Search

We present the first X-ray polarization measurements of GX 339-4. IXPE observed this source twice during its 2023-2024 outburst, once in the soft-intermediate state and again during a soft state. The observation taken during the intermediate state shows significant ($4\sigma$) polarization degree P = $1.3\% \pm 0.3\%$ and polarization angle $\theta$ = -74\degree $\pm$ 7\degree only in the 3 - 8 keV band. FORS2 at VLT observed the source simultaneously detecting optical polarization in the B, V, R, I bands (between $0.1%$ and $0.7\%$), all roughly aligned with the X-ray polarization. We also detect a discrete jet knot from radio observations taken later in time; this knot would have been ejected from the system around the same time as the hard-to-soft X-ray state transition and a bright radio flare occurred $\sim$3 months earlier. The proper motion of the jet knot provides a direct measurement of the jet orientation angle on the plane of the sky at the time of the ejection. We find that both the X-ray and optical polarization angles are aligned with the direction of the ballistic jet., Comment: Submitted to ApJ

We present mid-infrared (MIR) spectral-timing measurements of the prototypical Galactic microquasar GRS 1915+105. The source was observed with the Mid-Infrared Instrument (MIRI) onboard JWST in June 2023 at a MIR luminosity L(MIR)~10^{36} erg/s exceeding past IR levels by about a factor of 10. By contrast, the X-ray flux is much fainter than the historical average, in the source's now-persistent 'obscured' state. The MIRI low-resolution spectrum shows a plethora of emission lines, the strongest of which are consistent with recombination in the hydrogen Pfund (Pf) series and higher. Low amplitude (~1%) but highly significant peak-to-peak photometric variability is found on timescales of ~1,000 s. The brightest Pf(6-5) emission line lags the continuum. Though difficult to constrain accurately, this lag is commensurate with light-travel timescales across the outer accretion disc or with expected recombination timescales inferred from emission line diagnostics. Using the emission line as a bolometric indicator suggests a moderate (~5-30% Eddington) intrinsic accretion rate. Multiwavelength monitoring shows that JWST caught the source close in-time to unprecedentedly bright MIR and radio long-term flaring. Assuming a thermal bremsstrahlung origin for the MIRI continuum suggests an unsustainably high mass-loss rate during this time unless the wind remains bound, though other possible origins cannot be ruled out. PAH features previously detected with Spitzer are now less clear in the MIRI data, arguing for possible destruction of dust in the interim. These results provide a preview of new parameter space for exploring MIR spectral-timing in XRBs and other variable cosmic sources on rapid timescales., Comment: Dedicated to the memory of our colleague, Tomaso Belloni. Submitted 2024 June 21; Comments welcome

We report on the detection of X-ray polarisation in the black-hole X-ray binary Swift J1727.8$-$1613 during its dim hard spectral state by the Imaging X-ray Polarimetry Explorer (IXPE). This is the first detection of X-ray polarisation at the transition from the soft to the hard state in an X-ray binary. We find an averaged 2$-$8 keV polarisation degree of (3.3 ${\pm}$ 0.4) % and a corresponding polarisation angle of 3{\deg} ${\pm}$ 4{\deg}, which matches the polarisation detected during the rising stage of the outburst, in September$-$October 2023, within 1${\sigma}$ uncertainty. The observational campaign complements previous studies of this source and enables comparison of the X-ray polarisation properties of a single transient across the X-ray hardness-intensity diagram. The complete recovery of the X-ray polarisation properties, including the energy dependence, came after a dramatic drop in the X-ray polarisation during the soft state. The new IXPE observations in the dim hard state at the reverse transition indicate that the accretion properties, including the geometry of the corona, appear to be strikingly similar to the bright hard state during the outburst rise despite the X-ray luminosities differing by two orders of magnitude., Comment: 8 pages, 6 figures. Accepted for publication in A&A Letters

We present the results of the first X-ray polarimetric observation of the low-mass X-ray binary 4U 1957+115, performed with the Imaging X-ray Polarimetry Explorer in May 2023. The binary system has been in a high-soft spectral state since its discovery and is thought to host a black hole. The $\sim$571 ks observation reveals a linear polarisation degree of $1.9\% \pm 0.6\%$ and a polarisation angle of $-41^\circ.8 \pm 7^\circ.9$ in the 2-8 keV energy range. Spectral modelling is consistent with the dominant contribution coming from the standard accretion disc, while polarimetric data suggest a significant role of returning radiation: photons that are bent by strong gravity effects and forced to return to the disc surface, where they can be reflected before eventually reaching the observer. In this setting, we find that models with a black hole spin lower than 0.96 and an inclination lower than $50^\circ$ are disfavoured., Comment: 12 pages, 10 figures, 2 tables, accepted for publication in A&A

We report on the first simultaneous high-time resolution X-ray and infrared (IR) observations of a neutron star low mass X-ray binary in its hard state. We performed $\approx 2\,$h of simultaneous observations of 4U 1728-34 using HAWK-I@VLT, XMM-Newton and NuSTAR. The source displayed significant X-ray and IR variability down to sub-second timescales. By measuring the cross-correlation function between the infrared and X-ray lightcurves, we discovered a significant correlation with an infrared lead of $\approx 30-40\,$ms with respect to the X-rays. We analysed the X-ray energy dependence of the lag, finding a marginal increase towards higher energies. Given the sign of the lag, we interpret this as possible evidence of Comptonization from external seed photons. We discuss the origin of the IR seed photons in terms of cyclo-synchrotron radiation from an extended hot flow. Finally, we also observed the IR counterpart of a type-I X-ray burst, with a delay of $\approx7.2\,$s. Although some additional effects may be at play, by assuming that this lag is due to light travel time between the central object and the companion star, we find that 4U 1728-34 must have an orbital period longer than $3\,$h and an inclination higher than 8$^\circ$., Comment: Accepted for publication in MNRAS, 11 pages 7 figures

We present the long term analysis of GS 1826-238, a neutron star X-ray binary known as the "Clocked Burster", using data from NuSTAR StrayCats. StrayCats, a catalogue of NuSTAR stray light data, contains data from bright, off-axis X-ray sources that have not been focused by the NuSTAR optics. We obtained stray light observations of the source from 2014-2021, reduced and analyzed the data using nustar-gen-utils Python tools, demonstrating the transition of source from the "island" atoll state to a "banana" branch. We also present the lightcurve analysis of Type I X-Ray bursts from the Clocked Burster and show that the bursts from the banana/soft state are systematically shorter in durations than those from the island/hard state and have a higher burst fluence. From our analysis, we note an increase in mass accretion rate of the source, and a decrease in burst frequency with the transition.

Due to the structure of the NuSTAR telescope, photons at large off-axis (> 1deg) can reach the detectors directly (stray light), without passing through the instrument optics. At these off-axis angles NuSTAR essentially turns into a collimated instrument and the spectrum can extend to energies above the Pt k-edge (79 keV) of the multi-layers, which limits the effective area bandpass of the optics. We present the first scientific spectral analysis beyond 79 keV using a Cygnus X-1 observation in StrayCats, the catalog of stray light observations. This serendipitous stray light observation occurred simultaneously with an INTEGRAL observation. When the spectra are modeled together in the 30-120 keV energy band, we find that the NuSTAR stray light flux is well calibrated and constrained to be consistent with the INTEGRAL flux at the 90% confidence level. Furthermore, we explain how to treat the background of the stray light spectral analysis, which is especially important at high energies., Comment: Accepted for publication in ApJ

We present an updated catalog of StrayCats (a catalog of NuSTAR stray light observations of X-ray sources) that includes nearly 18 additional months of observations. StrayCats v2 has an added 53 sequence IDs, 106 rows, and 3 new identified stray light (SL) sources in comparison to the original catalog. The total catalog now has 489 unique sequence IDs, 862 entries, and 83 confirmed StrayCats sources. Additionally, we provide new resources for the community to gauge the utility and spectral state of the source in a given observation. We have created long term light curves for each identified SL source using MAXI and Swift/BAT data when available. Further, source extraction regions for 632 identified SL observations were created and are available to the public. In this paper we present an overview of the updated catalog and new resources for each identified StrayCats SL source., Comment: Accepted for publication in ApJ, 6 pages, 5 figures, 1 table

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 MNRAS

StrayCats, the catalog of NuSTAR stray light observations, contains data from bright X-ray sources that fall within crowded source regions. These observations offer unique additional data with which to monitor sources like X-ray binaries that show variable timing behavior. In this work, we present a timing analysis of stray light data of the high mass X-ray binary SMC X-1, the first scientific analysis of a single source from the StrayCats project. We describe the process of screening stray light data for scientific analysis, verify the orbital ephemeris, and create both time and energy resolved pulse profiles. We find that the orbital ephemeris of SMC X-1 is unchanged and confirm a long-term spin up rate of $\dot{\nu}=(2.52\pm0.03)\times10^{-11}$ Hz s$^{-1}$. We also note that the shape of SMC X-1's pulse profile, while remaining double-peaked, varies significantly with time and only slightly with energy., Comment: 10 pages, 8 figures

We present the results of X-ray spectral-timing analysis of a 90ks XMM-Newton observation of the nearby, broad line, early-type AGN NGC5273. The X-ray spectrum revealed the clear presence of a reflection component at high energies, with a clear signature of a narrow iron line at 6.4 keV, consistent with distant reflection. Applying a relativistic reflection model, we found only marginal evidence for a broader relativistic line component. However, cross-spectral analysis revealed that, between 4 and 6 $\times 10^{-4}$ Hz, the 5-8 keV band lagged the 2-3 keV band, implying reflection of the iron line from material close to the black hole. From the analysis of the lag-energy spectrum, we found a broad, but skewed line with a peak of $\approx$ 1000s at 7.5 keV relative to the continuum, which we interpret as the iron line in the reverberation spectrum from an illuminated accretion disc. From the asymmetry in the shape of lag-energy spectrum, we also found that the source is consistent with having an inclination $\geq 45^\circ$., Comment: Accepted for publication in MNRAS. 7 pages, 6 figures

Type-C QPOs in X-ray binaries have been often interpreted as a consequence of relativistic Lense-Thirring precession around a spinning black hole and they potentially offer a way to measure black hole spins and masses. The connection between relativistic precession and the resulting QPO has been made either in terms of a simplified model involving a single test particle producing the QPO, or in terms of a global model where a geometrically thick accretion flow precesses coherently as a rigid body. In this paper, we analyse similarities and differences between these two models, sometimes considered as in opposition to each other. We demonstrate that the former is the limiting case of the latter when the radial extent of the precessing flow is very small, and that solid lower limits to the black hole spin can be obtained by considering the test particle model alone. We also show that the global precession model naturally accounts for the range of frequencies observed for Type-C QPOs without the need to invoke a truncation of the inner accretion flow before it reaches the innermost stable circular orbit. Finally, we show that, in order to maintain rigid precession, the thick accretion flow should be radially narrow, and that if it extends beyond $10-10^2$ gravitational radii it aligns with the black hole spin too fast to produce a coherent QPO., Comment: 10 pages, 8 figures, accepted for publication in MNRAS