We present the results of ROSAT and XMM-Newton observations of the recurrent ultraluminous X-ray source ( ULX ) NGC~253~ULX1 .
This transient is one of the few ULXs that was detected during several outbursts .
The luminosity reached 1.4 \times 10 ^ { 39 } erg s ^ { -1 } and 0.5 \times 10 ^ { 39 } erg s ^ { -1 } in the detections by ROSAT and XMM-Newton , respectively , indicating a black hole X-ray binary ( BHXRB ) with a mass of the compact object of \geq 11 M _ { \odot } .
In the ROSAT detection NGC~253~ULX1 showed significant variability , whereas the luminosity was constant in the detection from XMM-Newton .
The XMM-Newton EPIC spectra are well-fit by a bremsstrahlung model ( kT = 2.24 keV , N _ { H } = 1.74 \times 10 ^ { 20 } cm ^ { -2 } ) , which can be used to describe a comptonized plasma .
No counterpart was detected in the optical I , R , B , NUV and FUV bands to limits of 22.9 , 24.2 , 24.3 , 22 and 23 mag , respectively , pointing at a XRB with a low mass companion .