A 58-year-old man from Delhi presents to the emergency department with sudden-onset severe headache, neck stiffness, and photophobia. On examination, he is drowsy with a blood pressure of 180/110 mmHg. Non-contrast CT (NCCT) head shows hyperdense material in the basal cisterns and sylvian fissures bilaterally, with no intraparenchymal hemorrhage. What is the most likely diagnosis and what is the next imaging modality of choice to identify the source of bleeding?