Open AccessMultiband study of NGC 7424 and its two newly discovered ultraluminous X‐ray sources
Author(s) -
Soria R.,
Kuncic Z.,
Broderick J. W.,
Ryder S. D.
Publication year - 2006
Publication title -
monthly notices of the royal astronomical society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.058
H-Index - 383
eISSN - 1365-2966
pISSN - 0035-8711
DOI - 10.1111/j.1365-2966.2006.10629.x
Subject(s) - physics , astrophysics , astronomy , spectral index , luminosity , supernova remnant , galaxy , pulsar , spiral galaxy , stars , supernova , spectral line
We have studied the face‐on, barred spiral NGC 7424 (site of the rare Type IIb SN 2001ig) with Chandra , Gemini and the Australia Telescope Compact Array. After giving revised X‐ray colours and luminosity of the supernova, here we focus on some other interesting sources in the galaxy: in particular, our serendipitous discovery of two ultraluminous X‐ray sources (ULXs). The brighter one (∼10 40 erg s −1 ) has a power‐law‐like spectrum with photon index Γ≈ 1.8 . The other ULX shows a spectral state transition or outburst between the two Chandra observations, 20‐d apart. Optical data show that this ULX is located in a young (age ≈7–10 Myr), bright complex rich with OB stars and clusters. An exceptionally bright, unresolved radio source (0.14 mJy at 4.79 GHz, implying a radio luminosity twice as high as Cas A) is found slightly offset from the ULX (≈80 pc). Its radio spectral index α≈−0.7 suggests optically thin synchrotron emission, either from a young supernova remnant or from a radio lobe powered by a ULX jet. An even brighter, unresolved radio source (0.22 mJy at 4.79 GHz) is found in another young, massive stellar complex, not associated with any X‐ray sources: based on its flatter radio spectral index (α≈−0.3) , we suggest that it is a young pulsar wind nebula, a factor of 10 more radio luminous than the Crab.