A redshifted Fe Kα\alpha line from the unusual gamma-ray source PMN J1603-4904

Multiwavelength observations have revealed the highly unusual properties of the gamma-ray source PMN J1603-4904, which are difficult to reconcile with any other well established gamma-ray source class. The object is either a very atypical blazar or compact jet source seen at a larger angle to the line of sight. In order to determine the physical origin of the high-energy emission processes in PMN J1603-4904, we study the X-ray spectrum in detail. We performed quasi-simultaneous X-ray observations with XMM-Newton and Suzaku in 2013 September, resulting in the first high signal-to-noise X-ray spectrum of this source. The 2-10 keV X-ray spectrum can be well described by an absorbed power law with an emission line at 5.44$\pm$0.05 keV (observed frame). Interpreting this feature as a K{\alpha} line from neutral iron, we determine the redshift of PMN J1603-4904 to be z=0.18$\pm$0.01, corresponding to a luminosity distance of 872$\pm$54 Mpc. The detection of a redshifted X-ray emission line further challenges the original BL Lac classification of PMN J1603-4904. This result suggests that the source is observed at a larger angle to the line of sight than expected for blazars, and thus the source would add to the elusive class of gamma-ray loud misaligned-jet objects, possibly a {\gamma}-ray bright young radio galaxy.Comment: 5 pages, 1 figure, A&A accepte

Cyclotron resonant scattering feature simulations. I. Thermally averaged cyclotron scattering cross sections, mean free photon-path tables, and electron momentum sampling

Electron cyclotron resonant scattering features (CRSFs) are observed as absorption-like lines in the spectra of X-ray pulsars. A significant fraction of the computing time for Monte Carlo simulations of these quantum mechanical features is spent on the calculation of the mean free path for each individual photon before scattering, since it involves a complex numerical integration over the scattering cross section and the (thermal) velocity distribution of the scattering electrons. We aim to numerically calculate interpolation tables which can be used in CRSF simulations to sample the mean free path of the scattering photon and the momentum of the scattering electron. The tables also contain all the information required for sampling the scattering electron's final spin. The tables were calculated using an adaptive Simpson integration scheme. The energy and angle grids were refined until a prescribed accuracy is reached. The tables are used by our simulation code to produce artificial CRSF spectra. The electron momenta sampled during these simulations were analyzed and justified using theoretically determined boundaries. We present a complete set of tables suited for mean free path calculations of Monte Carlo simulations of the cyclotron scattering process for conditions expected in typical X-ray pulsar accretion columns (0.01<B/B_{crit}<=0.12, where B_{crit}=4.413x10^{13} G and 3keV<=kT<15keV). The sampling of the tables is chosen such that the results have an estimated relative error of at most 1/15 for all points in the grid. The tables are available online at http://www.sternwarte.uni-erlangen.de/research/cyclo.Comment: A&A, in pres

NuSTAR observation of GRO J1744-28 at low mass accretion rate

We present the spectral analysis of the LMXB GRO J1744-28 using $\sim$29 ks of NuSTAR data taken in 2017 February at a low luminosity of $3.2\times 10^{36}$ erg/s (3-50 keV). The continuum spectrum is modeled with an absorbed power-law with exponential cut-off, and an additional iron line component. We find no obvious indications for a CRSF and therefore perform a detailed cyclotron line search using statistical methods on the pulse phase-averaged as well as phase-resolved spectra. The previously detected Type II X-ray bursts are absent. Clear pulsations at a period of 2.141124(9) Hz are detected. The pulse profile shows an indication of a secondary peak, which was not seen at higher flux. The 4$\sigma$ upper limit for the strength of a CRSF in the 3-20 keV band is 0.07 keV, lower than the strength of the line found at higher luminosity. The detection of pulsations shows that the source did not enter the "propeller" regime, even though the source flux of $4.15\times 10^{-10}$ erg/cm$^{2}$/s was almost one order of magnitude below the threshold for the propeller regime claimed in previous studies on this source. The transition into the propeller regime in GRO J1744-28 must therefore be below a luminosity of $3.2\times 10^{36}$ erg/s, which implies a surface magnetic field $\lesssim 2.9\times 10^{11}$ G and mass accretion rate $\lesssim 1.7\times 10^{16}$ g/s. A change of the CRSF depth as function of luminosity is not unexpected and has been observed in other sources. This result possibly implies a change in emission geometry as function of mass accretion rate to reduce the depth of the line below our detection limit

NuSTAR Observations of the Accreting Atolls GX 3+1, 4U 1702-429, 4U 0614+091, and 4U 1746-371

Atoll sources are accreting neutron star (NS) low-mass X-ray binaries. We present a spectral analysis of four persistent atoll sources (GX 3+1, 4U 1702−429, 4U 0614+091, and 4U 1746−371) observed for ~20 ks each with NuSTAR to determine the extent of the inner accretion disk. These sources range from an apparent luminosity of 0.006–0.11 of the Eddington limit (assuming the empirical limit of 3.8 × 10^(38) erg s^(−1)). Broad Fe emission features shaped by Doppler and relativistic effects close to the NS were firmly detected in three of these sources. The position of the disk appears to be close to the innermost stable circular orbit (ISCO) in each case. For GX 3+1, we determine R_(in) = 1.8^(+0.2)_(−0.6) R_(ISCO) (90% confidence level) and an inclination of 27°–31°. For 4U 1702−429, we find a R_(in) = 1.5^(+1.6)_(−0.4) R_(ISCO) and inclination of 53°–64°. For 4U 0614+091, the disk has a position of R_(in) = 1.3^(+5.4)_(−0.2) R_(ISCO) and inclination of 50°–62°. If the disk does not extend to the innermost stable circular orbit, we can place conservative limits on the magnetic field strength in these systems in the event that the disk is truncated at the Alfvén radius. This provides the limit at the poles of B ≤ 6.7 × 10^8 G, 3.3 × 10^8 G, and 14.5 × 10^8 G for GX 3+1, 4U 1702−429, and 4U 0614+091, respectively. For 4U 1746−371, we argue that the most plausible explanation for the lack of reflection features is a combination of source geometry and strong Comptonization. We place these sources among the larger sample of NSs that have been observed with NuSTAR

SIMULTANEOUS FITS IN ISIS ON THE EXAMPLE OF GRO J1008–57

Parallel computing and steadily increasing computation speed have led to a new tool for analyzing multiple datasets and datatypes: fitting several datasets simultaneously.  With this technique, physically connected parameters of individual data can be treated as a single parameter by implementing this connection directly into the fit. We discuss the terminology, implementation, and possible issues of simultaneous fits based on the Interactive Spectral Interpretation System (ISIS) X-ray data analysis tool. While all data modeling tools in X-ray astronomy in principle allow data to be fitted individually from multiple data sets, the syntax used in these tools is not often well suited for this task. Applying simultaneous fits to the transient X-ray binary GRO J1008–57, we find that the spectral shape is only dependent on X-ray flux. We determine time independent parameters e.g., the folding energy Efold, with unprecedented precision

NuSTAR and Suzaku X-ray Spectroscopy of NGC 4151: Evidence for Reflection from the Inner Accretion Disk

We present X-ray timing and spectral analyses of simultaneous 150 ks Nuclear Spectroscopic Telescope Array (NuSTAR) and Suzaku X-ray observations of the Seyfert 1.5 galaxy NGC 4151. We disentangle the continuum emission, absorption, and reflection properties of the active galactic nucleus (AGN) by applying inner accretion disk reflection and absorption-dominated models. With a time-averaged spectral analysis, we find strong evidence for relativistic reflection from the inner accretion disk. We find that relativistic emission arises from a highly ionized inner accretion disk with a steep emissivity profile, which suggests an intense, compact illuminating source. We find a preliminary, near-maximal black hole spin a>0.9 accounting for statistical and systematic modeling errors. We find a relatively moderate reflection fraction with respect to predictions for the lamp post geometry, in which the illuminating corona is modeled as a point source. Through a time-resolved spectral analysis, we find that modest coronal and inner disk reflection flux variation drives the spectral variability during the observations. We discuss various physical scenarios for the inner disk reflection model, and we find that a compact corona is consistent with the observed features.Comment: 20 pages, 12 figures, accepted for publication in Ap

NuSTAR reveals the hidden nature of SS433

SS433 is the only Galactic binary system known to accrete at highly super-critical rates, analogous to tidal disruption events, and needed to explain the mass of some high redshift quasars. Probing the inner regions of SS433 in the X-rays is crucial to understanding this system, and super-critical accretion in general, but has not yet been possible due to obscuration. NuSTAR observed SS433 in the hard X-ray band across multiple phases of its super-orbital precession period. Spectral-timing tools have allowed us to confirm that the hard X-ray emission from the inner regions is scattered towards us by the walls of the wind-cone. By comparing to numerical models, we determine an intrinsic X-ray luminosity of >= 3x10^37 erg/s and that, if viewed face on, the apparent luminosity would be > 1x10^39 erg/s, confirming its long-suspected nature as an ultraluminous X-ray source (ULX). A lag due to absorption by Fe XXV/XXVI in outflowing material travelling at least 0.14-0.29c matches absorption lines seen in ULXs and - in future - will allow us to map a super-critical outflow for the first time.Comment: 24 pages, 8 figures, submitted for publicatio

A multi-model approach to X-ray pulsars: Connecting spectral and timing models to pin down the intrinsic emission characteristics of magnetized, accreting neutron stars

The emission characteristics of X-ray pulsars are governed by magnetospheric accretion within the Alfvén radius, leading to a direct coupling of accretion column properties and interactions at the magnetosphere. The complexity of the physical processes governing the formation of radiation within the accreted, strongly magnetized plasma has led to several sophisticated theoretical modelling efforts over the last decade, dedicated to either the formation of the broad band continuum, the formation of cyclotron resonance scattering features (CRSFs) or the formation of pulse profiles. While these individual approaches are powerful in themselves, they quickly reach their limits when aiming at a quantitative comparison to observational data. Too many fundamental parameters, describing the formation of the accretion columns and the systems' overall geometry are unconstrained and different models are often based on different fundamental assumptions, while everything is intertwined in the observed, highly phase-dependent spectra and energy-dependent pulse profiles. To name just one example: the (phase variable) line width of the CRSFs is highly dependent on the plasma temperature, the existence of B-field gradients (geometry) and observation angle, parameters which, in turn, drive the continuum radiation and are driven by the overall two-pole geometry for the light bending model respectively. This renders a parallel assessment of all available spectral and timing information by a compatible across-models-approach indispensable. In a collaboration of theoreticians and observers, we have been working on a model unification project over the last years, bringing together theoretical calculations of the Comptonized continuum, Monte Carlo simulations and Radiation Transfer calculations of CRSFs as well as a General Relativity (GR) light bending model for ray tracing of the incident emission pattern from both magnetic poles. The ultimate goal is to implement a unified fitting model for phase-resolved spectral and timing data analysis. We present the current status of this project

High-density disc reflection spectroscopy of low-mass active galactic nuclei

The standard alpha-disc model predicts an anti-correlation between the density of the inner accretion disc and the black hole mass times square of the accretion rate, as seen in higher mass ($M_{\rm BH}>10^{6} M_{\odot}$) active galactic nuclei (AGNs). In this work, we test the predictions of the alpha-disc model and study the properties of the inner accretion flow for the low-mass end ($M_{\rm BH}\approx 10^{5-6}M_{\odot}$) of AGNs. We utilize a new high-density disc reflection model where the density parameter varies from $n_{\rm e}=10^{15}$ to $10^{20}$ cm$^{-3}$ and apply it to the broadband X-ray (0.3-10 keV) spectra of the low-mass AGN sample. The sources span a wide range of Eddington fractions and are consistent with being sub-Eddington or near-Eddington. The X-ray spectra reveal a soft X-ray excess below $\sim 1.5$ keV which is well modeled by high-density reflection from an ionized accretion disc of density $n_{\rm e}\sim 10^{18}$ cm$^{-3}$ on average. The results suggest a radiation pressure-dominated disc with an average of 70% fraction of the disc power transferred to the corona, consistent with that observed in higher mass AGNs. We show that the disc density higher than $10^{15}$ cm$^{-3}$ can result from the radiation pressure compression when the disc surface does not hold a strong magnetic pressure gradient. We find tentative evidence for a drop in black hole spin at low-mass regimes.Comment: 20 pages, 10 figures, 6 tables. Accepted for publication in MNRA

Detection of Reflection Features in the Neutron Star Low-Mass X-Ray Binary Serpens X-1 with NICER

We present Neutron Star Interior Composition Explorer (NICER) observations of the neutron star (NS) low-mass X-ray binary Serpens X-1 during the early mission phase in 2017. With the high spectral sensitivity and low-energy X-ray passband of NICER, we are able to detect the Fe L line complex in addition to the signature broad, asymmetric Fe K line. We confirm the presence of these lines by comparing the NICER data to archival observations with XMM-Newton/Reflection Grating Spectrometer (RGS) and NuSTAR. Both features originate close to the innermost stable circular orbit (ISCO). When modeling the lines with the relativistic line model relline, we find that the Fe L blend requires an inner disk radius of 1.4 [superscript +0.2][subscript -0.01] R ISCO and Fe K is at 1.03[superscript +0.13][subscript -0.03]R ISCO (errors quoted at 90%). This corresponds to a position of 17.3[superscript +2.5][subscript -0.1] km and 12.7[superscript +1.6][subscript -0.04] km for a canonical NS mass (M[subscript NS] = 1.4 M[superscript ⨀]) and dimensionless spin value of a = 0. Additionally, we employ a new version of the relxill model tailored for NSs and determine that these features arise from a dense disk and supersolar Fe abundance