NuSTAR observation of the Supergiant Fast X-ray Transient IGRJ11215-5952 during its 2017 outburst
Journal
Date Issued
2020
Author(s)
•
Postnov, K.
•
Tiengo, A.
•
Esposito, P.
•
•
•
Abstract
We report on the results of a NuSTAR observation of the Supergiant Fast X-ray
Transient pulsar IGRJ11215-5952 during the peak of its outburst in June 2017.
IGRJ11215-5952 is the only SFXT undergoing strictly periodic outbursts, every
165 days. NuSTAR caught several X-ray flares, spanning a dynamic range of 100,
and detected X-ray pulsations at 187.0 s, consistent with previous
measurements. The spectrum from the whole observation is well described by an
absorbed power-law (with a photon index of 1.4) modified, above 7 keV, by a
cutoff with an e-folding energy of 24 keV. A weak emission line is present at
6.4 keV, consistent with Kalpha emission from cold iron in the supergiant wind.
The time-averaged flux is 1.5E-10 erg/cm2/s (3-78 keV, corrected for the
absorption), translating into an average luminosity of about 9E35 erg/s (1-100
keV, assuming a distance of 6.5 kpc). The NuSTAR observation allowed us to
perform the most sensitive search for cyclotron resonant scattering features in
the hard X-ray spectrum, resulting in no significant detection in any of the
different spectral extractions adopted (time-averaged, temporally-selected,
spin-phase-resolved and intensity-selected spectra). The pulse profile showed
an evolution with both the energy (3-12 keV energy range compared with 12-78
keV band) and the X-ray flux: a double peaked profile was evident at higher
fluxes (and in both energy bands), while a single peaked, sinusoidal profile
was present at the lowest intensity state achieved within the NuSTAR
observations (in both energy bands). The intensity-selected analysis allowed us
to observe an anti-correlation of the pulsed fraction with the X-ray
luminosity. The pulse profile evolution can be explained by X-ray photon
scattering in the accreting matter above magnetic poles of a neutron star at
the quasi-spherical settling accretion stage.
Volume
638
Start page
A71
Issn Identifier
0004-6361
Ads BibCode
2020A&A...638A..71S
Rights
open.access
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