A 58-year-old man from Delhi presents to the emergency department with sudden-onset severe headache and neck stiffness. Blood pressure is 180/110 mmHg. Non-contrast CT head shows hyperdense material in the subarachnoid space, predominantly in the basal cisterns and sylvian fissures bilaterally. No intraparenchymal hemorrhage or hydrocephalus is noted. What is the most likely source of bleeding?