INTEGRAL follow up of the new outburst from SAX J1808.4-3658

ATel #15601; C. Ferrigno (ISDC), A. Sanna (Univ. of Cagliari), E. Bozzo, V. Savchenko (ISDC), L. Burderi, A. Riggio (Univ. of Cagliari), T. Di Salvo (Univ. of Palermo), D. Altamirano (Univ. of Southampton), J. Chenevez (DTU Space), E. Kuulkers (ESA-ESTEC), C. Sanchez-Fernandez (ESA-ESAC)
on 8 Sep 2022; 23:58 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Andrea Sanna (andrea.sanna@dsf.unica.it)

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On August 19, 2022, the MAXI/GSC nova alert system reported X-ray activity in the direction of the known accreting millisecond X-ray pulsar SAX J1808.4-3658 (Atel #15563). A rapid NICER follow-up observation performed on August 19, 14:12 UTC, detected X-ray pulsations at the expected pulse frequency of ~401 Hz (Atel #15559) hence confirming the beginning of a new outburst of the source. The INTEGRAL observations reported here were carried out in the period August 22, 00:39 UTC - August 23, 08:01 UTC.

The combined JEM-X1, JEM-X2 (3-20 keV), and IBIS/ISGRI (30-150 keV) energy spectrum (on-source good time ~70 ks) is reasonably well described by a power-law, with a best-fit chi-square of 25 for 48 degrees of freedom. We find a power-law index of 2.36±0.08, and a 1-100 keV flux of 1.2×10^-9erg/s/cm^-2.

We searched for thermonuclear type-I bursts in the JEM-X light curve (3-20 keV) extracted at 4-s time bins, but did not find any.

To search for pulsations, we split the IBIS/ISGRI events from 25 to 60 keV into two sets based on the pixel illumination fraction (PIF) by the source: pixels with PIF>0.5 are assigned to the source, otherwise to the background. We accumulated a pulse profile in every single point using the ephemeris obtained from the publicly available NICER observations. Then, we counted the number of pixels with PIF >0.5 and PIF <0.5 and normalized the pulse profiles. We summed the pulse profiles propagating the uncertainties and fit a model background + continuum * ( 1 + A sin (2 π ( phase - phase0)), where the phase is computed from 0 to 1. Using the ultranest MCMC sampling with Jeffreys priors on a continuum and A, we obtain a 90% upper limit on the pulsed fraction A of 3% compatible with the value reported from previous outbursts (see e.g., Falanga et al., ApJ, 2007, 661).

We thank the INTEGRAL team for promptly scheduling the reported observations.