Resonant scattering at the center of the galaxy cluster PKS 0745-191 with XRISM

Abstract

We report evidence for the resonant scattering effect at the center of the galaxy cluster PKS 0745-191 with XRISM. We analyzed XRISM/Resolve commissioning-phase observations of the distant cluster PKS 0745-191 (z = 0.103) with a 54 ks exposure. The gain drift was corrected using the onboard modulated X-ray source (MXS), and spectra were extracted from all pixels well illuminated by MXS, the core region (four central pixels, about 100 kpc), and the surrounding region. A single-temperature collisional ionization equilibrium (CIE) model fits the full field-of-view spectrum with kT about 6 keV and a turbulent velocity of about 120 km/s. From the core (r < 50 kpc) spectrum, we detect about 22 percent suppression of the Fe XXV He-alpha resonance (w) line relative to the CIE prediction. We performed Monte Carlo simulations to calculate the resonant scattering (RS) effect using radial profiles from Chandra data. The RS-inferred turbulence agrees with that inferred from Resolve line broadening, demonstrating that RS provides an independent and consistent constraint on ICM turbulence. These results highlight the potential of XRISM/Resolve for turbulence studies in galaxy clusters.

Figures (10)

• Figure 1: House keeping data during the PKS 0745$-$191 observation as a function of time. The origin of the time is 2023-11-08 10:21:04 UTC. From top to bottom: temperature of the cold stage of the adiabatic demagnetisation refrigerator (ADR), FW position, LED1 current driving the MXS, and 2--8 keV count rate of the Resolve array. The red vertical lines mark the start time of ADR recycling. The MXS LED current changed during the observation interval where $^{55}$Fe filter position was used. Alt text: Four line graphs showing housekeeping data during the observation. Each panel shows time variations of ADR temperature, FW position, LED current, and count rate.
• Figure 2: (Left): 2--8 keV Chandra exposure-corrected image of PKS 0745$-$191 overlaid with the 23 Resolve pixels used in the analysis. The remaining 12 pixels are not used due to low MXS counts to correct the gain drift (green line). See the text for the details. (Right): Resolve 2--8 keV counts map. We divided the 23 Resolve pixels into central (central four pixels) and outer (other 19 pixels) regions. Alt text: Two-panel color images of the Chandra and Resolve observations. The left panel is in sky coordinates (right ascension and declination), and the right panel is in detector coordinates.
• Figure 3: Energy spectra of the entire region (23 pixels) taken during the observation period with the FW OPEN position. (Top): 2--8 keV spectrum of PKS 0745$-$191. The spectrum is fitted with bvapec model. (Bottom): Zoomed up in Fe .8 .8 XXV He$\alpha$, which consists of the resonance line 'w', intercombination lines 'x' and 'y', and forbidden line 'z'. Alt text: Two line graphs showing the spectrum and fitting results. In the top panel, x axis shows the energy from 2 to 8 keV. The y axis shows the count per second and per keV. In the bottom panel, x axis shows the energy from 5.975 to 6.125 keV.
• Figure 4: Energy spectra of central and outer regions observed with Resolve. (a) 2--10 keV spectrum of the central region (central four pixels). (b) 5.975--6.125 keV spectrum of the central region covering the Fe .8 .8 XXV He$\rm \alpha$ line. (c) 2--10 keV spectrum of the outer region (outer 19 pixels). (d) 5.975--6.125 keV spectrum of the outer region. The orange solid line is the best-fitting results of bvapec+zgauss(w)+zgauss(z) model. The blue dash-dotted, green dashed, and gray solid lines show the components from the central, outer, and NXB model, respectively. The magenta solid lines are the bvapec model of $kT=5.33 \ \rm keV$ (central) and $6.27 \ \rm keV$ (outer). Alt text: Four line graphs showing the spectrum and best-fitting results with model components. The y axis is the count per second and per keV.
• Figure 5: (Left): The setting parameters of the RS simulation. Up to 500 kpc radius we used the results from sanders2014 and up to 1000 kpc we used the results from walker2012. The metal abundance was constant at 0.4 Solar value of anders1989 for sanders2014 and walker2012. (Right): The simulated spectrum of the turbulent velocity is 100 $\rm km\ s^{-1}$ in PKS 0745$-$191 central region ($r=0.5^{\prime}$). The black line shows without RS case (optically thin) and the red line shows with RS. The bottom panel shows count ratio of with and without the RS effect at $\sigma_{\rm turb}=100 \rm \ km \ s^{-1}$. Alt text: Three line graphs. In the left panel, the radial profile of the electro density and temperature are shown. The x axis shows radius from 6 kilo parsec to 3000 kilo parsec. In the right panel, the simulated spectrum of with and without RS are shown. The x axis shows the energy from 6.5 keV to 6.725 keV. The y axis shows the count. The bottom panel shows the count ratio of with and without the RS effect.