Discovery of high frequency quasi-periodic oscillations in 4U1915-05

The type I X-ray burster and dipper 4U1915-05 (also known as XB1916-053) wa s monitored by the Rossi X-ray Timing Explorer between February and October , 1996. The source was observed in various spectral states; the highest lum inosity state (L-X similar to 1.5 x 10(37) ergs s(-1), 10 kpc, 1-20 keV) is associated with a soft spectrum, whereas for the lower luminosity state (d own to similar to 5 x 10(36)ergs s(-1)) the spectrum is significantly harde r. 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 luminosit y was similar to 8 x 10(36)ergs s(-1). The QPO frequency ranges from 600 Hz up to similar to 1000 nz, with typical Full Width at Half Maximum (FWHM) s imilar to 50 - 100 Hz and Root Mean Squared (RMS) values similar to 15%. fn addition, by using the "shift and add" technique, we have detected a twin HFQPO (5.5 sigma level) separated from the main peak by similar to 355 Hz. 4U1915-05 is the eighth Atoll source displaying simultaneous twin HFQPOs, B ased on current knowledge of HFQPO sources, our observations suggest that 4 U1915-05 might contain a 2.8 (or 5.6) millisecond rotating neutron star.