The ultraluminous X-ray source ( ULX ) Holmberg II X-1 has been observed over 4 months in 2009/2010 by the Swift observatory .
The source luminosity varied by a factor of up to 14 , reaching a maximum 0.3–10 keV luminosity of \sim 3.0 \times 10 ^ { 40 } \mathrm { erg s ^ { -1 } } .
The spectral properties do not vary much over these 4 months , with only a slight monotonic increase of the hardness ratio with the count rate .
This means that the erratic flaring activity of the source is not associated with spectral changes , as seen in other ULXs .
Conversely , comparison with data obtained by Swift in 2006 shows a completely different picture : while at a luminosity also seen in the 2009/2010 data , the source appears with a hard spectrum .
Thus , it appears that , as in Galactic black hole binaries , spectral states in this ULX are not determined only by the X-ray luminosity .