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A high-resolution X-ray view of the ultra-fast outflow in MAXI J1810-222

C. Pinto, M. Del Santo, A. D'Aì, F. Pintore, T. D. Russell, M. Parra, J. Ferreira, P. -O. Petrucci, K. Fukumura, A. Marino, T. Muñoz-Darias, G. A. Rodríguez Castillo, A. Segreto

Abstract

In previous work, it was reported that the Galactic black hole candidate MAXI J1810-222 exhibited a notable absorption spectral feature at around 1 keV in low-resolution X-ray spectra of CCD-like detectors. The feature was correlated with the spectral state of the source, being stronger in the soft states, as it occurs in the typical Fe K winds of X-ray binaries (XRBs). However, the results hinted towards rather extreme wind velocities of up to ~0.1 c. We therefore requested and obtained an observation with XMM-Newton to take advantage of the 10-fold higher spectral resolution (R ~200-400) provided by the RGS detector in order to resolve the lines and break the degeneracy between different models and interpretations. We applied state-of-the-art models of plasma in photoionisation equilibrium and multiphase interstellar medium. Further comparisons are performed with a re-analysis of NICER and NuSTAR data. The XMM-Newton/RGS spectrum is consistent with the presence of a mildly relativistic wind, confirming the earlier indications obtained with NICER, but places tighter constraints on the outflow properties, with the lines being intrinsically broad. The data would then favour magnetically driven winds, although thermal effects may still contribute to mass loading. NuSTAR and XMM-Newton (EPIC) show a further hotter component indicating a stratified or multiphase outflow. Fe K spectra taken with calorimetric detectors (e.g., Resolve on XRISM) will enable a high-resolution view of the complex extreme outflow in this source and shed new light on outflow processes in XRBs.

A high-resolution X-ray view of the ultra-fast outflow in MAXI J1810-222

Abstract

In previous work, it was reported that the Galactic black hole candidate MAXI J1810-222 exhibited a notable absorption spectral feature at around 1 keV in low-resolution X-ray spectra of CCD-like detectors. The feature was correlated with the spectral state of the source, being stronger in the soft states, as it occurs in the typical Fe K winds of X-ray binaries (XRBs). However, the results hinted towards rather extreme wind velocities of up to ~0.1 c. We therefore requested and obtained an observation with XMM-Newton to take advantage of the 10-fold higher spectral resolution (R ~200-400) provided by the RGS detector in order to resolve the lines and break the degeneracy between different models and interpretations. We applied state-of-the-art models of plasma in photoionisation equilibrium and multiphase interstellar medium. Further comparisons are performed with a re-analysis of NICER and NuSTAR data. The XMM-Newton/RGS spectrum is consistent with the presence of a mildly relativistic wind, confirming the earlier indications obtained with NICER, but places tighter constraints on the outflow properties, with the lines being intrinsically broad. The data would then favour magnetically driven winds, although thermal effects may still contribute to mass loading. NuSTAR and XMM-Newton (EPIC) show a further hotter component indicating a stratified or multiphase outflow. Fe K spectra taken with calorimetric detectors (e.g., Resolve on XRISM) will enable a high-resolution view of the complex extreme outflow in this source and shed new light on outflow processes in XRBs.
Paper Structure (19 sections, 1 equation, 8 figures, 1 table)

This paper contains 19 sections, 1 equation, 8 figures, 1 table.

Table of Contents

  1. Introduction
  2. Observations and data reduction
  3. Swift data
  4. XMM-Newton
  5. NICER data
  6. Spectral modelling
  7. Baseline model
  8. Full ISM model
  9. Photoionised plasma
  10. The archival NICER spectra
  11. The archival NuSTAR spectrum
  12. Discussion
  13. The ISM along the line-of-sight
  14. Comparison between different epochs
  15. The nature of the outflow
  16. ...and 4 more sections

Figures (8)

  • Figure 1: Top panel: Swift/XRT (blue circles) and Swift/BAT (red diamonds) light curves. Bottom panel: XRT hardness defined as the ratio of the counts in the 2--10 keV and 0.5--2 keV energy bands. The dashed line indicates the XMM-Newton observing time.
  • Figure 2: XMM / RGS (blue data) + EPIC (pn/green and MOS2/orange data) + NICER (black data) spectra and best-fit continuum model. The EPIC data were ignored below 2.6 keV due to calibration issues.
  • Figure 3: XMM / RGS + EPIC + NICER spectra and best fit model (top two panels). The NICER (EPIC) data were ignored between 0.5-1.8(2.6) keV in order to fully employ the high resolving power of RGS and decrease degeneracy between models of line emission / absorption as well as calibration issues. Bottom two panels: residuals computed for the models without pion and without pion, ISM ions and dust.
  • Figure 4: Parameter-space scan throughout model grids of a photoionised plasma in absorption applied onto the XMM-Newton+NICER spectra adopting a velocity dispersion of 20,000 km/s. The dotted black lines identify the best-fit grid model. Structures are also present at high $\xi$.
  • Figure 5: Best-fit parameters of the outflowing plasma component (pion) for the five NICER stacked spectra sorted according to the intrinsic (unabsorbed) X-ray 0.3-10 keV luminosity (re-fit of the flux-resolved spectra with the RGS ISM model) as compared with the new RGS (open red circles) and NICER-only (blue star) results. The X-axis is inverted following the HID evolution from high-soft to low-hard states.
  • ...and 3 more figures