Abstract
On August 19 2022 the MAXI/GSC nova alert system reported a possible new outburst from the accreting millisecond X-ray pulsar SAX J1808.4-3658. We performed rapid targeted follow-up observations with NICER on August 19, 14:12 UTC, collecting 1.3 ks of exposure. We clearly detect the source at 217 counts per second in the 0.5-10 keV band, well above the 1~2 ct/s background level.
A power spectrum of our observation shows a strong pulse signal at the expected pulse frequency of ~401 Hz, confirming the beginning of the 10th outburst of SAX J1808.4-3658 since its discovery in 1996 (in't Zand et al. 1998, A&A, 331, L25). We proceeded to fold the data on the binary ephemeris measured during the 2019 outburst (Bult et al. 2020, ApJ, 898, 1). We detected the pulse at >20σ significance, with the local time of passage through the ascending node occurring about 15 seconds later than predicted, assuming a constant pulse frequency.
The 0.5-10 keV energy spectrum is reasonably well described using an absorbed disk blackbody and Comptonization continuum (nthcomp), with a best-fit chi-square of 297 for 128 degrees of freedom. We find an absorption column density of 1e21 cm^-2, a disk temperature of 0.9 keV and a Comptonization power-law index of 1.3. We note that spectrum residuals suggest the presence of emission lines at 1 keV and 7 keV, possibly due to Fe L and K lines. The unabsorbed bolometric (0.1-100 keV) flux is estimated at ~4e-9 erg/s/cm^2.
Further NICER observations of this source are underway. Additional multiwavelength follow-up is encouraged. NICER is a 0.2-12 keV X-ray telescope operating on the International Space Station. The NICER mission and portions of the NICER science team activities are funded by NASA.
A power spectrum of our observation shows a strong pulse signal at the expected pulse frequency of ~401 Hz, confirming the beginning of the 10th outburst of SAX J1808.4-3658 since its discovery in 1996 (in't Zand et al. 1998, A&A, 331, L25). We proceeded to fold the data on the binary ephemeris measured during the 2019 outburst (Bult et al. 2020, ApJ, 898, 1). We detected the pulse at >20σ significance, with the local time of passage through the ascending node occurring about 15 seconds later than predicted, assuming a constant pulse frequency.
The 0.5-10 keV energy spectrum is reasonably well described using an absorbed disk blackbody and Comptonization continuum (nthcomp), with a best-fit chi-square of 297 for 128 degrees of freedom. We find an absorption column density of 1e21 cm^-2, a disk temperature of 0.9 keV and a Comptonization power-law index of 1.3. We note that spectrum residuals suggest the presence of emission lines at 1 keV and 7 keV, possibly due to Fe L and K lines. The unabsorbed bolometric (0.1-100 keV) flux is estimated at ~4e-9 erg/s/cm^2.
Further NICER observations of this source are underway. Additional multiwavelength follow-up is encouraged. NICER is a 0.2-12 keV X-ray telescope operating on the International Space Station. The NICER mission and portions of the NICER science team activities are funded by NASA.
| Original language | English |
|---|---|
| Publication date | 19 Aug 2022 |
| Publication status | Published - 19 Aug 2022 |
| Series | The Astronomer's telegram |
|---|---|
| Number | ATel #15559 |
Keywords
- X-ray
- Binary
- Neutron Star
- Transient
- Pulsar