ABSTRACT The type I X-ray burster and dipper 4U1915-05 ( also known as XB1916-053 ) was monitored by the Rossi X-ray Timing Explorer between February and October , 1996 .
The source was observed in various spectral states ; the highest luminosity state ( L _ { X } \sim 1.5 \times 10 ^ { 37 } ergs s ^ { -1 } , 10 kpc , 1-20 keV ) is associated with a soft spectrum , whereas for the lower luminosity state ( down to \sim 5 \times 10 ^ { 36 } ergs s ^ { -1 } ) the spectrum is significantly harder .
Using the high time resolution data provided by the Proportional Counter Array ( PCA ) , we have discovered High-Frequency Quasi-Periodic Oscillations ( HFQPOs ) in the persistent X-ray emission of 4U1915-05 while its luminosity was \sim 8 \times 10 ^ { 36 } ergs s ^ { -1 } .
The QPO frequency ranges from 600 Hz up to \sim 1000 Hz , with typical Full Width at Half Maximum ( FWHM ) \sim 50 - 100 Hz and Root Mean Squared ( RMS ) values \sim 15 % .
In addition , by using the “ shift and add ” technique , we have detected a twin HFQPO ( 5.5 \sigma level ) separated from the main peak by \sim 355 Hz .
4U1915-05 is the eighth Atoll source displaying simultaneous twin HFQPOs .
Based on current knowledge of HFQPO sources , our observations suggest that 4U1915-05 might contain a 2.8 ( or 5.6 ) millisecond rotating neutron star .