Abstract
MAXI/GSC reported on an X-ray outburst from M15 on 2022 September 3 (ATel #15586). Follow-up observations by Swift/XRT on the same day at 13:09 UT confirmed the outburst but were unable to distinguish whether it originates from one of the known nearby X-ray sources AC 211 or M15 X-2 (ATel #15589). However, we note that the outburst is not compatible with the known very faint X-ray transient M15 X-3, which is over 20 arcsec away from those other two known sources. NICER observed the M15 field starting on 2022 September 4 at 17:34 UT for a total exposure time of 1.6 ks. The average 0.3-10.0 keV persistent count rate is 575.0 +/- 0.7 c/s.
NICER observed one type I (thermonuclear) X-ray burst from the field. The burst lasted for about 50 seconds with a peak count rate of about 2700 c/s in the 0.5-10 keV band. The burst decay can be described by a cooling blackbody, confirming its thermonuclear origin. We measured the peak bolometric flux of the X-ray burst to be 1.4 × 10-8 erg/s/cm2. No signs of photospheric radius expansion were observed. Assuming a distance of 10.9 kpc, the peak flux corresponds to a luminosity of 2 × 1038 erg/s, below the Eddington limit of the neutron star for a hydrogen-rich fuel composition. The pre-burst persistent emission is well described by an absorbed power-law spectral model with a blackbody component and absorption column density 1.49 (+0.13, -0.13) × 1021 cm-2, photon index of 1.78 (+0.04, -0.04), and blackbody temperature of 0.51 (+0.06, -0.04) keV (chi-squared/d.o.f. = 450/470). The 0.5-10 keV unabsorbed flux (pre-burst) from the total field is 1.6 × 10-9 erg/s/cm2. All uncertainties reported are 90% confidence limits.
As NICER is a non-imaging instrument, we advise caution on using the spectral parameters at face value for any individual M15 source. AC 211 is a known accretion disk corona source, where the neutron star primary is obscured by the edge-on accretion disk. Past studies suggest AC 211 is unlikely to be the source of type I X-ray bursts (White & Angelini 2001, ApJ, 561, 101; Kuulkers et al. 2003, ApJ, 399, 663); however, this cannot be completely ruled out. Meanwhile M15 X-2 is generally at least two times brighter than AC 211 (White & Angelini 2001, ApJ, 561, 101; Panurach et al. 2021, ApJ, 923, 88).
Coherent pulsation searches over the 0.3-2.0 keV and 2.0-10.0 keV energy ranges with averaged power spectra and acceleration searches did not detect any significant coherent periodicity. The 0.3-10.0 keV averaged power spectrum (64 s segments) is consistent with white noise (i.e., flat). We also did not detect burst oscillations from the source.
NICER will continue to observe the M15 field. We encourage further observations of this field with other observatories.
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.
NICER observed one type I (thermonuclear) X-ray burst from the field. The burst lasted for about 50 seconds with a peak count rate of about 2700 c/s in the 0.5-10 keV band. The burst decay can be described by a cooling blackbody, confirming its thermonuclear origin. We measured the peak bolometric flux of the X-ray burst to be 1.4 × 10-8 erg/s/cm2. No signs of photospheric radius expansion were observed. Assuming a distance of 10.9 kpc, the peak flux corresponds to a luminosity of 2 × 1038 erg/s, below the Eddington limit of the neutron star for a hydrogen-rich fuel composition. The pre-burst persistent emission is well described by an absorbed power-law spectral model with a blackbody component and absorption column density 1.49 (+0.13, -0.13) × 1021 cm-2, photon index of 1.78 (+0.04, -0.04), and blackbody temperature of 0.51 (+0.06, -0.04) keV (chi-squared/d.o.f. = 450/470). The 0.5-10 keV unabsorbed flux (pre-burst) from the total field is 1.6 × 10-9 erg/s/cm2. All uncertainties reported are 90% confidence limits.
As NICER is a non-imaging instrument, we advise caution on using the spectral parameters at face value for any individual M15 source. AC 211 is a known accretion disk corona source, where the neutron star primary is obscured by the edge-on accretion disk. Past studies suggest AC 211 is unlikely to be the source of type I X-ray bursts (White & Angelini 2001, ApJ, 561, 101; Kuulkers et al. 2003, ApJ, 399, 663); however, this cannot be completely ruled out. Meanwhile M15 X-2 is generally at least two times brighter than AC 211 (White & Angelini 2001, ApJ, 561, 101; Panurach et al. 2021, ApJ, 923, 88).
Coherent pulsation searches over the 0.3-2.0 keV and 2.0-10.0 keV energy ranges with averaged power spectra and acceleration searches did not detect any significant coherent periodicity. The 0.3-10.0 keV averaged power spectrum (64 s segments) is consistent with white noise (i.e., flat). We also did not detect burst oscillations from the source.
NICER will continue to observe the M15 field. We encourage further observations of this field with other observatories.
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 | 6 Sept 2022 |
| Publication status | Published - 6 Sept 2022 |
| Series | The Astronomer's telegram |
|---|---|
| Number | #15593 |
Keywords
- X-ray
- Neutron Star
- Transient,
- Pulsars