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A broadband look at the old and new ULXs of NGC 6946 | | Hannah P. Earnshaw
; Brian W. Grefenstette
; Murray Brightman
; Dominic J. Walton
; Didier Barret
; Felix Fürst
; Fiona A. Harrison
; Marianne Heida
; Sean N. Pike
; Daniel Stern
; Natalie A. Webb
; | | Date: |
9 May 2019 | | 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
$Gamma=3.6^{+0.4}_{-0.3}$ 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 $L_{
m X}=(6.5pm0.1) imes10^{39}$
erg s$^{-1}$, and has a power-law spectrum with $Gamma=2.51pm0.05$. 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
characterise the new source, ULX-4, which is only detected in the joint
XMM-Newton and NuSTAR observation, at $L_{
m X}=(2.27pm0.07) imes10^{39}$
erg s$^{-1}$, and is absent in a Chandra observation ten days later. It has a
very hard cut-off power-law spectrum with $Gamma=0.7pm0.1$ and $E_{
m
cut}=11^{+9}_{-4}$ 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. | | Source: | arXiv, 1905.3383 | | Services: | Forum | Review | PDF | Favorites | |
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