This is a White Paper in support of the mission concept of the Large Observatory for X-ray Timing (LOFT), proposed as a medium-sized ESA mission. We discuss the potential of LOFT for the study of thermonuclear X-ray bursts on accreting neutron stars. For a summary, we refer to the paper.
When a thermonuclear X-ray burst ignites on an accreting neutron star, the accretion disk undergoes sudden strong X-ray illumination, which can drive a range of processes in the disk. Observations of superbursts, with durations of several hours, prov
ide the best opportunity to study these processes and to probe accretion physics. Using detailed models of ionized reflection, we perform time resolved spectroscopy of the superburst observed from 4U 1636-536 in 2001 with RXTE. The spectra are consistent with a blackbody reflecting off a photoionized accretion disk, with the ionization state dropping with time. The evolution of the reflection fraction indicates that the initial reflection occurs from a part of the disk at larger radius, subsequently transitioning to reflection from an inner region of the disk. Even though this superburst did not reach the Eddington limit, we find that a strong local absorber develops during the superburst. Including this event, only two superbursts have been observed by an instrument with sufficient collecting area to allow for this analysis. It highlights the exciting opportunity for future X-ray observatories to investigate the processes in accretion disks when illuminated by superbursts.
Broad-line radio galaxies (BLRGs) are active galactic nuclei that produce powerful, large-scale radio jets, but appear as Seyfert 1 galaxies in their optical spectra. In the X-ray band, BLRGs also appear like Seyfert galaxies, but with flatter spectr
a and weaker reflection features. One explanation for these properties is that the X-ray continuum is diluted by emission from the jet. Here, we present two NuSTAR observations of the BLRG 3C 382 that show clear evidence that the continuum of this source is dominated by thermal Comptonization, as in Seyfert 1 galaxies. The two observations were separated by over a year and found 3C 382 in different states separated by a factor of 1.7 in flux. The lower flux spectrum has a photon-index of $Gamma=1.68^{+0.03}_{-0.02}$, while the photon-index of the higher flux spectrum is $Gamma=1.78^{+0.02}_{-0.03}$. Thermal and anisotropic Comptonization models provide an excellent fit to both spectra and show that the coronal plasma cooled from $kT_e=330pm 30$ keV in the low flux data to $231^{+50}_{-88}$ keV in the high flux observation. This cooling behavior is typical of Comptonizing corona in Seyfert galaxies and is distinct from the variations observed in jet-dominated sources. In the high flux observation, simultaneous Swift data are leveraged to obtain a broadband spectral energy distribution and indicates that the corona intercepts $sim 10$% of the optical and ultraviolet emitting accretion disk. 3C 382 exhibits very weak reflection features, with no detectable relativistic Fe K$alpha$ line, that may be best explained by an outflowing corona combined with an ionized inner accretion disk.
Recent studies have shown that runaway thermonuclear burning of material accreted onto neutron stars, i.e. Type I X-ray bursts, may affect the accretion disk. We investigate this by performing a detailed time-resolved spectral analysis of the superbu
rst from 4U 1636-536 observed in 2001 with the Rossi X-ray Timing Explorer. Superbursts are attributed to the thermonuclear burning of carbon, and are approximately 1000 times more energetic than the regular short Type I bursts. This allows us to study detailed spectra for over 11 ks, compared to at most 100 s for regular bursts. A feature is present in the superburst spectra around 6.4 keV that is well fit with an emission line and an absorption edge, suggestive of reflection of the superburst off the accretion disk. The line and edge parameters evolve over time: the edge energy decreases from 9.4 keV at the peak to 8.1 keV in the tail, and both features become weaker in the tail. This is only the second superburst for which this has been detected, and shows that this behavior is present even without strong radius expansion. Furthermore, we find the persistent flux to almost double during the superburst, and return to the pre-superburst level in the tail. The combination of reflection features and increased persistent emission indicates that the superburst had a strong impact on the inner accretion disk, and it emphasizes that X-ray bursts provide a unique probe of accretion physics.
Finding and characterizing the population of active galactic nuclei (AGNs) that produces the X-ray background (XRB) is necessary to connect the history of accretion to observations of galaxy evolution at longer wavelengths. The year 2012 will see the
deployment of the first hard X-ray imaging telescope that, through deep extragalactic surveys, will be able to measure the AGN population at the energies where the XRB peaks (~20-30 keV). Here, we present predictions of AGN number counts in three hard X-ray bandpasses: 6-10 keV, 10-30 keV and 30-60 keV. Separate predictions are presented for the number counts of Compton thick AGNs, the most heavily obscured active galaxies. The number counts are calculated for five different models of the XRB that differ in the assumed hard X-ray luminosity function, the evolution of the Compton thick AGNs, and the underlying AGN spectral model. The majority of the hard X-ray number counts will be Compton thin AGNs, but there is a >10x increase in the Compton thick number counts from the 6-10 keV to the 10-30 keV band. The Compton thick population show enough variation that a hard X-ray number counts measurement will constrain the models. The computed number counts are used to consider various survey strategies for the NuSTAR mission, assuming a total exposure time of 6.2 Ms. We find that multiple surveys will allow a measurement of Compton thick evolution. The predictions presented here should be useful for all future imaging hard X-ray missions.
