NewXMM-Newtonobservation of the thermally emitting isolated neutron star 2XMM J104608.7-594306

Abstract

Context. The isolated neutron star (INS) 2XMM J104608.7-594306 is one of the only two to be discovered through their thermal emission since the ROSAT era. Possibly a remnant of a former generation of massive stars in the Carina nebula, the exact nature of the source is unclear, and it might be unique amongst the several classes of Galactic INSs. Aims. In a first dedicated XMM-Newton observation of the source, we found intriguing evidence of a very fast spin period of P ∼ 18.6 ms at the 4σ confidence level. Moreover, spectral features in absorption have also been identified. We re-observed 2XMM J104608.7-594306 with XMM-Newton to better characterise the spectral energy distribution of the source, confirm the candidate spin period, and possibly constrain the pulsar spin-down. Methods. We used the two XMM-Newton observations of 2XMM J104608.7-594306 to perform detailed timing and spectral X-ray analysis. Both the spin-down rate and the energy of the spectral features provide estimates on the neutron star magnetic field, which are crucial for investigating the evolutionary state of the neutron star. Results. Statistically acceptable spectral fits and meaningful physical parameters for the source are only obtained when the residuals at energies 0.55 keV and 1.35 keV are taken into account by the spectral modelling. While the former can result from the inhomogeneous temperature distribution on the surface of the neutron star or can be related to a local overabundance of oxygen in the Carina nebula, the one at 1.35 keV is only satisfactorily accounted for by invoking a line in absorption. In this case, the best-fit neutron star atmosphere models constrain the hydrogen column density, the effective temperature, and the luminosity of the source within N H = (2.5−3.3) × 10 21 cm −2 , T eff = (6−10) × 10 5 K, and L X = (1.1−7.4) × 10 32 erg s −1 . The implied distance is consistent with a location in (or in front of) the Carina nebula, and radiation radii are compatible with emission originating on most of the surface. Non-thermal X-ray emission is ruled out at levels above 0.5% (3σ) of the source luminosity. Unfortunately, the second XMM-Newton observation proved inconclusive in terms of confirming (discarding) the fast candidate spin, providing an upper limit on the pulsed fraction of the source that is very close to the limiting sensitivity for detecting the modulation found previously. Conclusions. In the absence of an unambiguous period determination and an estimate of the magnetic field, the nature of the source remains open to interpretation. Its likely association with the Carina cluster and its overall spectral properties (only shared by a handful of other peculiar INSs) disfavour a standard evolutionary path or one in which the source was previously recycled by accretion in a binary system. The star 2XMM J104608.7-594306 may be similar to Calvera (1RXS J141256.0+792204), a neutron star for which the scenario of an evolved anti-magnetar has been discussed. A better age estimate and deeper radio and γ-ray l...

Candidate isolated neutron stars in the 4XMM-DR10 catalog of X-ray sources

“…Strong spectral variability is also visible in the magnetar SGR J0418+5729 (Guillot et al 2015). All the other INSs (black stars) have steady HRs: We found the central compact object 1E 1207.4−5209 (Bignami et al 2003), the high magnetic field pulsar J0726−2612 (Rigoselli et al 2019a), two thermally-emitting pulsars (McGowan et al 2006;Ng et al 2007), the INS candidate of Pires et al (2015) and, as expected, six of the seven known XDINSs (the lack of RX J0420.0−5022 is due to the fact that its spectrum is so soft that its HR 2 had error >0.1). We also found PSR J1400−1431, a binary system composed of a millisecond pulsar and a white dwarf (Swiggum et al 2017).…”

“…In fact, further searches using these satellites led only to a small increase of this sample. For example, Pires et al (2009b) analyzed the 2XMMp catalog of serendipitous sources discovered with XMM-Newton and found a few possible INS candidates, among which the most promising is 2XMM J104608.7−594306 (Pires et al 2009a(Pires et al , 2015.…”

Follow‐up of isolated neutron star candidates from the eROSITA survey

“…Therefore, excluding the bright end of the population, which was already probed by ROSAT, around 27% of the eROSITA INSs are still sufficiently bright in X‐rays to be identified and investigated in the years following the eROSITA survey. Interestingly, these sources will be at a similar flux level (on average, f X = 1.26(29) × 10 − 13 erg s − 1 cm − 2 ) as that of the only thermally emitting INS identified to‐date in non‐ROSAT data, namely 2XMM J104608.7‐594306 (Pires et al , ). In this regard, the fact that the XMM‐Newton and Chandra observatories show good prospects for the next decade of operation is timely news for neutron star physics as well.…”

“…Cooling–age diagram for different isolated neutron star ( INS ) groups (data points; see legend). Details can be found in Pires et al (). The standard neutrino candle is shown as a solid black line; effects of proton superfluidity in the core and of a light‐element accreted envelope are shown in turquoise.…”

Atmospheres and radiating surfaces of neutron stars with strong magnetic fields

“…2XMM J104608.7-594306 is a thermally emitting INS, but unlike the Magnificent Seven it has a short spin period of only 18.6 ms. Its XMM-Newton spectrum has been analyzed in [131], using the blackbody model and hydrogen atmosphere models NSA and NSMAXG. Statistically acceptable spectral fits and meaningful physical parameters for the source are only obtained when the purely thermal spectrum is modified by absorption lines at E ≈ 0.55 keV and 1.3 keV.…”