We demonstrate a novel technique for calibrating the energy scale of the EPIC-pn detector on XMM-Newton , which allows us to measure bulk flows in the intracluster medium ( ICM ) of the Perseus and Coma galaxy clusters .
The procedure uses the fluorescent instrumental background lines present in all observations , in particular , Cu-K \alpha .
By studying their spatial and temporal variations , in addition to incorporating calibration observations , we refined the absolute energy scale of the detector to better than 150 \ > \textrm { km s } ^ { -1 } at the Fe-K line , a large improvement over the nominal calibration accuracy of 550 \ > \textrm { km s } ^ { -1 } .
With our calibration , we mapped the bulk motions over much of the central 1200 and 800 kpc of Perseus and Coma , respectively , in spatial regions down to 65 and 140 kpc size .
We cross-checked our procedure by comparing our measurements with those found in Perseus by Hitomi for an overlapping 65 kpc square region , finding consistent results .
For Perseus , there is a relative line-of-sight velocity increase of 480 \pm 210 \ > \textrm { km s } ^ { -1 } ( 1 \sigma ) at a radius of 250 kpc east of the nucleus .
This region is associated with a cold front , providing direct evidence of the ICM sloshing in the cluster potential well .
Assuming the intrinsic distribution of bulk motions is Gaussian , its width is 214 \pm 85 \ > \textrm { km s } ^ { -1 } , excluding systematic uncertainties .
Removing the sloshing region , this is reduced to 20 – 150 \ > \textrm { km s } ^ { -1 } , which is similar in magnitude to the Hitomi line width measurements in undisturbed regions .
In Coma , the line-of-sight velocity of the ICM varies between the velocities of the two central galaxies .
Maps of the gas velocity and metallicity provide clues about the merger history of the Coma , with material to the north and east of the cluster core having a velocity similar to NGC 4874 , while that to the south and west has velocities close to NGC 4889 .
Our results highlight the difference between a merging system , such as Coma , where we observe a \sim 1000 km s ^ { -1 } range in velocity , and a relatively relaxed system , such as Perseus , with much weaker bulk motions .