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On The Ultra-Luminous X-Ray Source In ESO 243-49: HLX-1 (2XMM J011028.1−460421)

06/07/2012

The X-ray source 2XMM J011028.1−460421 (a.k.a HLX-1) is the brightest known ultraluminous X-ray source in the sky (ULX; Feng & Soria 2011) and is suspected to be an intermediate-mass black hole (IMBH) candidate. With the recent discovery  by Wiersema et al. 2010 in the optical HLX-1 spectrum of an emission line consistent with Hα at the redshift of ESO 243-49 (z = 0.0223) irrevocably confirms its association with this galaxy at a distance of 95 Mpc. As it is located within the outskirts of the S0 galaxy ESO 243-49, approximately 0.8 kpc out of the plane and ~3.3 kpc away from the nucleus, HLX-1 can not be considered as the supermassive black hole (SMBH) that resides within the nucleus of its host galaxy. Yet, considering it has comparable X-ray luminosities to that of many modest SMBHs, such observations make it a rather interesting object in the night sky.

Fig. 1: Composite Far-UV/Near-UV/C/V/IH band image of ESO 243-49 taken by the Hubble Space Telescope, HLX-1 is centered within the white circle. (Credit: Farrell et al. 2009)

Most black holes found thus far are either comparable to stellar masses (e.g. 3 – 10) or belong to an extreme brand of SMBH with masses of billions of times the mass of our Sun. However, estimates of the mass of HLX-1, based on its X-ray spectrum, range from 500 to 100000 (Farrell et al. 2009; Davis et al. 2011; Servillat et al. 2011), placing HLX-1 into the IMBH classification. The mass of a black hole is often associated to the amount of energy is emitted from the accretion disc. Due to the massive gravitational potentials created by black holes, gases being accreted (falling) onto a black hole are heated. As X-rays are highly energetic (second only to γ-rays) the flux associated with an object will always be proportional to its mass. Hence, it is possible to measure the mass of an object such as a black hole through its X-ray emission.

2XMM J011028.1–460421, referred to hereafter as HLX-1, was discovered serendipitously by XMM-Newton on November 23rd, 2004 in the outskirts of the edge on spiral galaxy ESO 243-49, at a redshift of z = 0.0224 (Afonso et al. 2005) is classified as an ultra-luminous X-ray source, with a peak X-ray luminosity of  (0.2-10 keV) (Servillat et al. 2011).

With observed X-ray luminosities reaching above  , HLX-1 is super-Eddington if the black-hole’s mass is less than ~100000. Beaming effects (e.g. King 2008; Kording et al. 2002) have been proposed as viable mechanisms for producing the apparent super-Eddington luminosities seen from other ULXs. However, beaming is unlikely to explain HLX-1’s extreme luminosity due to the observed large-scale variability (which appears similar to that seen from Galactic stellar mass black hole binaries that are not viewed down the jet-axis) and the luminosity of the Hα line, which is an order of magnitude above that expected from reprocessing in the local absorbing material (Wiersema et al. 2010). Hence, we are dealing with no differing effects, where the variability likely confirms HLX-1 as a clear IMBH (Lasota et al. 2011).

What is also very interesting to note is that the X-ray luminosity of HLX-1 varies from ~10 – 400 times the Eddington limit of a 20 black hole, where mass estimates for HLX-1 from Eddington scaling, accretion disc continuum fitting, and jet flare luminosity all support a mass of 100000, placing it quite clearly within the IMBH classification: approximately one hundred thousand times as large as a stellar sized black hole but not yet massive enough to be considered a “super massive”.

A rather interesting point to make, which is something I have not always felt plausible, is that if ESO 243-49 contains a SMBH at its heart and if we were to fast forward a couple of billion years is that HLX-1 may merge with such a possible SMBH. Supporting evidence is supplied by many studies of HLX-1, using derived stellar ages to show that the age of HLX-1 inconsistent with globular cluster, instead implying HLX-1 could be nucleus of stripped dwarf galaxy accreted by ESO 243-49. In essence, the observation of a very young cluster of stars indicates that the intermediate-mass black hole may have originated as the central black hole in a very low-mass dwarf galaxy. The dwarf galaxy was then gravitationally bound to and swallowed by ESO 243-49. Hence, many millions or perhaps billions of years from now could see the merger of HLX-1 and the SMBH at the centre of ESO 243-49.

Journal References:

  • Davis, S.W. et al. (2010) The Cool Accretion Disk In ESO 243-49 HLX-1: Further Evidence Of An Intermediate-Mass Black HoleThe Astrophysical Journal, 734 (2): Article I.D. #111.
  • Lasota, J.P. et al. (2011) The Origin of Variability Of The Intermediate-Mass Black-Hole ULX System HLX-1 In ESO 243-49The Astrophysical Journal, 735 (2):  Article I.D. #89.
  • Servillat, M. et al. (2011) X-Ray Variability & Hardness Of ESO 243-49 HLX-1: Clear Evidence for Spectral State TransitionsThe Astrophysical Journal743 (1): Article I.D. #6.
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The 2012 Solar Transit Of Venus

06/06/2012

As everyone may well know, the 2012 Transit of Venus occurred on 5th to the 6th June, with the whole event lasting slightly under seven hours. The transit started around at 23:04 British Summer Time (22:04 UTC) on 5 June through various North American based telescope streams, a little after the sun has set in the UK.

Being based here in Cornwall in the U.K., my attempts at seeing the sun came from two possible sources. My first source was the SLOOH SpaceCamera’s live online feed, URL: [http://events.slooh.com/], which hosted the full transit. This gave me an excellent chance to document and record the main phases of this much anticipated solar transit (please see my observations below). The second was setting my alarm for 04:45 BST and using my newly purchased Lunt 60mm H-Alpha Telescope (9B600 – 2″ Crayford Focuser) with my sketchpad and drawing what I saw, the result can be seen above.

From the SLOOH live feeds I was able to record the various phases of the transit as well as some key parameters of the evenings events. It took, by my estimations, 19 minutes and 42 seconds from the point when Venus first encroached onto the disk of the Sun (‘ingress exterior’) until the planet was fully silhouetted (‘ingress interior’). The planet then took a curved path across the northern hemisphere of the Sun. Mid-transit was recorded to be at 02:32 BST (01:32 UTC) on the 6th June. Venus began to leave the Sun (‘egress interior’) at about 05:37 BST (04:37 UTC), and the transit was (‘egress exterior’) over by 05:57 BST (04:57 UTC).

Further Reading:

  • Espenek, F. (2012) The 2012 Transit Of Venus: [http://eclipse.gsfc.nasa.gov/OH/transit12.html]. The Observer’s Handbook 2012, Royal Astronomical Society Of Canada.

The Peculiar Evolutionary History Of The Terzan 5 Transient IGR J17480-2446

22/05/2012

During its Galactic bulge monitoring program, performed between 08:45UTC and 12:27UTC on October 10th 2010 (Bordas et al. 2010) and again on October 11th (Chenevez et al. 2010), the IBIS/ISGRI instrument onboard INTEGRAL discovered an outburst from a hard X-ray source originating from within the globular cluster Terzan 5. This source, now known as IGR J17480-2446, is an analogue to the clock-like burster GS 1826−238 (Chakraborty et al. 2011) and was swiftly identified as a neutron star (NS) low-mass X-ray binary (LMXB) with a relative slow rotation frequency of 11Hz (Strohmayer & Markwardt 2010). The LMXB system containing IGR J17480-2446 has an orbital period of 21.3 hr around a companion star with a mass of 0.4 (Papitto et al. 2011).

N.B. Terzan 5, discovered in 1968, is a globular cluster lying at approximately 18000 light years or 5.5±0.9 kpc (Ortolani et al. 2007) from Earth rich in a class of pulsar known as the millisecond pulsar (MSP). This cluster contains at least 33 of this pulsar type (Freire 2007). It is worth mentioning that the MSP population inside Terzan 5 includes the millisecond pulsar “PSR J1748–2446ad”, the fastest known millisecond pulsar spinning at 716 Hz (Hessels et al. 2006). Although IGR J17480-2446 is not an MSP, recent work by Papitto et al. (2010) coupled the high rate of MSPs in Terzan 5, has indicated that through accretion from its 0.4 companion IGR J17480-2446 is in the process of spinning-up. Being in this stage in the pulsar evolutionary track will reveal key information about the development of millisecond pulsars from slower spinning neutron stars via the recycling mechanism.

Millisecond radio pulsars are believed to be a result of the recycling scenario: a transfer of angular momentum via accretion of matter from a main sequence companion star to a compact neutron star (Alpar et al. 1982, Radhakrishnan & Srinivasan 1982). Such scenarios cause the spin-up of millsecond pulsars in binary star systems known as low mass X-ray binaries (LMXBs). The relative slow rotation period for IGR J17480-2446 is quite the peculiarity. Such a slowly spinning neutron star is unusual for LMXBs, because we expect accretion-induced spin-up of such stars (Bhattacharya & van den Heuvel 1991). IGR J17480-2446 shows thermonuclear bursts (Chenevez et al. 2010, Linares et al. 2011b) with burst oscillations phase locked with the accretion powered pulations (Cavecchi et al. 2011), and the magnetic field is within the range  (Cavecchi et al. 2011, Papitto et al. 2011). Such a field measurement is indicative of a pulsar which has started the recycling mechanism phase. However, the 11Hz rotation frequency is curiously slow, where the majority of rotation periods for accretion powered pulsars in globular clusters being of the order of milliseconds (Patruno et al. 2012). So what is this cause of such a low rotation frequency?

As mentioned, understanding this anomaly is valuable because IGR J17480–2446 can be the only accreting pulsar discovered so far which is in the process of becoming an accreting millisecond pulsar. Recent work by Patruno et al. (2012) concluded that IGR J17480-2446 is in an exceptionally early Roche-lobe overflow stage (RLOF) in its evolution, where the 0.4 companion star to IGR J17480-2446 is spilling its outer layers over the L1 point onto the compact neutron star. They predict that the total spin-up timescale to transform IGR J17480-2446 from it’s current state as a slow pulsar (~1 Hz) into a millisecond pulsar (>100 Hz) is approximately a billion years. Since the current RLOF is predicted to last for approximately one billion years they expect to observe perhaps 1 to 10 accreting millisecond pulsars that have followed a similar evolutionary history as IGR J17480-2446.

The prior spin-down history gives results which are not compatible with a binary having an age of several billion years as expected if IGR J17480–2446 is primordial or if it has not suffered exchange interactions in the last few billion years. Different formation scenarios are available to explain the apparent discrepancy between the age of the cluster and the binary. A possibility is that a recent dynamical encounter has played a role in forming the binary or in accelerating the onset of the RLOF phase. If an exchange interaction has taken place in the last 10-100 million years then this would explain why the apparent age of IGR J17480–2446 is so small in comparison with the age of the cluster. Coupled with this, the location of IGR J17480–2446 in the high mass and high central density cluster Terzan 5 suggests that the interaction rate might be particularly high in this environment. Such peculiarities in the work conducted by Patruno et al. (2012) has opened up a whole wealth of new questions regarding the formation of MSPs in the wider Universe.

Journal References:

  • Chakraborty, M.; Bhattacharyya, S.; Mukherjee, A. (2011) Terzan 5 Transient IGR J17480-2446: Variation Of Burst & Spectral Properties. Monthly Notices Royal Astronomical Society, 418 (1): pp.490-499.
  • Boyles, J.; Lorimer, D. R.; Turk, P. J.; Mnatsakanov, R.; Lynch, R. S.; Ransom, S. M.; Freire, P. C.; Belczynski, K. (2011) Young Radio Pulsars in Galactic Globular Clusters. The Astrophysical Journal, 742 (1): Article I.D. #51.
  • Patruno, A.; Alpar, M. A.; van der Klis, M.; van den Heuvel, E.P. J. (2012) The Peculiar Evolutionary History of IGR J17480-2446 In Terzan 5The Astrophysical Journal, 752 (1): Article I.D. #33.

Related Articles:

The Most Distant Known Quasar In The Universe: ULAS  J112001.48+064124.3

21/05/2012

Some galaxies like our very own Milky Way seem to be in a dormant state, existing in a seemingly quiet epoch of the Universe. However, what lies at the centre of our Galaxy reveals our not so distant past: our Galaxy was most likely to be active due to the accretion of gas and dust onto the central super massive black hole (SMBH) (Lynden-Bell 1969; Antonucci 1993). This accretion can power certain galaxies to emit very high-energy photons, known as active galaxies. There is no real phenomenological distinction between quasars and active galactic nuclei. They are both believed to be powered by the same accretion processes, where the distinction comes that a quasar is considered to be a distant object. The issue of the activity of nuclei of galaxies (AGN) was first raised by the Armenian physicist Victor Ambartsumian in the early 1950s. However, it wasn’t until the discovery of 3C 273 in 1959, the first confirmed of the quasi-stellar class, that the paradigm of active galactic nuclei was taken with less skepticism.

Since the late 1950s many objects resembling quasars and radio galaxies have been observed. Many have been observed locally, with redshifts equalling z ~ 0.5. But it wasn’t until the Hubble Space Telescope and other modern space telescopes that the super distant objects, such as ULAS  J112001.48+064124.3, were detected with redshifts of  z > 6.   The quasar ULAS  J112001.48+064124.3 was fi rst identifi ed in the United Kingdom Infrared Telescope (UKIRT) Infrared Deep Sky Survey (UKIDSS) (Lawrence et al. 2007) and published in the Eighth Data Release back in September of 2010, where follow-up observations confirmed it as a quasar with a redshift of z ≥ 6.5 (Mortlock et al. 2011).

In November of 2010 a spectrum was obtained for ULAS J112001.48+064124.3 using the Gemini Multi-Object Spectrograph on the Gemini North Telescope. The absence of significant emission blueward of a sharp break at the λ = 0.98 μm wavelength confirmed that ULAS J112001.48+064124.3 was indeed a quasar with a redshift of z ~ 7.08. With that tentative figure in mind and assuming that the standard model of cosmology (Dunkley et al. 2009) is the most correct (i.e. a flat Universe with a Hubble parameter of  at the present epoch) that would mean the light we are seeing from ULAS J112001.48+064124.3 has taken 12.9 billion years (Gyr) to reach us, starting it’s journey when the Universe was as little as 0.77 billion years old.

Further spectroscopic observations of ULAS J112001.48+064124.3 were made using the FOcal Reducer/Low Dispersion Spectrograph 2 (FORS2) on the Very Large Telescope (VLT) Antu and the Gemini Near-Infrared Spectrograph (GNIRS) on the Gemini North Telescope. The spectrum obtained of ULAS J112001.48+064124.3 shows remarkable similarity to lower redshift quasars of comparable luminosity, and comparison to a rest-frame template spectrum (Hewett & Wild 2010) over the wavelength range including the strong Si [III]+C [III] and Mg[II] emission features yielding an accurate redshift of z = 7.085±0.003. The most unusual feature of the spectrum is the 2800±250 km s-1 blueshift of the C [IV] emission line, which is greater than that seen in 99.9 % of z ≳ 2 quasars (Richards et al. 2011). There is associated absorption at various other spectral doublets indicating the presence of material in front of the quasar flowing out at 1100±200 km s-1. There is also a narrow absorption line at the Lyman-α emission wavelength that is consistent with a cloud of H[I] gas close to the quasar.  Aside from its existence, the most striking aspect of ULAS J112001.48+064124.3 is the almost complete lack of observed flux blueward of its Lyman-α  emission line, which can be attributed to absorption by H[I] along the line of sight.

Although this is not the highest redshift object in the Universe [The Discovery Of UDFy-38135539: Z ~ 8 In The Hubble Ultra-Deep Field], ULAS  J112001.48+064124.3 is the furthest known quasar in the Universe. Quasars are not expected to have formed so early in the galaxy, due to the current understanding of the formation of SMBHs. The light from ULAS J112001.48+064124.3 was emitted during a time period before the end of the theoretically predicted transition of the interstellar medium from an electrically neutral to an ionized state. Quasars may have been an important energy source in this process, known as reionization, which marked the end of an epoch known as the Cosmic Dark Ages, so a quasar from before the transition is of significant theoretical interest in both how it interacted with the Universe around it but also in the manner of the formation of the central SMBH engine, e.g. do they form slowly and gradually over time or do they form instantaneously at approximately the same mass that they are recorded at the present epoch? The disovery of ULAS  J112001.48+064124.3 and the subsequent research it will enable will almost certainly allow astrophysicists to shed light on these age old questions of SMBH formation.

Journal References:

  • Loeb, A; Barkana, R. (2001) The Reionization Of The Universe By The First Stars & Quasars. Annual Review: Astronomy & Astrophysics 39: pp. 19-66.
  • Mortlock, D.J.; Warren, S.J.; Venemans, B.P; et al. (2011). A Luminous Quasar At A Redshift Of z = 7.085Nature 474 (7353): pp.616–619.
  • Willott, C. (2011). A Monster In The Early UniverseNature 474 (7353): pp.583–584.

The Progenitor System Of Nova V2491 Cygni (SDSS J194301.96+321913.8)

03/05/2012

V2491 Cygni (Nova Cygni 2008 #2; SDSS J194301.96+321913.8) was discovered in outburst on 2008 April 10.8 UT (Nakano et al. 2008). Its nature was confirmed spectroscopically by Ayani & Matsumoto (2008), with a Hα FHWM* of ~4500 km s-1 and was latter classified as a Helium/Nitrogen (He/N) nova (Helton et al. 2008; Munari et al. 2011). The rapid decline from maximum light classifies V2491 Cyg as a very fast classical nova. Rather unusually, the optical decline of V2491 Cyg exhibited a secondary maximum at day ~15 (Munari et al. 2011). Such significant re-brightenings have been seen in a number of other novae (V2362 Cyg and V1493 Aql; Strope et al. 2010), but are unusual and still poorly understood. Hachisu & Kato (2009) have proposed magnetic activity as an additional energy source and a possible cause of the re-brightening, a requirement being a highly magnetised white dwarf known as a “polar”.

N.B.* FWHM is short for full-width at half maximum, a measure astronomers use to describe resolution of star images.

Classical novae (CNe) systems typically consist of a compact white dwarf (WD) primary and a main-sequence secondary star which usually resembling the same properties of our own Sun (Iben & Tutukov 1996). The system is in a tight binary orbit where the secondary star swells and fills its Roche lobe spilling material onto the surface of the WD primary. This situation leads to a thermonuclear runaway on the surface of the WD, forming an accreted layer. This layer is then expelled and heated to very high temperatures: this is seen by us on Earth as a nova eruption (Iben & Livio 1993; Hellier 2001; Bode & Evans 2008).

Following the initial April 2008 outburst, it was discovered that there was a pre-existing X-ray source coincident with the position of V2491 Cyg (Ibarra & Kuulkers 2008; Ibarra et al. 2008; Ibarra et al. 2009). This is only the second nova for which pre-outburst X-ray emission has been detected, after V2487Oph (Hernanz & Sala 2002). V2487 Oph was classified as a recurrent nova (RN) when a previous outburst was indentified from 1900 (Pagnotta et al. 2009).

The progenitor system of V2491 Cyg was identified as USNO-B1.0 1223-042965 (Henden & Munari 2008) and there is no evidence of previous outbursts at this position (Jurdana-Sepic & Munari 2008). Rudy et al. (2008) determined a sharp reddening of V2491 Cyg via O [I] line ratios. Helton et al. (2008) used the CN “MMRD”† relation (Livio & della Valle 1995) with this reddening to determine d  = 10.5 kpc. Munari et al. (2011) independently derived both the extinction and distance to V2491 Cyg using the interstellar Na [I] line and the van den Bergh & Younger (1987) relation, finding a distance of d = 14 kpc. Ribeiro et al. (2011) reported an ejecta morphology consisting of polar blobs and an equatorial ring, with expansion velocities of ~3000 km s-1 and an inclination of 80°: i.e. close to edge on.

The first indication that V2491 Cyg may be recurrent in nature came from Tomov et al. (2008) who noted the spectral similarities to the RNe U Sco and V394 CrA early after outburst. Additionally, Bode et al. (2009) noted that the V2491 Cyg spectra were similar to the early spectra of RS Oph and in particular those ofM31N 2007-12b, a RS Oph-class RN candidate in M31.

Many properties of V2491 Cyg are directly comparable with the RN U Sco; the optical and near-IR luminosities at quiescence are almost identical, as is the outburst amplitude of the system. As the spectral energy distributions of both V2491 Cyg and U Sco are so similar, we must conclude that the composition of the system is similar; an accretion disc dominating the short wavelength emission the secondary being important at longer wavelengths. Based with the quiescent photometric evidence Darnley et al. (2011) concluded that the V2491 Cyg system is likely itself to be a “clone” of the U Sco system. That is, highly inclinded (edge-on), containing a high mass WD primary, a sub-giant secondary and a bright accretion disc.

Such a system is however incompatible with an orbital period as short as 0.1 days as has been widely assumed for this system, as the radius of the sub-giant star would be larger than the orbital separation. We are therefore given two alternatives: either, V2491 Cyg is a U Sco class RN or, the system is much closer at ~2 kpc. However, there is one parameter of the nova outburst that is both distance and extinction independent, the outburst amplitude; for V2491 Cyg  ~10 magnitudes. Such a small amplitude is incompatible with a very fast CN and is only achievable for the slowest and faintest of novae. Using the outburst amplitude versus rate of decline relationship for CNe (e.g. Warner 2008), the amplitude for a nova with a decline time of ~4.8 days (Munari et al. 2011) and inclination ~80° (Ribeiro et al. 2011) is almost 16 magnitudes. As such, the only scenarios compatible with such a small outburst amplitude are that V2491 Cyg is a (nearby) severely under-luminous and very unusual CN or that it is a (distant) U Sco-class RN. In either case V2491 Cyg is an interesting and important system worthy of further study.

Journal References:

  • Darnley, M. J.; et al. (2011) On The Progenitor System Of Nova V2491 CygniAstronomy & Astrophysics, 530: Article I.D.: 70.
  • Ibarra, A.; et al. (2009) Pre-Nova X-Ray Observations Of V2491 Cygni (Nova Cyg 2008b)Astronomy & Astrophysics, 497 (1): pp.L5-L8.

A Giant Concurrent Radio & γ-Ray Flare From Cygnus X-3 (FLT J2032.2578+4057.279)

02/05/2012

The high-mass X-ray binary* (HMXB) Cygnus X-3 (a.k.a V* V1521 Cyg; RX J2032.2578+4057.279) is one of the brightest objects in the X-ray sky, third brightest in the Cygnus constellation (Seward & Charles 2010). Cygnus lies 27000 light years from Earth and is heavily obscured by dust and interstellar gas, but if one were to remove the obscuration then it would be appear to be one of the most intrinsically luminous extrasolar objects in our Galaxy after the stars Sirius and Canopus. Coupled with frequent flaring activity of its relativistic jets, Cygnus X-3 (Cyg X-3) is one of the most active microquasars and is the only Galactic black hole candidate with confirmed high-energy γ-ray emission in the Galaxy (Corbel et al. 2012).

N.B.*A HMXB is a binary star system that is strong in X rays where the stellar component is a massive star: usually a Harvard spectral class O or B star, a Be star, or a blue supergiant. The X-ray emission is caused by the accretion of the gases of the massive star onto a compact object, perhaps a neutron star (NS), a black hole (BH) or perhaps more exotic, rarer variants. As Cygnus X-3 is considered a HMXB because the optical counterpart to the compact object in the Cyg X-3 system, V1521 Cyg, is a high-mass Wolf-Rayet star of mass as inferred from the observation of broad emission lines of He [I] and He [II] and a lack of strong hydrogen spectral features in its emission profile (van Kerkwijk et al. 1992).

Cygnus X-3 was discovered almost 50 years ago back in 1967 by Giacconi et al. (1967). The compact object within Cyg X-3 is thought to be a black hole, which mutually orbits its Wolf-Rayet companion star every 4.8 hrs (Parsignault et al. 1972). Like most other accreting X-ray binaries, Cyg X-3 has variability in the hard (10-100keV) and soft (1-10keV) X-ray energy bands which is partially absorbed by a surrounding dense stellar wind (Szostek, Zdziarski & McCollough 2008; Hjalmarsdotter et al. 2009). Cyg X-3 is also known for the recurrent activity of its relativistic jets that make it one of the brightest Galactic transient radio sources (Mioduszewski et al. 2001; Miller-Jones et al. 2004; Corbel et al. 2012).

Through simultaneous AGILE and Fermi/LAT detections (Tavani et al. 2009; Abdo et al. 2009) of high energy γ–rays (>100MeV) Cygnus X-3 has now been classified as the first accreting microquasar† related to γ–ray emission (Corbel et al. 2012). The γ-ray emission measured by the Fermi/LAT was found to be modulated on the orbital period, securing the identification as Cygnus X-3 (Abdo et al. 2009).

N.B. †A microquasar is a radio emitting X-ray binary with an associated variable radio emission with an accompanying accretion disc surrounding a compact object (either a neutron star or a black hole) that emits particles and high-energy photons as relativistic jets (Tavani et al. 2009). Galactic ‘microquasars’ also produce relativistic jets, generally together with radio flares.

After this first emission detection above the 100MeV threshold both Bulgarelli et al. (2010) and Williams et al. (2011) reported a very short γ-ray flare during a short transient softening of Cygnus X-3, however no evidence for emission above 250 GeV has been found (Aleksíc et al. 2010). In early 2011 February, Cyg X-3 was observed to transit to a soft X-ray state, a period known to be associated with high energy gamma-ray emission. A giant (20 Jy) optically thin radio flare marked the end of the soft state. In addition, the observations unambiguously showed that the γ-ray emission is not exclusively related to the spectacular and rare giant radio flares. A 3-week period of gamma-ray emission was also detected earlier during soft state, when Cyg X-3 was weakly flaring in radio, with 15 GHz peak flux density of 0.6 Jy.

Corbel et al. (2012) suggest transitions into and out of the ultra-soft X-ray (radio quenched) state trigger γ-ray emission, implying a connection to the accretion process and that the γ-ray activity is related to the level of radio flux (and possibly shock formation), strengthening the connection of both events to the relativistic jets emitting from Cygnus X-3.

Journal References:

  • Tavani, M.; et al. (2009) Extreme Particle Acceleration In The Microquasar Cygnus X-3. Nature462 (7273): pp. 620-623.
  • Bulgarelli, A.; et al. (2012) AGILE Detection Of Cygnus X-3 γ-Ray Active States During The Period Mid-2009-Mid-2010Astronomy & Astrophysics, 538: Article I.D.: 63.
  • Zdziarski, A.A; et al. (2012) The γ-Ray Emitting Region Of The Jet In Cyg X-3Monthly Notices Royal Astronomical Society, 421 (3): pp.1611-1620.
  • Corbel, S.; et al. (2012) A Giant Radio Flare From Cygnus X-3 With Associated γ-Ray EmissionMonthly Notices Royal Astronomical Society, 421 (4): pp.2947–2955.

The Possible Future Merger Of The WD-WD Binary SDSS J010657.30-100003.3

01/05/2012

Isolated bodies similar to our solitary, lonely Sun are something of a rarity in the Universe. Stars often form inside huge stellar nurseries (Hayashi 1961; Wolfire & Cassinelli 1987; Prialnik 2000) out of the primordial dense regions of hydrogen gas clouds e.g. N11 or the Orion Nebula, forming in groups of two, but sometimes three or more, locked in a mutual gravitationally bound system. These systems often exhibit more exotic and chaotic behaviour than solitary stars: and as such have light profiles which vary on short (e.g. [~8hrs: W Ursae Majoris]; [19.63hrs: SZ Herculis]) to long timescales (e.g. [2.867 days: β Persei/Algol]; [12.94 days: β Lyrae]) as determined by the interacting period of the system. However, there are some binary systems which undergo an altogether more complex and interesting evolution, dependent on a slight difference in their initial masses (Gänsicke et al. 2001).

Once a main sequence star approaches the end of its nuclear burning phase it will begin to cool and expand, forming a red giant and eventual expelling it’s outer envelope leaving behind a white dwarf. However, extremely low mass white dwarfs (ELM WDs) are not thought to be the result of isolated stellar evolution due to the very slow nuclear fusion rates in very low mass main sequence stars (Brown et al. 2011). Hence, the formation of ELMs needs some sort of new driving mechanism, a way to evolve low mass main sequence stars faster.

The Universe is not old enough to produce ELM WDs through single star evolution. Therefore, the system that is the subject of this post, SDSS J010657.30-100003.3, begins its life as two ordinary main sequence stars with similar or slightly less mass than that of our Sun (Kilic et al. 2010) that will undergo significant mass loss during their formation in binary systems. The majority of ELM WDs have been identified as companions to milli-second pulsars. However, not all ELM WDs have such companions (van Leeuwen et al. 2007; Agüeros et al. 2009a). Radial velocity, radio and X-ray observations of the lowest gravity WD found in the Sloan Digital Sky Survey (SDSS), (e.g. SDSS J0917+4638), show that the companion is almost certainly another WD (Kilic et al. 2007a,b; Agüeros et al. 2009b).

Mergers of binary white dwarfs (WDs) have been proposed to explain supernovae (SNe) Ia events, extreme helium stars including R Coronae Borealis (RCrB), single sub-dwarf B and sub-dwarf O stars (Iben & Tutukov 1984; Webbink 1984; Heber 2009). However, radial velocity surveys of WDs prior to the work conducted on  SDSS J010657.30-100003.3, have not revealed a large binary population that will merge within an approximate Hubble time,  (Marsh 1995; Maxted et al. 2000; Napiwotzki et al. 2001, 2002; Nelemans et al. 2005). Kilic et al. (2010) were particularly interested in the evolution of this system from the present day. They have theorised that this system will eventually merge in 37 Myr (37,000,000 years), a comparably short time scale compared to the Hubble time/age of the Universe. After this merger, instead of undergoing the catastrophic Type 1a explosion, they will form a new star: releasing huge amounts of gravitational wave energy, essentially sending out harmonic ripples in the fabric of space-time (Lorén–Aguilar et al. 2005).

The reason for this eventually rather than a Type Ia SNe is the extremely low masses of the two white dwarfs. Based on the work provided by Panei et al. (2007) for ELM WDs, the SDSS J010657.30-100003.3 system is likely to contain a 0.17 WD with a 0.37 WD companion at a separation of 0.32. Hence, when this system merges their combined mass will not make the Chandrasekhar limit (1.4) required to initiate a Type Ia supernova detonation. Instead, this new 0.54 WD creation will be gravitationally stable.

Hence, as we do not have a Type Ia SNe as the resulting fate of this dual WD system, an altogether different scenario may be possible. High-field magnetic white dwarfs, with magnetic fields in excess of 10 G and up to 10 G (Schmidt et al. 2003), have been long suspected to be the result of stellar mergers. García-Berro et al. (2012) have theorised that the hot, convective, differentially rotating corona present in the outer layers of the remnant of the merger of two degenerate cores can produce magnetic fields of the required strength that do not decay for long timescales. Hence, the rebirth of this system may be as a highly-magnetic white dwarf system or a magnetar (Wickramasinghe & Ferrario 2000; King, Pringle & Wickramasinghe 2001).

Hence, this end fate for this dual WD system is believed to be the first of its kind; although others will more than likely be discovered following the techniques employed by Kilic et al. (2010). Yet this discovery will allow our ancestors to witness on of the most the amazing spectacles: the rebirth of a two white dwarfs as one, although estimated from current period derivative of SDSS J010657.30-100003.3 to be sometime 37 million years from now!

Journal Reference:

  • Kilic, M.; Brown, W.R.; Kenyon, S. J. et al.  (2011) The Shortest Period Detached Binary White Dwarf SystemMonthly Notices of the Royal Astronomical Society: Letters, 413 (1): pp. L54-L60.

Suggested Further Reading:

  • Kilic, M.; Brown, W.R.; Kenyon, S. J. et al.  (2011) The Merger Rate of Extremely Low Mass White Dwarf Binaries: Links to the Formation of AM CVn Stars & Underluminous SupernovaeMonthly Notices of the Royal Astronomical Society: Letters, 411 (1): pp. L31-L35.
  • Kilic, M.; Brown, W.R.; Kenyon, S. J. et al.  (2010) The ELM Survey. I. A Complete Sample of Extremely Low-Mass White DwarfsThe Astrophysical Journal, 723 (2): pp. 1072-1081.
  • Kilic, M.; Brown, W.R.; Allende-Prieto, C.; Kenyon, S.J.; Panei, J.A. (2010) The Discovery of Merging Binary White Dwarfs Within 500 MyrThe Astrophysical Journal, 716 (1): pp. 122-130.
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