A broadband look at the old and new ULXs of NGC 6946
Abstract
Two recent observations of the nearby galaxy NGC 6946 with NuSTAR, one simultaneous with an XMM-Newton observation, provide an opportunity to examine its population of bright accreting sources from a broadband perspective. We study the three known ultraluminous X-ray sources (ULXs) in the galaxy, and find that ULX-1 and ULX-2 have very steep power-law spectra with Γ=3.6^(+0.4)_(−0.3) in both cases. Their properties are consistent with being super-Eddington accreting sources with the majority of their hard emission obscured and down-scattered. ULX-3 (NGC 6946 X-1) is significantly detected by both XMM-Newton and NuSTAR at L_X = (6.5 ± 0.1) × 10^(39) erg s^(−1), and has a power-law spectrum with Γ = 2.51 ± 0.05. We are unable to identify a high-energy break in its spectrum like that found in other ULXs, but the soft spectrum likely hinders our ability to detect one. We also characterize the new source, ULX-4, which is only detected in the joint XMM-Newton and NuSTAR observation, at L X = (2.27 ± 0.07) × 10^(39) erg s^(−1), and is absent in a Chandra observation 10 days later. It has a very hard cutoff power-law spectrum with Γ = 0.7 ± 0.1 and E_(cut)=11^(+9)_(−4) keV. We do not detect pulsations from ULX-4, but its transient nature can be explained either as a neutron star ULX briefly leaving the propeller regime or as a micro-tidal disruption event induced by a stellar-mass compact object.
Additional Information
© 2019 The American Astronomical Society. Received 2019 April 5; revised 2019 May 7; accepted 2019 May 8; published 2019 August 9. We thank our anonymous referee for useful comments on this paper. This work was supported under NASA contract NNG08FD60C. D.J.W. acknowledges financial support from STFC in the form of an Ernest Rutherford fellowship. This work made use of data from the NuSTAR mission, a project led by the California Institute of Technology, managed by the Jet Propulsion Laboratory, and funded by the National Aeronautics and Space Administration. This work has also made use of observations by XMM-Newton, an ESA science mission with instruments and contributions directly funded by ESA Member States and NASA. Results reported in this article are based in part on public data obtained from the Chandra and Swift data archives, and on observations made with the NASA/ESA Hubble Space Telescope, obtained from the Data Archive at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. Facilities: NuSTAR - The NuSTAR (Nuclear Spectroscopic Telescope Array) mission, XMM - , CXO - , Swift(XRT) - , HST. - Software: astropy (Astropy Collaboration et al. 2013, 2018), CIAO (Fruscione et al. 2006), HENDRICS (Bachetti 2015), HEASoft (NASA's High Energy Astrophysics Science Archive Research Center (HEASARC), 2014), NuSTARDAS, XMM-Newton SAS.Attached Files
Published - Earnshaw_2019_ApJ_881_38.pdf
Submitted - 1905.03383.pdf
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Additional details
- Eprint ID
- 96507
- Resolver ID
- CaltechAUTHORS:20190618-153955252
- NNG08FD60C
- NASA
- Science and Technology Facilities Council (STFC)
- NASA/JPL/Caltech
- ESA Member States
- NAS 5-26555
- NASA
- Created
-
2019-06-18Created from EPrint's datestamp field
- Updated
-
2021-11-16Created from EPrint's last_modified field
- Caltech groups
- NuSTAR, Space Radiation Laboratory, Astronomy Department