Probing large-scale wind structures in Vela X-1 using off-states with INTEGRAL

Vela X-1 is the prototype of the class of wind-fed accreting pulsars in high mass X-ray binaries hosting a supergiant donor. We have analyzed in a systematic way ten years of INTEGRAL data of Vela X-1 (22-50 keV) and we found that when outside the X-ray eclipse, the source undergoes several luminosity drops where the hard X-rays luminosity goes below 3x10^35 erg/s, becoming undetected by INTEGRAL. These drops in the X-ray flux are usually referred to as "off-states" in the literature. We have investigated the distribution of these off-states along the Vela X-1 ~8.9 d orbit, finding that their orbital occurrence displays an asymmetric distribution, with a higher probability to observe an off-state near the pre-eclipse than during the post-eclipse. This asymmetry can be explained by scattering of hard X-rays in a region of ionized wind, able to reduce the source hard X-ray brightness preferentially near eclipse ingress. We associate this ionized large-scale wind structure with the photoionization wake produced by the interaction of the supergiant wind with the X-ray emission from the neutron star. We emphasize that this observational result could be obtained thanks to the accumulation of a decade of INTEGRAL data, with observations covering the whole orbit several times, allowing us to detect an asymmetric pattern in the orbital distribution of off-states in Vela X-1.Comment: Accepted for publication in Monthly Notices of the Royal Astronomical Society (5 pages, 3 figures). A few typos fixed to match the published versio

Atomic carbon chains as spin-transmitters: an \textit{Ab initio} transport study

An atomic carbon chain joining two graphene flakes was recently realized in a ground-breaking experiment by Jin {\it et al.}, Phys. Rev. Lett. {\bf 102}, 205501 (2009). We present {\it ab initio} results for the electron transport properties of such chains and demonstrate complete spin-polarization of the transmission in large energy ranges. The effect is due to the spin-polarized zig-zag edge terminating each graphene flake causing a spin-splitting of the graphene $\pi_z$ bands, and the chain states. Transmission occurs when the graphene $\pi$-states resonate with similar states in the strongly hybridized edges and chain. This effect should in general hold for any $\pi$-conjugated molecules bridging the zig-zag edges of graphene electrodes. The polarization of the transmission can be controlled by chemically or mechanically modifying the molecule, or by applying an electrical gate

Spectral Changes in the Hyperluminous Pulsar in NGC 5907 as a Function of Super-Orbital Phase

We present broad-band, multi-epoch X-ray spectroscopy of the pulsating ultra-luminous X-ray source (ULX) in NGC 5907. Simultaneous XMM-Newton and NuSTAR data from 2014 are best described by a multi-color black-body model with a temperature gradient as a function of accretion disk radius significantly flatter than expected for a standard thin accretion disk (T(r) ~ r^{-p}, with p=0.608^{+0.014}_{-0.012}). Additionally, we detect a hard power-law tail at energies above 10 keV, which we interpret as being due to Comptonization. We compare this observation to archival XMM-Newton, Chandra, and NuSTAR data from 2003, 2012, and 2013, and investigate possible spectral changes as a function of phase over the 78d super-orbital period of this source. We find that observations taken around phases 0.3-0.4 show very similar temperature profiles, even though the observed flux varies significantly, while one observation taken around phase 0 has a significantly steeper profile. We discuss these findings in light of the recent discovery that the compact object is a neutron star and show that precession of the accretion disk or the neutron star can self-consistently explain most observed phenomena.Comment: 7 pages, 5 figures, submitted to ApJ; comments welcom

Discovery of a new source of rifamycin antibiotics in marine sponge actinobacteria by phylogenetic prediction

Phylogenetic analysis of the ketosynthase (KS) gene sequences of marine sponge-derived Salinispora strains of actinobacteria indicated that the polyketide synthase (PKS) gene sequence most closely related to that of Salinispora was the rifamycin B synthase of Amycolatopsis mediterranei. This result was not expected from taxonomic species tree phylogenetics using 16S rRNA sequences. From the PKS sequence data generated from our sponge-derived Salinispora strains, we predicted that such strains might synthesize rifamycin-like compounds. Liquid chromatography-tandem mass spectrometry (LC/MS/MS) analysis was applied to one sponge-derived Salinispora strain to test the hypothesis of rifamycin synthesis. The analysis reported here demonstrates that this Salinispora isolate does produce compounds of the rifamycin class, including rifamycin B and rifamycin SV. A rifamycin-specific KS primer set was designed, and that primer set increased the number of rifamycin-positive strains detected by PCR screening relative to the number detectable using a conserved KS-specific set. Thus, the Salinispora group of actinobacteria represents a potential new source of rifamycins outside the genus Amycolatopsis and the first recorded source of rifamycins from marine bacteria

Disentangling the Complex Broadband X-ray Spectrum of IRAS 13197-1627 with NuSTAR, XMM-Newton and Suzaku

We present results from a coordinated $XMM$-$Newton$+$NuSTAR$ observation of the type 1.8 Seyfert galaxy IRAS 13197-1627. This is a highly complex source, with strong contributions from relativistic reflection from the inner accretion disk, neutral absorption and further reprocessing by more distant material, and ionised absorption from an outflow. We undertake a detailed spectral analysis combining the broadband coverage provided by $XMM$-$Newton$+$NuSTAR$ with a multi-epoch approach incorporating archival observations performed by $XMM$-$Newton$ and $Suzaku$. Our focus is on characterising the reflection from the inner accretion disk, which previous works have suggested may dominate the AGN emission, and constraining the black hole spin. Using lamppost disk reflection models, we find that the results for the inner disk are largely insensitive to assumptions regarding the geometry of the distant reprocessor and the precise form of the illuminating X-ray continuum. However, these results do depend on the treatment of the iron abundance of the distant absorber/reprocessor. The multi-epoch data favour a scenario in which the AGN is chemically homogeneous, and we find that a rapidly rotating black hole is preferred, with $a^* \geq 0.7$, but a slowly-rotating black hole is not strongly excluded. In addition to the results for the inner disk, we also find that both the neutral and ionised absorbers vary from epoch to epoch, implying that both have some degree of inhomogeneity in their structure.Comment: 15 pages, 7 figures, accepted for publication in MNRA

Spectral and Timing Properties of IGR J17091-3624 in the Rising Hard State During its 2016 Outburst

We present a spectral and timing study of the NuSTAR and Swift observations of the black hole candidate IGR J17091-3624 in the hard state during its outburst in 2016. Disk reflection is detected in each of the NuSTAR spectra taken in three epochs. Fitting with relativistic reflection models reveals that the accretion disk is truncated during all epochs with $R_{\rm in}>10~r_{\rm g}$, with the data favoring a low disk inclination of $\sim 30^{\circ}-40^{\circ}$. The steepening of the continuum spectra between epochs is accompanied by a decrease in the high energy cut-off: the electron temperature $kT_{\rm e}$ drops from $\sim 64$ keV to $\sim 26$ keV, changing systematically with the source flux. We detect type-C QPOs in the power spectra with frequency varying between 0.131 Hz and 0.327 Hz. In addition, a secondary peak is found in the power spectra centered at about 2.3 times the QPO frequency during all three epochs. The nature of this secondary frequency is uncertain, however a non-harmonic origin is favored. We investigate the evolution of the timing and spectral properties during the rising phase of the outburst and discuss their physical implications.Comment: 11 pages, 9 figures, accepted by Ap

Studying the accretion geometry of EXO 2030+375 at luminosities close to the propeller regime

The Be X-ray binary EXO 2030+375 was in an extended low luminosity state during most of 2016. We observed this state with NuSTAR and Swift, supported by INTEGRAL observations as well as optical spectroscopy with the NOT. We present a comprehensive spectral and timing analysis of these data here to study the accretion geometry and investigate a possible onset of the propeller effect. The H-alpha data show that the circumstellar disk of the Be-star is still present. We measure equivalent widths similar to values found during more active phases in the past, indicating that the low-luminosity state is not simply triggered by a smaller Be disk. The NuSTAR data, taken at a 3-78 keV luminosity of ~6.8e35 erg/s (for a distance of 7.1 kpc), are well described by standard accreting pulsar models, such as an absorbed power-law with a high-energy cutoff. We find that pulsations are still clearly visible at these luminosities, indicating that accretion is continuing despite the very low mass transfer rate. In phase-resolved spectroscopy we find a peculiar variation of the photon index from ~1.5 to ~2.5 over only about 3% of the rotational period. This variation is similar to that observed with XMM-Newton at much higher luminosities. It may be connected to the accretion column passing through our line of sight. With Swift/XRT we observe luminosities as low as 1e34 erg/s during which the data quality did not allow us to search for pulsations, but the spectrum is much softer and well described by either a blackbody or soft power-law continuum. This softer spectrum might be due to the fact that accretion has been stopped by the propeller effect and we only observe the neutron star surface cooling.Comment: 11 pages, 6 figures, accepted for publication in A&A (v2 including language edits

NuSTAR Observations of the Black Hole GS 1354-645: Evidence of Rapid Black Hole Spin

We present the results of a NuSTAR study of the dynamically confirmed stellar-mass black hole GS 1354-645. The source was observed during its 2015 "hard" state outburst; we concentrate on spectra from two relatively bright phases. In the higher-flux observation, the broadband NuSTAR spectra reveal a clear, strong disk reflection spectrum, blurred by a degree that requires a black hole spin of a = cJ/GM^2 > 0.98 (1 sigma statistical limits only). The fits also require a high inclination: theta = 75(2) degrees. Strong "dips" are sometimes observed in the X-ray light curves of sources viewed at such an angle; these are absent, perhaps indicating that dips correspond to flared disk structures that only manifest at higher accretion rates. In the lower-flux observation, there is evidence of radial truncation of the thin accretion disk. We discuss these results in the context of spin in stellar-mass black holes, and inner accretion flow geometries at moderate accretion rates.Comment: Accepted for publication in ApJ Letter

The nature and origin of Seyfert warm absorbers

We collate the results of recent high resolution X-ray spectroscopic observations of 23 AGN, and use the resulting information to try to provide answers to some of the main open questions about warm absorbers: where do they originate, what effect do they have on their host galaxies, and what is their importance within the energetics and dynamics of the AGN system as a whole? We find that the warm absorbers of nearby Seyferts and certain QSOs are most likely to originate in outflows from the dusty torus, and that the kinetic luminosity of these outflows accounts for well under 1% of the bolometric luminosities of the AGN. Our analysis supports, however, the view that the relativistic outflows recently observed in two PG quasars have their origin in accretion disc winds, although the energetic importance of these outflows is similar to that of the Seyfert warm absorbers. We find that the observed soft X-ray absorbing ionisation phases fill less than 10% of the available volume. Finally, we show that the amount of matter processed through an AGN outflow system, over the lifetime of the AGN, is probably large enough to have a significant influence on the evolution of the host galaxy and of the AGN itself.Comment: v2: 15 pages, 9 figures, accepted for publication in A&A, minor typos correcte