The X-ray properties and structure of A3571 up to $R_{500}$

TL;DR

Abell 3571 is examined with wide-field X-ray imaging and optical data to probe its outskirts beyond $R_{500}$ and to diagnose merger-driven gas motions. Using EP-FXT, the authors detect northern and southern surface-brightness excesses and map the temperature, pressure, and entropy, finding a cooler northern region and a hotter southern region—signatures consistent with gas sloshing triggered by a past off-axis minor merger. No evidence for shocks or cold fronts is found, reinforcing a post-merger, partially relaxed state. The study demonstrates the power of EP-FXT for exploring cluster outskirts and clarifies the dynamical history of A3571, while noting calibration-related uncertainties that will be mitigated with ongoing mission data.

Abstract

Abell 3571 is a nearby, X-ray bright galaxy cluster located in the Shapley Supercluster. Although it appears morphologically relaxed in X-ray images, multiwavelength observations reveal subtle indications of residual dynamical activity, likely associated with past merger events. Using wide-field ($1^{\circ} \times 1^{\circ}$) data from the Einstein Probe Follow-up X-ray Telescope (EP-FXT), we extend measurements of the cluster's properties beyond its $R_{500}$ radius. We detect surface-brightness excesses on both the northern and southern sides, consistent with previous XMM-Newton results. The temperature, pressure, and entropy in the northern excess region are lower than the average values, whereas those on the southern side are slightly higher. However, we find no evidence for cold fronts or shocks. These features can be interpreted as sloshing motions triggered by an off-center minor merger. Our findings suggest that, despite its symmetric appearance, A3571 is still recovering from a minor merger and is currently in a post-merger phase. This work also demonstrates the excellent capability of EP-FXT for probing the outskirts of galaxy clusters.

Figures (9)

• Figure 1: Particle background-subtracted, vignetting-corrected, and smoothed EP-FXT image of A3571 in the 0.3–7.0 keV energy band. The green circle is $R_{500}$ (1.29 Mpc).
• Figure 2: The galaxy luminosity density map, generated from optical photometric data (galaxies with $r$-band magnitudes $R_{c} < 22$ and redshifts in the range 0.019–0.079), reveals a north–south extension. The red cross indicates the BCGs, the yellow triangle mark the X-ray peak, the purple star shows the position of A3572, the green rectangles denote the field of EP-FXT , and the black contours are the X-ray isophotes.
• Figure 3: The black points represent the EP-FXT data, and the red solid line shows the best-fit double-$\beta$ model including background components. The blue points correspond to the EP-FXT surface brightness after subtraction of both the particle and sky backgrounds, fitted with a double-$\beta$ model without backgrounds, and the best-fit curve is shown as the black solid line, while the orange and green dashed lines indicate the two individual components of this model. The cyan points are the XMM-Newton data. The blue and green solid horizontal lines denote the particle background from fxtbkggen and sky background levels for EP-FXT, respectively, while the blue and green dashed horizontal lines indicate the particle and sky background levels for XMM-Newton.
• Figure 4: The spectrum of A3571 extracted within a $20^{\prime}$ radius centered on the X-ray peak. The blue and red points represent the observational data from FXT-A and FXT-B, respectively. The green and brown points correspond to their particle backgrounds, and the orange and purple points represent the sky backgrounds after subtraction of the particle background.
• Figure 5: The radial temperature profile of A3571. The black circles show the temperature distribution measured using EP-FXT in this study. The blue dots and the green squares correspond to the XMM-Newton data and the ASCA data reported by 2005xmm3571. The purple and yellow triangles indicate the temperature profile measured by EP-FXT in the southern and northern directions, respectively. The gray dashed vertical line indicates the location of $R_{2500}$.