A search for thermally emitting isolated neutron stars in the 2XMMp catalogue

📝 Abstract

The relatively large number of nearby radio-quiet and thermally emitting isolated neutron stars (INSs) discovered in the ROSAT All-Sky Survey, dubbed the ``Magnificent Seven’’ (M7), suggests that they belong to a formerly neglected major component of the overall INS population. So far, attempts to discover similar INSs beyond the solar vicinity failed to confirm any reliable candidate. The EPIC cameras onboard the XMM-Newton satellite allow to efficiently search for new thermally emitting INSs. We used the 2XMMp catalogue to select sources with no catalogued candidate counterparts and with X-ray spectra similar to those of the M7, but seen at greater distances and thus undergoing higher interstellar absorptions. Identifications in more than 170 astronomical catalogues and visual screening allowed to select fewer than 30 good INS candidates. In order to rule out alternative identifications, we obtained deep ESO-VLT and SOAR optical imaging for the X-ray brightest candidates. We report here on the optical follow-up results of our search and discuss the possible nature of 8 of our candidates. A high X-ray-to-optical flux ratio together with a stable flux and soft X-ray spectrum make the brightest source of our sample, 2XMM J104608.7-594306, a newly discovered thermally emitting INS. The X-ray source 2XMM J010642.3+005032 has no evident optical counterpart and should be further investigated. The remaining X-ray sources are most probably identified with CVs and AGN, as inferred from the colours and flux ratios of their likely optical counterparts. Beyond the finding of new thermally emitting INSs, our study aims at constraining the space density of this Galactic population at great distances and at determining whether their apparently high density is a local anomaly or not.

💡 Analysis

The relatively large number of nearby radio-quiet and thermally emitting isolated neutron stars (INSs) discovered in the ROSAT All-Sky Survey, dubbed the ``Magnificent Seven’’ (M7), suggests that they belong to a formerly neglected major component of the overall INS population. So far, attempts to discover similar INSs beyond the solar vicinity failed to confirm any reliable candidate. The EPIC cameras onboard the XMM-Newton satellite allow to efficiently search for new thermally emitting INSs. We used the 2XMMp catalogue to select sources with no catalogued candidate counterparts and with X-ray spectra similar to those of the M7, but seen at greater distances and thus undergoing higher interstellar absorptions. Identifications in more than 170 astronomical catalogues and visual screening allowed to select fewer than 30 good INS candidates. In order to rule out alternative identifications, we obtained deep ESO-VLT and SOAR optical imaging for the X-ray brightest candidates. We report here on the optical follow-up results of our search and discuss the possible nature of 8 of our candidates. A high X-ray-to-optical flux ratio together with a stable flux and soft X-ray spectrum make the brightest source of our sample, 2XMM J104608.7-594306, a newly discovered thermally emitting INS. The X-ray source 2XMM J010642.3+005032 has no evident optical counterpart and should be further investigated. The remaining X-ray sources are most probably identified with CVs and AGN, as inferred from the colours and flux ratios of their likely optical counterparts. Beyond the finding of new thermally emitting INSs, our study aims at constraining the space density of this Galactic population at great distances and at determining whether their apparently high density is a local anomaly or not.

📄 Content

arXiv:0906.4966v1 [astro-ph.GA] 26 Jun 2009 Astronomy & Astrophysics manuscript no. pires˙2xmm c⃝ESO 2018 June 1, 2018 A search for thermally emitting isolated neutron stars in the 2XMMp catalogue A. M. Pires1,2, C. Motch2, and E. Janot-Pacheco1 ⋆ 1 Instituto de Astronomia, Geof´ısica e Ciˆencias Atmosf´ericas, Universidade de S˜ao Paulo, R. do Mat˜ao 1226, 05508-090 S˜ao Paulo, Brazil, e-mail: apires@astro.iag.usp.br 2 CNRS, Universit´e de Strasbourg, Observatoire Astronomique, 11 rue de l’Universit´e, 67000 Strasbourg, France Received …; accepted … ABSTRACT The relatively large number of nearby radio-quiet and thermally emitting isolated neutron stars (INSs) discovered in the ROSAT All- Sky Survey, dubbed the “Magnificent Seven”, suggests that they belong to a formerly neglected major component of the overall INS population. So far, attempts to discover similar INSs beyond the solar vicinity failed to confirm any reliable candidate. The good positional accuracy and soft X-ray sensitivity of the EPIC cameras onboard the XMM-Newton satellite allow to efficiently search for new thermally emitting INSs. We used the 2XMMp catalogue to select sources with no catalogued candidate counterparts and with X-ray spectra similar to those of the Magnificent Seven, but seen at greater distances and thus undergoing higher interstellar absorptions. Identifications in more than 170 astronomical catalogues and visual screening allowed to select fewer than 30 good INS candidates. In order to rule out alternative identifications, we obtained deep ESO-VLT and SOAR optical imaging for the X-ray brightest candidates. We report here on the optical follow-up results of our search and discuss the possible nature of 8 of our candidates. A high X-ray-to-optical flux ratio together with a stable flux and soft X-ray spectrum make the brightest source of our sample, 2XMM J104608.7-594306, a newly discovered thermally emitting INS. The X-ray source 2XMM J010642.3+005032 has no evident optical counterpart and should be further investigated. The remaining X-ray sources are most probably identified with cataclysmic variables and active galactic nuclei, as inferred from the colours and flux ratios of their likely optical counterparts. Beyond the finding of new thermally emitting INSs, our study aims at constraining the space density of this Galactic population at great distances and at determining whether their apparently high density is a local anomaly or not. Key words. stars: neutron – X-rays: individual: 2XMM J104608.7-594306 – Catalogs

1. Introduction Around ten years ago, seven X-ray bright thermally emitting and radio-quiet isolated neutron stars (INSs) sharing similar prop- erties were identified in the ROSAT All-Sky Survey data. This group is now commonly referred to as the “Magnificent Seven” (or M7, for simplicity; see Haberl 2007, for a review) for the reasons that they clearly stand apart from the population of rotation-powered radio pulsars. In spite of many searches for similar objects in the ROSAT data (e.g. Rutledge et al. 2003; Chieregato et al. 2005; Ag¨ueros et al. 2006), no new candidate was identified since the discovery of the last member, RX J2143.0+0654 (Zampieri et al. 2001). A soft blackbody spectrum undergoing low interstellar ab- sorption (kT ∼40 – 100 eV and NH ∼few 1020 cm−2) is com- mon to the seven sources, as well as the absence of a non- thermal component extending towards higher energies. We note, however, the detection in the optical/UV of a possible non- thermal power-law component in the spectrum of RX J0720.4- 3125 (Kaplan et al. 2003). Very faint optical counterparts with Send offprint requests to: A. M. Pires ⋆Based on the public data archive of XMM-Newton an ESA Science Mission with instruments and contributions directly funded by the ESA Member States and the USA (NASA) and a Chandra Legacy programme. Optical observations were performed at the European Southern Observatory, Paranal, Chile, under programme ID 079.D- 0633(A), and at the Southern Astrophysical Research Telescope, Cerro Pach´on, Chile. blue magnitudes of mB ∼25 – 28 were detected for several sources, implying high logarithmic X-ray-to-optical flux ratios of ∼4 – 5. Relatively to radio pulsars, the neutron star spin peri- ods are longer and distributed in a much narrower range, P ∼3 – 10 s. Six of the sources show sinusoidal X-ray pulsations with pulsed fractions between ∼1% and 18%. The M7 are believed to be nearby (d <∼500 pc), as inferred from the distribution of the interstellar medium in the line-of-sight and the equiva- lent hydrogen column densities measured in their X-ray spec- tra (Posselt et al. 2007). Furthermore, HST parallaxes are avail- able for the two X-ray brightest members (Kaplan et al. 2002,

2007. with results largely consistent with those estimated by Posselt et al. (2007). The seven sources do not show either per- sistent or transient radio emission to a rather sensitive limiting flux of ∼10 µJy (Kondratiev et al. 2008) and ar

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