Title: “The accreting millisecond pulsar SAX J1808.4-3658 during its 2022 outburst: hints of an orbital shrinking”

Seminar by Giulia Illiano, PhD – Astronomical Observatory of Rome (INAF-OAR)

Tuesday 7th of March, at 2.30 pm

Abstract: “Accreting millisecond pulsars (AMSPs) are rapidly rotating neutron stars hosted in a tight binary system with a low-mass companion. Their millisecond periods result from a Gyr-long phase in which old radio pulsars are spun up by accreting matter from a donor via a Roche lobe overflow. SAX J1808.4-3658 was the first AMSP discovered in 1998. Since then, the source has undergone ten ~1-month-long outbursts with ~2-3 years recurrence, making it the most thoroughly investigated of its type. When the onset of a new outburst was detected in August 2022, we performed a multi-wavelength campaign with three X-ray telescopes (XMM-Newton, NuSTAR, and NICER), the fast optical photometer TNG/SiFAP2, and the HST. I will present a coherent timing analysis of X-ray pulsations during this latest outburst, confirming the long-term spin-down rate compatible with the expected energy losses from a rotating magnetic dipole. This may indicate that a radio pulsar is active in the system during quiescence. From the long-term orbital evolution, for the first time in the last twenty years we found hints of an orbital decay. During the previous outburst in 2019, optical and UV pulsations from this source were discovered: I will also discuss optical observations performed during the 2022 outburst and I will compare the results of these two latest outbursts.”

Google Meet Link: https://meet.google.com/ots-rija-hyc