We present timing and spectral results of PSR B1937+21 , the fastest known millisecond pulsar ( P \simeq 1.56 ms ) , observed with RXTE .
The pulse profile , detected up to \sim 20 keV , shows a double peak with the main component much stronger than the other .
The peak phase separation is 0.526 \pm 0.002 and the pulsed spectrum over the energy range 2–25 keV is well described by a power law with a photon index equal to 1.14 \pm 0.07 .
We find that the X-ray pulses are closely aligned in phase with the giant pulses observed in the radio band .
This results suggest that giant radio pulses and X-ray pulses originate in the same region of the magnetosphere due to a high and fluctuating electron density that occasionally emits coherently in the radio band .
The X-ray events , however , do not show any clustering in time indicating that no X-ray flares are produced .