NuSTAR discovery of a luminosity dependent cyclotron line energy in Vela X-1
- Cahill Center for Astronomy and Astrophysics, California Institute of Technology, Pasadena, CA 91125 (United States)
- Center for Space Science and Technology, University of Maryland Baltimore County, Baltimore, MD 21250 (United States)
- Dr. Karl-Remeis-Sternwarte and ECAP, Sternwartstr. 7, D-96049 Bamberg (Germany)
- Space Sciences Laboratory, University of California, Berkeley, CA 94720 (United States)
- Université de Toulouse, UPS-OMP, IRAP, Toulouse (France)
- DTU Space, National Space Institute, Technical University of Denmark, Elektrovej 327, DK-2800 Lyngby (Denmark)
- Columbia Astrophysics Laboratory, Columbia University, New York, NY 10027 (United States)
- Department of Astronomy, The University of Michigan, Ann Arbor, MI 48109 (United States)
- Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109 (United States)
- NASA Goddard Space Flight Center, Astrophysics Science Division, Code 662, Greenbelt, MD 20771 (United States)
We present NuSTAR observations of Vela X-1, a persistent, yet highly variable, neutron star high-mass X-ray binary (HMXB). Two observations were taken at similar orbital phases but separated by nearly a year. They show very different 3-79 keV flux levels as well as strong variability during each observation, covering almost one order of magnitude in flux. These observations allow, for the first time ever, investigations on kilo-second time-scales of how the centroid energies of cyclotron resonant scattering features (CRSFs) depend on flux for a persistent HMXB. We find that the line energy of the harmonic CRSF is correlated with flux, as expected in the sub-critical accretion regime. We argue that Vela X-1 has a very narrow accretion column with a radius of around 0.4 km that sustains a Coulomb interaction dominated shock at the observed luminosities of L {sub x} ∼ 3 × 10{sup 36} erg s{sup –1}. Besides the prominent harmonic line at 55 keV the fundamental line around 25 keV is clearly detected. We find that the strengths of the two CRSFs are anti-correlated, which we explain by photon spawning. This anti-correlation is a possible explanation for the debate about the existence of the fundamental line. The ratio of the line energies is variable with time and deviates significantly from 2.0, also a possible consequence of photon spawning, which changes the shape of the line. During the second observation, Vela X-1 showed a short off-state in which the power-law softened and a cut-off was no longer measurable. It is likely that the source switched to a different accretion regime at these low mass accretion rates, explaining the drastic change in spectral shape.
- OSTI ID:
- 22348249
- Journal Information:
- Astrophysical Journal, Vol. 780, Issue 2; Other Information: Country of input: International Atomic Energy Agency (IAEA); ISSN 0004-637X
- Country of Publication:
- United States
- Language:
- English
Similar Records
Observing the onset of the accretion wake in Vela X-1
3C 273 with NuSTAR: Unveiling the active galactic nucleus