Shocking Behavior: Evidence for Additional Photoionization

Title: Very Large Telescope MUSE Observations of the Bubble Nebula around NGC 1313 X-2 and Evidence for Additional Photoionization

Authors: Changxing Zhou, Fuyan Bian, Hua Feng, and Jiahui Huang

First Author’s Institution: Department of Engineering Physics, Tsinghua University, Beijing, People’s Republic of China

Status: Published in ApJ [open access]

Ultraluminous X-ray Sources (ULXs) continue to be among the hot topics in X-ray astronomy, puzzling observers and theorists alike. These extragalactic X-ray sources emit unusually high X-ray luminosities, exceeding those produced by typical stellar black holes of ten times the solar masses (luminosities more than ~ 1039 erg/s). Although the nature of these objects remains unclear (are they intermediate mass black holes?), the current favored model suggests that ULXs are close binary systems with stellar-mass black holes (or neutron stars!).

Shock-ionized Bubble Nebulae around Ultraluminous X-ray Sources

To gain a more comprehensive understanding of ULXs beyond what can be obtained from the analysis of X-ray data, multiwavelength studies are necessary. In the optical wavelength, bubble-shaped diffuse emissions, known as bubble nebulae, have been found around many ULXs (see Figure 1). These nebulae have sizes ranging from several tens to hundreds of parsecs and expand at velocities of about 100 km/s. Most of them appear to be shock-ionized, powered by outflows (they could be strong winds or relativistic jets) colliding with the interstellar medium. By studying bubble nebulae, astronomers can constrain various properties of the ULX systems, such as the strength of the outflows, the characteristic ages, and the formation and mass-loss history. Such studies may also help bring an avenue to end the debate over the nature of these puzzling objects! 

Figure 1: H-alpha images of the bubble nebulae around 3 different ULXs system (left to right): ULX HolmbergIX X-1, ULX M81 X-6, and ULX NGC 1313 X-2 (the object of today’s bite), taken with various optical instruments. The ULXs are marked with the plus sign and the black circles represent the Chandra position errors (taken from Pakull and Mirioni, 2003, Figures 1-3).

The Shocking Case of NGC 1313 X-2

The authors of today’s paper analyzed the NGC 1313 X-2 data, taken with the Multi-Unit Spectroscopic Explorer (MUSE), an integral field spectrograph (IFS) mounted on the Very Large Telescope (VLT). NGC 1313 X-2 was one of the first ULXs to be discovered and has been extensively studied. It has a bubble nebula that overlaps with the X-ray source position with an expansion velocity of 80 – 100 km/s. The high velocity of the expansion suggests that the emissions originate from radiative shocks, which is further supported by the presence of the [O I] and [S II] forbidden lines. Through their analysis of the MUSE data, the authors were able to gain new insights into the properties of this ULX system.

IFS instruments such as MUSE allow imaging and spectroscopy observations to be done at the same time, generating a final data product that contains spectral information at each spatial pixel (spaxel). Using MUSE data, the authors presented the first spatially resolved spectroscopic studies of the bubble nebula around NGC 1313 X-2. Each emission line on the nebula’s spectrum of each spaxel was modeled (or “fitted”) with a Gaussian distribution to derive the line properties, and to produce the flux, radial velocity, and velocity dispersion maps of the nebula (see Figure 2). They found that the doubly-ionized oxygen [O III] emission is enhanced at the region of the central X-ray source. To produce such a high-ionization emission line, either high velocity shocks or photoionization is required. 

Figure 2: The derived properties of NGC 1313 X-2 bubble nebula from MUSE data: H-alpha flux (left), radial velocity (middle), and velocity dispersion (right) maps. The black cross indicates the X-ray position of NGC 1313 X-2. Each map is overlaid with the flux contours and pixels are not shown if the emission line has significance less than 2-sigma (Figure 3 of the paper).

The authors then modeled the integrated H-alpha spectrum of the ULX region, assuming a shock velocity of 80 km/s from previous studies. They also take into account that the line kinematics observed are resulted from both approaching and receding motions of the nebula. Shockingly, a simple model with two Gaussian components failed to fit the line profile: this model’s line width is significantly larger than the observed line width. The same issue persisted even when the shock velocity was changed to any values between 50 – 100 km/s. When a third Gaussian component of low velocity was added, the model fit the observed line profile (see Figure 3). This low-velocity component in the spectrum could be evidence for additional photoionization happening in the bubble nebula.

Figure 3: Left – The observed (solid black line) integrated Hα spectrum of the ULX region, with the 2-component model (dashed red line and each component represented by the solid green line). Right – The same integrated spectrum with a 3-component model (Figure 5 of the paper).

To test whether photoionization could occur in addition to shocks ionization, the authors used MAPPINGS V, a plasma modeling code that handles both shocks and photoionization modeling. The simulations suggested that a pure shocks model could not explain the observed line ratios, and that photoionization needed to be added to reproduce the observations. This finding could indicate that the distance of the bubble nebula to its ionizing source may be closer than previously assumed, or that there are more ionizing sources, such as nearby O-type stars, contributing to the total luminosity of the ULX system. The authors also noted that this additional photoionization could affect the wind power calculation of the system, which is currently based on a shock-only model. This study proves that even though NGC 1313 X-2 is among the most extensively studied ULXs, there are still many shocking mysteries waiting to be explored! 

Astrobite edited by Emma Clarke and Pratik Gandhi

Featured image credit: X-ray: NASA/CXC/Caltech/M. Brightman et al.; Optical: NASA/STScI

About Janette Suherli

Janette is a PhD student at University of Manitoba in Winnipeg (Winterpeg!), Canada. Her research focuses on the utilization of integral field spectroscopy for the studies of supernova remnants and their compact objects in the optical. She grew up in Indonesia where it is summer all year round! Before pursuing her PhD in astrophysics, Janette worked as a data analyst for a big Indonesian tech company, combating credit card fraud.


  1. Awesome work!!

    • Indeed, the authors work is very interesting! You could also check out their group’s work on another ULXs here:

  2. Great paper summary. Can’t wait to read another shocking findings about the interstellar world.

    • Thank you!


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