During last week, NuSTAR performed an observation of the intermediate mass X-ray binary Her X-1 together with ESA’s INTErnational Gamma-ray Astrophysics Laboratory (INTEGRAL). Her X-1 is one of the first celestial emitters of X-rays discovered about half a century ago and it is formed by a neutron star stealing the outer atmosphere of its companion star. Neutron stars are the outcome of stellar evolution of massive stars and enclose the mass of our Sun in a radius of 10 km. They are so dense that any further compression would lead to the formation of a black-hole. Owing to the immense gravitational field, the matter accreted onto them is accelerated at half the speed of light and when it crashes onto their surface, it emits high energy X-rays which are optimally detected by NuSTAR. Her X-1 was the first source in which a peculiar signature on its energy-dependent emission, a cyclotron spectral line, was detected, leading to the direct measurement of an immense magnetic field, trillions of times larger than the Sun's. Measuring the energy of this spectral line over the years has led to the discovery that the surface magnetic field of a neutron star accreting matter is not stable: it shows jumps, trends that are complicating scientists’ understanding of these exotic objects and require regular monitoring. NuSTAR and INTEGRAL have been instrumental in performing these measurements with their long life and ideal sensitivity in the 20-70 keV energy range. Moreover, in this late phase of the INTEGRAL mission, it has become fundamental for calibration of the energy scale of the INTEGRAL-ISGRI detector after being exposed to cosmic-ray radiation for so long. For this purpose, a spectral line at 35 keV is an ideal benchmark. Last week’s observation was one of the final occasions for INTEGRAL to cross-calibrate with NuSTAR, as INTEGRAL will be decommissioned on 4 March 2025 after more than 23 years of operations, during which INTEGRAL has studied the X- and gamma-ray sky with unprecedented sensitivity.

Also over the past week, NuSTAR conducted multiple observations of the Seyfert-1 galaxy NGC 5548, coordinated with ESA’s XMM-Newton and NASA’s HST observatories, to investigate the long-term evolution of this active galactic nucleus (AGN)’s obscuring winds and their impact on the surrounding environment. Thanks to its high-energy coverage, NuSTAR enables a measurement of the parameters of the hard X-ray continuum, which is critical for disentangling the intrinsic X-ray emission from the effects of absorption by the intervening outflows. XMM-Newton, with its Reflection Grating Spectrometer instrument, provides high-resolution X-ray spectra at lower energies, allowing analysis of the properties and evolution of the ionized outflows. Meanwhile, HST’s Cosmic Origins Spectrograph captures ultraviolet absorption features, offering insights into the changes in the speed and ionization of these outflows. Combining all of these multi-wavelength observations provides the information needed to investigate how the obscuration has evolved over time, its influence on the surrounding environment, and whether the obscuring winds and ionized outflows have a common origin. This multi-wavelength approach is useful for gaining additional insight into the mechanisms that drive AGN winds and ultimately better understanding the role of black hole-driven winds in shaping their host galaxies.

Authors: Karl Forster (NuSTAR Science Operations Lead, Caltech), Hannah Earnshaw (NuSTAR Project Scientist, Caltech), Carlo Ferrigno (Senior Scientist, Universite de Geneve and PI of INTEGRAL science data centre), Missagh Mehdipour (postdoctoral researcher, STScI)