Super-critical accretion onto black holes and neutron stars

Accretion is one of the most important processes in astrophysics. Accretion discs are observed in a very large wavelength range, from infrared to X-rays, and in a very large range of scales: from several kilometers (accretion onto a neutron star) to parsecs (gas tori in active galactic nuclei). Physics of accretion discs is diverse, but the basic equations, equations of magnetohydrodynamics, remain the same. Accretion can also be a radiatively efficient process. As a consequence, radiation pressure easily becomes dynamically important, and a large number of accreting sources exceed their Eddington limit – the luminosity limit beyond which radiation pressure is sufficient to destroy the accretion flow itself. Ultraluminous X-ray sources (ULXs) are extragalactic non-nuclear sources with huge luminosities L È few£1039 erg s¡1 exceeding the Eddington luminosity limits for a stellar mass black hole. Though studied for already about 30 years, they remain interesting both as an extreme case of accreting sources and as an outcome of violent and poorly understood stellar evolution. Besides, recent discoveries have shown many ULXs to contain neutron stars with strong magnetic fields, making them even more extreme both in the sense of fundamental physics and from the point of view of the luminosity excess over the Eddington limit. To meaningfully interpret ULX observations, detailed models of super-Eddington accretion onto compact objects, both of the accretion disc and of the disc-magnetosphere interaction, are highly desirable. However, theoretical models describing accretion at such high rates are complicated by the dominance of radiation pressure, the need to account for advection of energy towards the compact object, and the possible presence of powerful outflows. We have started our studies by deriving the critical luminosity for a disc around a black hole taking into account disc finite thickness, heat advection, and general relativity (GR) effects. GR effectively makes vertical gravity stronger, and so does inward heat advection. At the same time, normally ignored non-linear vertical gravity dependence on height in a thick disc slightly lowers the limit, resulting in an overall correction by about a factor of two with respect to the classical approach. More accurate result surprisingly depends on the two-dimensional rotation profile of the disc. For the case of a neutron star with a strong magnetic field, the structure of the disc is not that important by itself as it is essentially invisible and unimportant for the energy budget. However, the position of the disc-magnetosphere boundary is the most important link between the fundamental parameters of the system (magnetic moment of the star, mass accretion rate, viscosity parameter etc.) and the observables (such as spin period and its derivative, power density spectrum of the variability of the source). We developed a model of an accretion disc around a neutron star taking into account the effects of advection and wind. This model may be applied to a large variety of magnetized neutron stars accreting close to or above their Eddington limits: ULX pulsars, Be/X-ray binaries in outbursts, and other systems.Massan kertyminen painovoimakentässä on yksi tärkeimmistä astrofysiikan prosesseista. Kertymäkiekkoja voidaan havaita laajalla aallonpituusalueella aina infrapunasta röntgensäteisiin, ja useissa kokoluokissa useista kilometreistä (kertyminen neutronitähdelle) parsekeihin (aktiivisten galaksiydinten kaasurenkaat). Kertymäkiekkojen fysiikka on monipuolista, mutta perusyhtälöt eli magnetohydrodynamiikan yhtälöt pysyvät samoina. Massan kertyminen voi myös olla säteilyllisesti tehokas prosessi. Tämän seurauksena säteilypaineesta tulee helposti dynaamisesti tärkeä, ja moni kerryttävistä kohteista ylittää Eddingtonin rajansa, luminositeettiraja, jota kirkkaammissa kohteissa säteilyn paine estää massan kertymisen. Ylikirkkaat röntgenkohteet (ultraluminous X-ray sources, ULX:t), ovat galaksin ulkopuolisia kohteita, joiden huimat luminositeetit L È few£1039 erg s¡1 ylittävät Eddingtonin luminositeetit tähdenmassaiselle mustalle aukolle. Vaikka kohteita on tutkittu jo noin 30 vuotta, ne ovat edelleen kiinnostavia äärimmäisiä massaa kerryttäviä kohteita ja rajun sekä huonosti ymmärretyn tähtien elinkaaren lopputuloksia. Viimeaikaiset havainnot osoittavat usean ULX:n sisältävän voimakkaasti magneettisen neutronitähden, mikä tekee niistä entistäkin äärimmäisempiä sekä perustavanlaatuisen fysiikan että Eddingtonin rajan ylittävän kirkkauden näkökulmasta. ULX-havaintojen ymmärtäminen edellyttääkin yksityiskohtaisia malleja Eddingtonin rajan ylittävälle massan kertymiselle, sekä kertymäkiekon että kiekon ja magnetosfäärin vuorovaikutuksen suhteen. Suurilla massankertymismäärillä säteilypaineen dominoiva rooli, tarve ottaa huomioon energian advektio kompaktista kohdetta kohti ja mahdolliset voimakkaat ulosvirtaukset kuitenkin monimutkaistavat teoreettisia malleja. Olemme aloittaneet tutkimuksemme johtamalla kriittisen luminositeetin mustaa aukkoa ympäröivälle kiekolle ottaen huomioon kiekon rajatun paksuuden, lämmön advektion ja yleisen suhteellisuusteorian (general relativity, GR) vaikutukset. Käytännössä GR voimistaa pystysuuntaista painovoimaa, kuten myös lämmön advektio sisäänpäin. Samaan aikaan, tavallisesti huomiotta jätetty epälineaarinen pystysuuntainen painovoiman riippuvuus korkeudesta paksussa kiekossa hieman alentaa rajaa johtaen kaiken kaikkiaan kahden kokoluokkaa olevaan korjaukseen klassiseen verrattuna. Tarkempi tulos yllättäen riippuu kiekon kaksiulotteisesta pyörimisprofiilista. Voimakkaasti magnetisoidun neutronitähden tapauksessa kiekon rakenne ei ole niin tärkeä itsessään, sillä se on käytännössä piilossa ja merkityksetön energiabudjetille. Kiekon ja magnetosfäärin rajan sijainti on kuitenkin tärkein linkki järjestelmän perustavanlaatuisten parametrien (tähden magneettinen momentti, massan kertymismäärä, viskositeetti, jne.) ja havaittavien suureiden välillä (mm. pyörimisaika ja sen derivaatta sekä kohteen kirkkauden muutoksen tehospektri). Kehitimme neutronitähden kertymäkiekon mallin ottaen huomioon advektion ja tuulen vaikutukset. Tätä mallia voi soveltaa lukuisiin erityyppisiin magneettisiin neutronitähtiin, jotka kerryttävät massaa lähellä tai yli niiden Eddingtonin rajan: ULX-pulsarit, Be-/röntgenkaksoistähdet purkauksissa ja muut järjestelmät

Super-Eddington accretion discs with advection and outflows around magnetized neutron stars

We present a model for a super-Eddington accretion disc around a magnetized neutron star taking into account advection of heat and the mass loss by the wind. The model is semi-analytical and predicts radial profiles of all basic physical characteristics of the accretion disc. The magnetospheric radius is found as an eigenvalue of the problem. When the inner disc is in radiation-pressure-dominated regime but does not reach its local Eddington limit, advection is mild, and the radius of the magnetosphere depends weakly on the accretion rate. Once approaching the local Eddington limit, the disc becomes advection-dominated, and the scaling for the magnetospheric radius with the mass accretion rate is similar to the classical Alfven relation. Allowing for the mass loss in a wind leads to an increase of the magnetospheric radius. Our model may be applied to a large variety of magnetized neutron stars accreting close to or above their Eddington limits: ultra-luminous X-ray pulsars, Be/X-ray binaries in outbursts, and other systems. In the context of our model we discuss the observational properties of NGC 5907~X-1, the brightest ultra-luminous pulsar known so far, and NGC 300~ULX-1 which is apparently a Be/X-ray binary experiencing a very bright super-Eddington outburst.Comment: Submitted to Astronomy and Astrophysics, 15 pages, 16 figure

On the Eddington limit for relativistic accretion discs

Standard accretion disc model relies upon several assumptions, the most important of which is geometrical thinness. Whenever this condition is violated, new physical effects become important such as radial energy advection and mass loss from the disc. These effects are important, for instance, for large mass accretion rates when the disc approaches its local Eddington limit. In this work, we study the upper limits for standard accretion disc approximation and find the corrections to the standard model that should be considered in any model aiming on reproducing the transition to super-Eddington accretion regime. First, we find that for thin accretion disc, taking into account relativistic corrections allows to increase the local Eddington limit by about a factor of 2 due to stronger gravity in general relativity (GR). However, violation of the local Eddington limit also means large disc thickness. To consider consequently the disc thickness effects, one should make assumptions upon the two-dimensional rotation law of the disc. For rotation frequency constant on cylinders r sin &theta; = const, vertical gravity becomes stronger with height on spheres of constant radius. On the other hand, effects of radial flux advection increase the flux density in the inner parts of the disc and lower the Eddington limit. In general, the effects connected to disc thickness tend to increase the local Eddington limit even more. The efficiency of accretion is however decreased by advection effects by about a factor of several.</p

Super-Eddington accretion discs with advection and outflows around magnetized neutron stars

We present a model for a super-Eddington accretion disc around a magnetized neutron star taking into account advection of heat and the mass loss by the wind. The model is semi-analytical and predicts radial profiles of all the basic physical characteristics of the accretion disc. The magnetospheric radius is found as an eigenvalue of the problem. When the inner disc is in radiation-pressure-dominated regime but does not reach its local Eddington limit, advection is mild, and the radius of the magnetosphere depends weakly on the accretion rate. Once it approaches the local Eddington limit the disc becomes advection-dominated, and the scaling for the magnetospheric radius with the mass accretion rate is similar to the classical Alfven relation. Allowing for the mass loss in a wind leads to an increase in the magnetospheric radius. Our model can be applied to a wide variety of magnetized neutron stars accreting close to or above their Eddington limits: ultra-luminous X-ray pulsars, Be/X-ray binaries in outbursts, and other systems. In the context of our model we discuss the observational properties of NGC5907 X-1, the brightest ultra-luminous pulsar currently known, and NGC300 ULX1, which is apparently a Be/X-ray binary experiencing a very bright super-Eddington outburst

Superorbital variability of the X-ray flux in the Be-donor binaries SXP 138, GX-304, and γ Cas

© 2015, Pleiades Publishing, Ltd. RXTE observations of the X-ray binary systems SXP 138, GX-304, and γ Cas in 1997–2011 have shown for the first time that these objects (X-ray binaries with Be donors) display X-ray flux variations on timescales of ∼1000 days. This timescale is about 10 times longer than their orbital periods, and is comparable to the total time of the observations. The observed variations are apparently not strictly periodic and represent stochastic variability, as is characteristic of such systems in the optical. γ Cas is considered as an example. The series of optical observations of this system available in the AAVSO database covers 78 years, and is much longer than the timescale of the variability studied. Our analysis of this series has shown that γ Cas variability on a timescale of tens of years is predominantly stochastic with a power-law spectrum