Citation V/Ultra/Encore (C560)
Flight Crew Air Transport Pilot (ATP)
Distraction
Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural Published Material / Policy; Deviation / Discrepancy - Procedural FAR
Air Traffic Control Issued New Clearance; Flight Crew Landed As Precaution
Aircraft
While enroute at 41;000 ft.; both the crew and passenger noticed a loud pop.  There was also some 'movement'. Everything seemed OK. After a few moments; it was noted that the cabin was rising around 500 fpm. We assumed that the pop was associated with a pressurization issue. Immediately requested 25;000 ft with ATC. Started an expeditious decent. Pilot Monitoring (Pilot In Command) donned his O2 mask. ATC was having trouble getting lower. Did not declare an emergency; but made it clear we needed to get lower immediately.  ATC provided us a turn and decent to 11;000 ft. We continued a quick decent with speed brakes deployed. The cabin was still staying around a 500 fpm climb. The highest the cabin ever got was 9000 ft as we were reaching 10;000 ft. With the uncertainty of the issue; the weather deviations in front of us at the lower altitude and passenger anxiety; we decided to divert. Cabin altitude descended with us. Arrival; approach and landing were uneventful. Looking back; the one item I missed was donning my mask in case the situation escalated. It was a very slow leak that demanded immediate attention.  Not an emergency; and we descended as quick as we could. The cabin was never out of control. But if it had let loose; even with pilot monitoring having his mask; I should of had mine also; just in case. We got busy with trying to diagnose the issue; dealing with ATC and our passenger was somewhat distracting with questions.
Citation crew experienced a slow loss of pressurization and immediately requested and received a lower altitude from ATC and diverted. Pilot monitoring failed to don his O2 mask.