NICER observations of the new outburst from Aql X-1
ATel #16153; Jeroen Homan (Eureka Scientific), Kevin Alabarta, David M. Russell, M. Cristina Baglio, Payaswini Saikia, D. M. Bramich (NYU Abu Dhabi), Yasmine Heikal (UAEU, Al Ain), and Fraser Lewis (Faulkes Telescope Project & Astrophysics Research Institute, LJMU)
on 26 Jul 2023; 10:29 UT
Credential Certification: Jeroen Homan (jeroenhoman@icloud.com)
Subjects: X-ray, Neutron Star, Transient
Following reports of optical and X-ray outburst activity from the neutron-star transient Aql X-1 (ATel #16147), we have observed the source with NICER. The observation started on July 24 14:15 at UTC and ended on July 25 at 02:50 UTC, with a total exposure of 3.4 ks.
The source is clearly detected, with 0.5-10 keV count rates increasing from ~90-95 cts/s (for 52 detectors) at the beginning of the observation to 110-115 cts/s at end. The time-averaged spectrum (0.5-10 keV) was fitted well by an absorbed (nH ~6e21 cm^-2) continuum consisting of a power law (index ~1.66) and a black body (kT ~0.67 keV). The unabsorbed 0.5-10 keV flux was ~4.9e-10 erg/cm^2/s, with the power law contributing ~80% of the flux. For a distance of 5.5 kpc this corresponds to a luminosity of ~1.8e36 erg/s. The 0.5-10 keV power spectrum shows strong noise at low frequencies (~20% rms; 1/128 - 100 Hz). No QPOs or pulsations were detected in the power spectrum and no type-I X-ray bursts were observed in the light curve. The variability and spectral properties are consistent with the source being in a hard state.
NICER will continue observing the evolution of the outburst on a daily basis. We encourage observations at other wavelengths as well.
We thank the NICER team for rapidly executing our ToO program.
