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XMM-Newton Observations of Flares and a Possible Pulse Dropout in the Supergiant X-ray binary 4U 1909+07

Joel B. Coley, Ralf Ballhausen, McKinley Brumback, Robin H. D. Corbet, Camille M. Diez, Felix Fuerst, Nazma Islam, Gaurava K. Jaisawal, Peter Kretschmar, Christian Malacaria, Katja Pottschmidt, Pragati Pradhan

Abstract

We report on a pair of X-ray Multi-Mirror Mission (XMM-Newton) observations of the Supergiant X-ray binary 4U 1909+07, which were performed on 2021 October 3 and 2021 October 8, respectively. We measure the neutron star rotation period in each observation to be $\sim$602.62 s. This continues a long spin-up trend that has persisted since 2001 where the neutron star spin period was found to be $\sim$604.66 s. In our timing analysis, we observe strong variations in the amplitude of the 1--10 keV pulse profile as a function of time, and for the first time we find a low flux interval extending for a single pulse period in which pulsations are no longer detected. We interpret this low flux interval as a pulse dropout similar to those observed in Vela X-1 and GX 301-2, which were each explained by a low-density cavity in the wind driving the propeller effect. In our time-resolved spectral analysis, we observed the spectral continuum, which can be described as an absorbed power law modified by a high-energy cutoff, to significantly soften during the pulse-dropout phase. No evidence of an increasing absorption column density was found. The observed softening in 4U 1909+07 also supports an interpretation that the observed pulse dropout may be driven by the propeller effect, but the quasi-spherical settling accretion regime cannot be ruled out.

XMM-Newton Observations of Flares and a Possible Pulse Dropout in the Supergiant X-ray binary 4U 1909+07

Abstract

We report on a pair of X-ray Multi-Mirror Mission (XMM-Newton) observations of the Supergiant X-ray binary 4U 1909+07, which were performed on 2021 October 3 and 2021 October 8, respectively. We measure the neutron star rotation period in each observation to be 602.62 s. This continues a long spin-up trend that has persisted since 2001 where the neutron star spin period was found to be 604.66 s. In our timing analysis, we observe strong variations in the amplitude of the 1--10 keV pulse profile as a function of time, and for the first time we find a low flux interval extending for a single pulse period in which pulsations are no longer detected. We interpret this low flux interval as a pulse dropout similar to those observed in Vela X-1 and GX 301-2, which were each explained by a low-density cavity in the wind driving the propeller effect. In our time-resolved spectral analysis, we observed the spectral continuum, which can be described as an absorbed power law modified by a high-energy cutoff, to significantly soften during the pulse-dropout phase. No evidence of an increasing absorption column density was found. The observed softening in 4U 1909+07 also supports an interpretation that the observed pulse dropout may be driven by the propeller effect, but the quasi-spherical settling accretion regime cannot be ruled out.
Paper Structure (20 sections, 13 equations, 9 figures, 3 tables)

This paper contains 20 sections, 13 equations, 9 figures, 3 tables.

Table of Contents

  1. Introduction
  2. Data and Analysis
  3. Timing Analysis
  4. Light Curve and Pulse Profiles
  5. Hardness Ratios
  6. Fractional RMS
  7. Spectral Analysis
  8. Time-Averaged Spectral Analysis
  9. ks-Integrated Spectral Analysis
  10. Pulse-to-Pulse Spectral Analysis
  11. Discussion
  12. Geometry of the Absorbing Medium
  13. Pulse Profile Evolution
  14. Comparison with Vela X-1
  15. Pulse Dropout Mechanisms
  16. ...and 5 more sections

Figures (9)

  • Figure 1: Swift BAT light curve (black) folded on the $\sim$15.2 day superorbital period using 16 bins. Phase zero corresponds to the time of maximum flux 2023ApJ...948...45I. The XMM-Newton EPIC-pn 1--10 keV light curves for observations I and II binned to a time resolution of 602.62 s are overplotted in red (see text for details).
  • Figure 2: Long-term evolution of the neutron star rotation period between 2001--2021. The pulse period measured by RXTE 2004ApJ...617.1284L, INTEGRAL 2011AA...525A..73F, Suzaku 2012AA...547A...2F2013ApJ...779...54J, NuSTAR 2020MNRAS.498.4830J2023ApJ...948...45I, Astrosat 2020MNRAS.498.4830J and XMM-Newton (this work) are indicated by the gold triangles, red circles, magneta diamond, green squares, blue squares and cyan plus, respectively.
  • Figure 3: (a) XMM-Newton EPIC-pn 1--10 keV light curves of 4U 1909$+$07 for Observations I (left) and II (right). The time of the off-state is indicated by the blue arrow. Short flares are indicated by green arrows. The time resolution is the local pulse period (see text for details). Count rate is plotted on a logarithmic scale to emphasize the energy-dependence of the variability. The hardness ratios of the 2--4 keV and 8--10 keV, 4--6 keV and 8--10 keV, and 6--8 keV and 8--10 keV bands are shown in panels (b), (c) and (d), respectively.
  • Figure 4: Energy-resolved XMM-Newton EPIC-pn pulse profiles for Observations I (red) and II (blue). The pulse profiles are normalized such that their mean value is zero and their standard deviation is unity. The modulation appears to show a broad plateau and a narrow dip feature.
  • Figure 5: 3 ks segments of the 1--10 keV EPIC-pn light curve for Observations I (top) and II (bottom). Panels (a) and (b) show the off state and a flare, respectively. Panels (c) and (d) show a flare and a drop in the count rate that is not coincident with a pulse dropout, respectively. The light curves binned at a resolution of 10 s and 40 s are indicated in black and red, respectively. The average pulse profile for the full observation is overplotted in blue. Note the $y-$axes are not the same scale.
  • ...and 4 more figures