The observational manifestation of a neutron star is strongly connected with
the properties of its magnetic field. During the star's lifetime, the field
strength and its changes dominate the thermo-rotational evolution and the
source phenomenology across the electromagnetic spectrum. Signatures of
magnetic field evolution are best traced among elusive groups of X-ray emitting
isolated neutron stars (INSs), which are mostly quiet in the radio and
$\gamma$-ray wavelengths. It is thus important to investigate and survey INSs
in X-rays in the hope of discovering peculiar sources and the long-sought
missing links that will help us to advance our understanding of neutron star
evolution. The Extended R\"ontgen Survey with an Imaging Telescope Array
(\eROS), the primary instrument on the forthcoming Spectrum-RG mission, will
scan the X-ray sky with unprecedented sensitivity and resolution. The survey
has thus the unique potential to unveil the X-ray faint end of the neutron star
population and probe sources that cannot be assessed by standard pulsar
surveys.Comment: 5 pages, 3 figures; to appear in the proceedings of "Physics of
  Neutron Stars 2017" eds. G.G. Pavlov, J.A. Pons, P.S. Shternin & D.G.
  Yakovle

Pires, Adriana Mancini

English

arXiv

The observational manifestation of a neutron star is strongly connected with the properties of its magnetic field. During the star's lifetime, the field strength and its changes dominate the thermo-rotational evolution and the source phenomenology across the electromagnetic spectrum. Signatures of magnetic field evolution are best traced among elusive groups of X-ray emitting isolated neutron stars (INSs), which are mostly quiet in the radio and γ-ray wavelengths. It is thus important to investigate and survey INSs in X-rays in the hope of discovering peculiar sources and the long-sought missing links that will help us to advance our understanding of neutron star evolution. The Extended Röntgen Survey with an Imaging Telescope Array (eROSITA), the primary instrument on the forthcoming Spectrum-RG mission, will scan the X-ray sky with unprecedented sensitivity and resolution. The survey has thus the unique potential to unveil the X-ray faint end of the neutron star population and probe sources that cannot be assessed by standard pulsar surveys

Pires, A.M.

Repositorium für Naturwissenschaften und Technik  (TIB Hannover)

Journal of Physics: Conference SeriesPAPER • OPEN ACCESSThe missing links of neutron star evolution in theeROSITA all-sky X-ray surveyTo cite this article: A M Pires 2017 J. Phys.: Conf. Ser. 932 012008 View the article online for updates and enhancements.You may also likeThe X-Ray and Radio Loud Fast BlueOptical Transient AT2020mrf: Implicationsfor an Emerging Class of Engine-drivenMassive Star ExplosionsYuhan Yao, Anna Y. Q. Ho, PavelMedvedev et al.-Probing the Cross-power of UnresolvedCosmic Infrared and X-Ray Backgroundswith Upcoming Space MissionsA. Kashlinsky, R. G. Arendt, N. Cappellutiet al.-Implications from Late-time X-RayDetections of Optically Selected TidalDisruption Events: State Changes,Unification, and Detection RatesP. G. Jonker, N. C. Stone, A. Generozovet al.-This content was downloaded from IP address 89.245.22.239 on 20/04/2023 at 05:521Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distributionof this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.Published under licence by IOP Publishing Ltd1234567890International Conference Physics of Neutron Stars - 2017. 50 years after. IOP PublishingIOP Conf. Series: Journal of Physics: Conf. Series 932 (2017) 012008  doi :10.1088/1742-6596/932/1/012008The missing links of neutron star evolution in theeROSITA all-sky X-ray surveyA M PiresLeibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam,GermanyE-mail: apires@aip.deAbstract. The observational manifestation of a neutron star is strongly connected withthe properties of its magnetic field. During the star’s lifetime, the field strength and itschanges dominate the thermo-rotational evolution and the source phenomenology across theelectromagnetic spectrum. Signatures of magnetic field evolution are best traced among elusivegroups of X-ray emitting isolated neutron stars (INSs), which are mostly quiet in the radio andγ-ray wavelengths. It is thus important to investigate and survey INSs in X-rays in the hopeof discovering peculiar sources and the long-sought missing links that will help us to advanceour understanding of neutron star evolution. The Extended Röntgen Survey with an ImagingTelescope Array (eROSITA), the primary instrument on the forthcoming Spectrum-RG mission,will scan the X-ray sky with unprecedented sensitivity and resolution. The survey has thus theunique potential to unveil the X-ray faint end of the neutron star population and probe sourcesthat cannot be assessed by standard pulsar surveys.1. IntroductionAn isolated neutron star (INS) radiates at the expense of its rotational, thermal, and magneticenergy. The emission is produced through various mechanisms, with photon energies covering theentire electromagnetic spectrum. Fundamental properties inherited at birth (mass, composition,spin rate, magnetic field, etc), as well as their subsequent temporal evolution, determine thedominant emission mechanism at a given age. These, however, are subject to many theoreticaluncertainties and remain poorly constrained by observations.The population of over 2,600 neutron stars observed in our Galaxy is dominated by radiopulsars [1]. Of those, about 50 peculiar X-ray emitting sources are not detected by radio andγ-ray pulsar surveys: they are the young and energetic magnetars, usually identified by theirremarkable spectral properties and bursts of high energy emission [2]; the local group of thermallyemitting middle-aged INSs discovered by ROSAT and dubbed the magnificent seven (M7) [3];and the young and seemingly weakly-magnetised central compact objects (CCOs), which arethermal X-ray sources located near the centre of supernova remnants [4]. These groups areimportant as they provide a privileged view of a variety of emission processes and evolutionarychannels that cannot be probed through the bulk of the pulsar population.On the one hand, this is realised by the correlation of a neutron star’s thermal luminosityand its inferred magnetic field [5], which holds true for sources with B  1013G (Figure 1). Thestandard cooling theory [6] falls short at describing the temperature of these INSs (Figure 2).Whereas for most neutron stars magneto-rotational and thermal evolution proceed almost21234567890International Conference Physics of Neutron Stars - 2017. 50 years after. IOP PublishingIOP Conf. Series: Journal of Physics: Conf. Series 932 (2017) 012008  doi :10.1088/1742-6596/932/1/0120080.1 1 10 100Magnetic field (1014 G)0.010.11101001000Luminosity (1033 erg s-1 cm-2)magnetarpulsarM7Figure 1. Correlation between thermal luminosityand magnetic field intensity for high magnetic fieldINSs (data extracted from the Magnetar OutburstOnline Catalog, http://magnetars.ice.csic.es).1 2 3 4 5 6 7log Age (years)5.56.06.57.0log Temperature (K)magnetarpulsarCCOM7Figure 2. Cooling-age diagram for INSs withthermal emission, in relation to the coolingscenario with no magnetic field decay (solid,dashed, dotted, and dotted-dahsed lines).independently, the cooling of strongly magnetised sources is affected by field decay and magneto-rotational energy dissipation [7, 8]. Evolutionary models that take these effects into account[9] show that strong fields at birth can brake the INS spin to an asymptotic value over arelatively short timescale; moreover, field dissipation keeps the stellar crust hot for a longer timethan expected from the standard cooling theory. While accounting for the spectral and timingproperties of individual sources, the model implies an evolutionary link between magnetars andthe M7.At the opposite end of the magnetic field distribution, CCOs challenge the stereotype of ayoung INS. The very low measured spin-down values of three CCOs (dubbed anti-magnetars)imply very low magnetic field strengths and characteristic ages much older than expected fromtheir association with a supernova remnant. Moreover, the lack of detected pulsations in mostmembers indicates that the atmosphere is weakly magnetised [10]. This contradicts the evidencethat some sources conceal strong magnetic fields [11]. The fact that no old (> 10 kyr) anti-magnetar with similar timing properties is recognised in either radio or X-ray surveys [12, 13]implies that their magnetic field is not constant at secular timescales.CCOs might represent a different outcome of neutron star evolution, where they are recoveringfrom an early episode of field burial by hypercritical accretion [14]. In this scenario, if a largeamount of debris falls back onto the neutron star after the supernova explosion, accretion islikely to occur at such high rates that any field can be hidden [15]. This may delay the onset ofradio pulsar activity for several 1− 100 kyr, until the field can diffuse back to the surface [16].2. Evolutionary scenarios: issues and open questionsThe role played by the decay of the magnetic field in heating the neutron star crust cannot beoverlooked, as it partially explains the observed neutron star diversity. However, even state-of-31234567890International Conference Physics of Neutron Stars - 2017. 50 years after. IOP PublishingIOP Conf. Series: Journal of Physics: Conf. Series 932 (2017) 012008  doi :10.1088/1742-6596/932/1/012008the-art models of field decay are built over uncertain assumptions, concerning, for instance, theinitial field configuration and the level of impurity of the crust [9]. At present, the observedradio and X-ray pulsar population is not sufficient to constrain different hypotheses [17]. Whilethe phenomenology of CCOs has triggered a lively debate over their formation and fate, to ourknowledge no attempt has been made in population synthesis to include anti-magnetars as apossible outcome of neutron star evolution.The paucity of these INS groups has hindered their use in population syntheses on the Galacticscale. While particular aspects of neutron star evolution and emissivity have been investigatedthrough an extensive number of population syntheses of radio pulsars1, overall progress can onlybe hoped for when the various groups are unified in a ‘grand scheme’ of neutron star evolution[19]. There are the works which attempt to join the standard radio pulsar population with thoseof magnetars and the M7, through models of field decay [20, 17]. A crucial result is that, toreproduce the number of X-ray thermally emitting INSs we observe to date, the mean strengthof the magnetic field distribution at birth has to be significantly higher (and the distributionwider) than one would expect from radio pulsar studies alone. Moreover, the models usuallypredict a large number of very long spin period neutron stars (P > 20 s), which are not observed.Also important is to assess the relative contributions of the various INS groups to the totalnumber of neutron stars populating the Milky Way. If each INS group is treated independently, alikely consequence is that the Galactic core-collapse supernova rate cannot account for the entireestimated population of neutron stars [21]. The neutron star birthrate derived from populationsynthesis is even more discrepant from the expected value when field decay is taken into account[22, 17]. Moreover, while active magnetars are not expected to contribute significantly to theabsolute number of observable neutron stars in the Galaxy, the existence of transient sources,which are believed to possess low quiescent X-ray luminosities, also has implications for boththe total magnetar birthrate as for the ensemble of the population. The birthrate of CCOsis similarly unclear, either as anti-magnetars (that is, as neutron stars born with intrinsicallyweak magnetic fields), or within the ‘hidden magnetic field’ scenario. The fraction of neutronstars that undergo a phase of fallback accretion and field submergence is unknown, althoughthese events could commonly take place in the early life of a neutron star (see e.g. [15], for adiscussion).3. A forecast for the eROSITA surveyAs long as only the X-ray bright end of the radio-quiet INS population is known, theobservational and theoretical advances seen in recent years may reach a halt. The ExtendedRöntgen Survey with an Imaging Telescope Array (eROSITA) [23] is the primary instrumenton the forthcoming Spectrum-RG mission. The four-year eROSITA all-sky survey (eRASS)is expected to detect a large sample of clusters of galaxies, active galactic nuclei, transientphenomena, and Galactic compact objects, among other interesting case studies. eRASS offersa timely opportunity for a better sampling of a considerable number of neutron stars thatcannot be assessed by radio and γ-ray surveys. Eventually, the identification and investigationof eROSITA sources at faint X-ray fluxes will help us to test alternative neutron star evolutionaryscenarios and constrain the rate of core-collapse supernovae in our Galaxy.To estimate the number of INSs to be detected by eRASS through their thermal X-rayemission, we performed Monte Carlo simulations of a population synthesis model [24]. Briefly,neutron stars are created from a progenitor population of massive stars distributed in the spiralarms of the Galactic disk; after the supernova explosion and the imparted ‘kick’ velocity, theirevolution in the Galactic potential is followed while they cool down, isotropically emitting soft1 Interestingly, recent results considering only the radio pulsar population show that pulsar evolutionary pathsin P − Ṗ space can signficantly deviate from those expected along lines of constant field, when the decay of theangle between spin and magnetic axes is included in the population synthesis [18].41234567890International Conference Physics of Neutron Stars - 2017. 50 years after. IOP PublishingIOP Conf. Series: Journal of Physics: Conf. Series 932 (2017) 012008  doi :10.1088/1742-6596/932/1/012008Figure 3. Histogram of the optical Vmagnitudes required for ruling out otherclasses of X-ray emitter among INS can-didates, based on X-ray-to-optical flux ra-tios. We considered in the simulations twopossible configurations of optical blockingfilters and the resulting total effective areaof the seven telescopes, averaged over thefield of view [24]. A minimum X-ray-to-optical flux ratio of 103.5 is assumed forthe eROSITA-detected sample of neutronstars. Distances to the detected sourcesare within 400 pc and 8 kpc and the accu-mulated survey exposure ranges from 1.6to 8 ks, with a median around 1.9 ks. Thelimiting flux is around 10−14 erg s−1 cm−2,which is 50 times deeper than that of theROSAT Bright Star Catalogue.X-rays. The expected source count rates and total flux are then computed in the eROSITAdetectors, taking into account the absorbing material in the line-of-sight, the detector andsurvey properties, and the celestial exposure after four years of all-sky observation. We applyan analytical description of the interstellar medium, based on layers of hydrogen in both atomicand molecular form, and commonly adopted cross-sections. All neutron stars in our simulationscool down at the same fiducial rate.Our study indicates an expected number of up to ∼ 85− 95 thermally emitting INSs2 to bedetected in the eRASS with more than 30 counts (0.2−2 keV). In Figure 3 we show the histogramof V magnitudes required to select promising INS candidates for deep follow-up investigations(assuming a logarithmic X-ray-to-optical flux ratio of log(FX/FV ) = 3.5, which is sufficientto exclude other classes of X-ray emitters). Although optical follow-up will require very deepobservations – in particular, the identification of the faintest candidates will have to wait for thenext generation of extremely large telescopes – sources at intermediate fluxes can be selectedfor follow-up investigations using current observing facilities. We anticipate ∼ 25 discoveries inthe first years after the completion of the all-sky survey (see [24], for details).4. Summary and conclusionsThe observed neutron star phenomenology is arguably governed by the properties of the magneticfield: specifically, by its magnitude at birth, whether it decays or grows during the star’s lifetime,and how these characteristics affect the rotational and thermal history of the neutron star.Overall, crucial aspects of neutron star evolution and emissivity cannot be probed through theradio and γ-ray pulsar population alone, and are not yet described by theory. It is thus importantto investigate and survey INSs in X-rays in the hope of discovering missing links in theirevolution, and to gain insight into neutron star physics and phenomenology. The forthcoming all-sky survey of eROSITA, expected for launch in September 2018, is a unique oportunity to unveilthe X-ray faint end of the neutron star population. Both coordination with photometric andspectroscopic surveys at other wavelengths and catalogue cross-correlation procedures yielding2 Depending on the exact configuration of optical blocking filters [24].51234567890International Conference Physics of Neutron Stars - 2017. 50 years after. IOP PublishingIOP Conf. Series: Journal of Physics: Conf. Series 932 (2017) 012008  doi :10.1088/1742-6596/932/1/012008actual probabilities of source identification are required to pinpoint promising INS candidatesamong the millions of eROSITA-detected sources. Dedidated follow-up investigations will tacklethe evolutionary state of individual targets to evaluate the impact of alternative scenarios onthe evolution and observability of the neutron star population in the Milky Way.AcknowledgmentsThe collaboration of Axel Schwope and Christian Motch is acknowledged. 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The missing links of neutron star evolution in the eROSITA all-sky X-ray survey

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