Discovery of accretion-driven pulsations in the prolonged low X-ray luminosity state of the Be/X-ray transient GX 304-1
Abstract
We present our Swift monitoring campaign of the slowly rotating neutron star Be/X-ray transient GX 304-1 (spin period of ∼275 s) when the source was not in outburst. We found that between its type I outbursts, the source recurrently exhibits a slowly decaying low-luminosity state (with luminosities of 1034 - 35 erg s-1). This behaviour is very similar to what has been observed for another slowly rotating system, GRO J1008-57. For that source, this low-luminosity state has been explained in terms of accretion from a non-ionised ("cold") accretion disc. Because of the many similarities between the two systems, we suggest that GX 304-1 enters a similar accretion regime between its outbursts. The outburst activity of GX 304-1 ceased in 2016. Our continued monitoring campaign shows that the source is in a quasi-stable low-luminosity state (with luminosities a few factors lower than previously seen) for at least one year now. Using our NuSTAR observation in this state, we found pulsations at the spin period, demonstrating that the X-ray emission is due to accretion of matter onto the neutron star surface. If the accretion geometry during this quasi-stable state is the same as during the cold-disc state, then matter indeed reaches the surface (as predicted) during this later state. We discuss our results in the context of the cold-disc accretion model.
- Publication:
-
Astronomy and Astrophysics
- Pub Date:
- December 2018
- DOI:
- 10.1051/0004-6361/201834572
- arXiv:
- arXiv:1811.01453
- Bibcode:
- 2018A&A...620L..13R
- Keywords:
-
- X-rays: binaries;
- stars: neutron;
- accretion;
- accretion disks;
- pulsars: individual: GX 304-1;
- Astrophysics - High Energy Astrophysical Phenomena
- E-Print:
- 5 pages, 2 figures, accepted for A&