chandra data were taken with P13 off-axis).

The latest Chandra flux for P13 is significantly brighter than the upper limits placed by the XRT prior to this observation, which imply Lx < 5e37 erg/s (ATEL #13751), i.e. similar to the 2011/12 data. It may be that this low-state was only short-lived in comparison to the 2011/12 event, and P13 is already returning to its typical ULX state.

We thank the Chandra director/team for approving and executing this DDT observation.