Largest Observed SMBH In The Compact Lenticular Galaxy NGC1277

It is a now just over a decade since the supermassive black hole (SMBH) paradigm began to take weight in the astronomical community. The paradigm essentially consists of the observational confirmation that our Galaxy, and many others, harbour an extremely massive object within the core nucleus (Schödel et al. 2002). Within the paradigm sits a very slight detail: the central SMBH rarely exceeds 0.1% of the host galaxy’s mass (Häring et al. 2004; Gültekin et al. 2009; Sani et al. 2011). However, for a few very select galaxies the central SMBH mass can exceed upwards of 10% of the host galaxy’s total mass e.g. NGC4486B (Magorrian et al. 2009), Henize 2-10 (Reines et al. 2011) and OJ 287. Recently, in November of last year, the largest observed fractional SMBH was measured, surpassing all the previous observed holders.

Fig. 1: Optical Hubble Space Telescope image of the compact lenticular galaxy NGC1277. (Credit: Hubble Space Telescope)

Sitting at approximately 73Mpc away, the lenticular* galaxy, rather forgettably titled “NGC 1277″, is thought to be a staggering ~14% supermassive black hole (van den Bosch et al. 2012). Hopefully the more eagle eyed readers amongst you will now be beginnign to wonder just how a black hole is measured. The answer is quite simple: spatially resolved stellar and/or gas kinematics within the region known as the “sphere-of-influence”; where the mass of the central black hole dominates the gravitational potential. Essentially, measuring and observing how objects move within the central region. The faster, and perhaps the more elliptical, the movement of the gas, dust and stars around the central region: the greater the mass of the central SMBH (Djurić et al. 2010).

N.B.* Lenticular refers to galaxies which are visually similar in characteristics and morphology to both spiral (Binney & Merrifield 1998), like the Andromeda galaxy, and elliptical galaxies (to varying degrees), an intermediate Hubble-type if you will.

However, van den Bosch (2012) seemed like they were on the hunt for only the most massive. So how is this done? Firstly, van den Bosch et al. (2012) selected galaxies that were likely to have a very dominant Galactic mass fractions within the central core. This is done by measuring two key parameters: velocity dispersion, σ_c, and half-light ratio, R_{e, K}; where the larger the σ and the smaller the R_e, K value, the better. These galaxies were ARK90, NGC1270, NGC1277, UGC 1859,UGC 2698 and MRK1216, each with velocity dispersion values of between 350-410 km s^-1 and half-light ratios of between 1.6kpc and 2.7kpc. These six galaxies targeted were observed with the Marcario Low Resolution Spectrograph (Hill et al. 1998) on the Hobby-Eberly Telescope (HET) as part of a large survey program known as HETMGS, and selected from the Two Micron All Sky Survey (2MASS) extended source catalogue (Jarrett et al. 2000) that are expected to have the largest sphere-of-influence.

Using observationally obtained measurements of the velocity dispersion and the half-light ratio, van den Bosch et al. (2012) focused on NGC1277, a galaxy 73Mpc from the Milky Way in the Perseus constellation with a stellar population older than ≳ 8Gyr. NGC 1277 was first observed in Ireland by the astronomer Lawrence Parsons in 1875. NGC1277 had a specific velocity disperson, σ = 403 ± 4 km s^-1 and a half-ligh t ratio of R_e, K = 1.6 kpc. These values lend itself to the possibility that the mass of the central SMBH is quite large!

These parameters can then be used to find the mass of the black hole by fitting a self-consistent Schwarzschild model; see [http://mpia-hd.mpg.de/~bosch/Schwarzschild...] for further details] (Schwarzschild 1972) to spatially resolved spectroscopy and high resolution imaging. Conveniently for van den Bosch et al. (2012), archival Hubble Space Telescope (HST) imaging is available for one of these six dense galaxies, NGC1277 (and can be seen in the greyscale image of Fig. 1, at the top of this post).

van den Bosch et al. (2012) NGC1277 obtained a value of 14% mass fraction value, as a percentage of the total stellar mass (1.2±0.4 ×10¹¹), where the central black hole component was measured at 1.7±0.3 ×10¹⁰, making the black hole at the centre of NGC1277 one of the largest to ever be dynamically confirmed in the known Universe.

N.B. The HET Massive Galaxy Survey has allowed van den Bosch and his team to find galaxies with extremely big black holes and they are currently following those up with the Calor Alto 3.5m, Harlan J. Smith, Keck, Gemini and Hubble Space Telescopes.

Journal References:

• van den Bosch, R.C. E.; Gebhardt, K.; Gültekin, K.; et al. (2012). An Over-Massive Black Hole In The Compact Lenticular Galaxy: NGC1277. Nature, 491 (7426): pp. 729-731.
• McConnell, N. J. et al. (2011) Two 10-Billion Solar Mass Black Holes At The Centres Of Giant Elliptical Galaxies. Nature 480: pp.215–218.

• Häring, N. & Rix, H.-W. (2004) On The Black Hole Mass-Bulge Mass Relation. The Astrophys. Jrnl. 604, L89–L92.

Suggested Further Reading:

• Dr. Remco van den Bosch (Max Planck Institut für Astronomie, Heidelberg) “An Over-Massive Black Hole in the Compact Lenticular Galaxy NGC1277″: [http://mpia.de/~bosch/blackholes.html].

• NASA/IPAC Extra-Galactic Database “NGC1277″ Entry: [http://ned.ipac.caltech.edu/.../nph-objsearch?objname=NGC+1277...]. 

Oldest Galaxy Ever Observed: Z ~ 10 Candidate UDFj-39546284 In The HUDF & XDF

The Hubble Ultra Deep Field (H-UDF) and the recently released eXtreme Deep Field (XDF) are perhaps the two most remarkable astronomical surveys ever conducted by humans. The light which has travelled from these regions has taken an eternity, light which has never been seen before by humans. In recent years, the identification of some of the oldest galaxies in the Universe with redshifts of z ~ 8 has become more routine (e.g. Lorenzoni et al. 2011; Bouwens et al. 2011b; Oesch et al. 2012b; Bradley et al. 2012).

Hence, the hunt for high-redshift objects is now focused on much more distant galaxies at redshifts > 9, with a small number of candidates being proposed (e.g. Bouwens et al. 2011a; Oesch et al. 2012a; Zheng et al. 2012; Coe et al. 2012; Bouwens et al. 2012a). This lack of observational prospect makes finding an object at extreme distances incredibly hard.

Still, a probable z  ~ 10 candidate “UDFj-39546284″ (Bouwens et al. 2011a, Oesch et al. 2012a) was found in the original WFC3/IR observations of the Hubble Ultra Deep Field (Bouwens et al. 2011b). UDFj-39546284 is believed to be the oldest observed galaxy to date. UDFj-39546284 had all the expected properties of a star-forming galaxy at z  10, i.e., a demonstrated absence of flux blue-ward of 1.6μm and a blue spectral slope red-ward of the break as evidenced by its faintness in the available IRAC observations. However, the source was only detected in a single band (Ellis et al. 2013).

Fig.1: Gray-scale image of high redshift object UDFj-39546284 from HUDF WFC3/IR (Credit: NASA, ESA, G. Illingworth (University of California, Santa Cruz), R. Bouwens (University of California, Santa Cruz, and Leiden University), and the HUDF09 Team.)

This slight problem led to a new study. Using new, deep WFC3/IR F160W observations over the HUDF09/XDF field, Bouwens et al. (2012) tested the reality of the oldest galaxy ever known candidate. They found that UDFj-39546284 is detected at 4.3 in the new observations, and at 7.1 in the full 77-orbit H160-band image. This demonstrates at > 99.99% confidence that this z  ~ 10 candidate corresponds to a real source.

N.B. The previous record holder was UDFy-38135539 (a.k.a “HUDF.YD3″): [The Discovery Of UDFy-38135539: Z ~ 8 In The Hubble Ultra-Deep Field].

Journal References:

• Bouwens, R. J. et al. (2012) Confirmation Of The z~10 Candidate UDFj-39546284 Using Deeper WFC3/IR+ACS+IRAC Observations Over The HUDF09/XDF. ArXix: [http://arxiv.org/abs/1211.3105].

• Ellis, R.S. et al. (2013) The Abundance Of Star-Forming Galaxies In The Redshift Range 8.5-12: New Results From The 2012 Hubble Ultra Deep Field Campaign. The Astrophysical Journal: Letters, 763 (1): Article I.D.: L7.

Saturn Vortex Bigger Than Jupiter’s Great Red Spot Continues To Amaze

The remarkable disturbance that began in Saturn’s northern hemisphere late in 2010 was initiated by a single discrete outburst of bright white cloud material, which spread with the prevailing zonal winds to completely encircle the planet within a matter of weeks. The disturbance generated the largest stratospheric thermal anomalies ever detected on Saturn (infrared ‘beacons’ that dominate the planetary emission), revealing dynamical coupling over hundreds of kilometers from the troposphere to the stratosphere (Orton et al. 2012). Observations made between January and March of 2011 revealed the presence of two stratospheric ‘beacons’ and only tropospheric cooling associated with the upwelling regions of the disturbance and a mysterious explosion in ethylene production (Hesman et al. 2012; Tejfel et al. 2012; Baines et al. 2012).

At one point NASA’s Cassini-Huygens orbiter detected on Saturn an “almost unbelievable” regional temperature spike of 84°C (84K) above the normal stratospheric temperature levels: the biggest jump ever recorded in our solar system (Fletcher et al. 2011). Temporal variations of Saturn’s equatorial jet and magnetic field hint at rich dynamics coupling the atmosphere and the deep interior. However, it has been assumed that rotation of the interior dynamo must be steady over tens of years of modern observations. The planetary-scale storm that perturbed Saturn’s seasonal cycle in its northern hemisphere in 2010-2011 has left a hot stratospheric vortex at 40∘N. This large vortex is still observable as of mid-2012.

Fig. 1: NASA’s Cassini-Huygens Saturn orbiter detected an “almost unbelievable” regional temperature spike of 84°C (84K) above the normal stratospheric temperature level. (Credit: NASA JPL.)

The size and power of the storm at its strongest was revealed largely through infrared imaging. What that instrument found, and what optical telescopes could not have, is that, as the visible storm spread in cloud deck of Saturn’s troposphere, waves of energy shot up hundreds of miles, creating enormous beacons of hot air which pushed into the stratosphere. Saturn completes an orbit of the sun every 30 Earth years and has a major storm at roughly the same interval. The 2011 monster storm came ten years early and lasted for more than half a year. It was also the first to be monitored by a sophisticated orbiting satellite.

As outlined by Fletcher et al. (2012) the evolutionary sequence of the Saturn storm can be divided into three phases: (I) the formation and intensification of two distinct warm airmasses near 0.5 mbar between 25 and 35N (one residing directly above the convective storm head) between January-April 2011, moving westward with different zonal velocities; (II) the merging of the warm airmasses to form the large single `stratospheric beacon’ near 40N between April and June 2011, dissociated from the storm head and at a higher pressure (2mbar) than the original beacons; and (III) the mature phase characterised by slow cooling and longitudinal shrinkage of the anticyclone since July 2011, moving west with a near-constant velocity of 2.70±0.04 deg/day (-24.5±0.4 m/s at 40N). Peak temperatures of 220K at 2mbar were measured on May 5th 2011 immediately after the merger, some 80K warmer than the quiescent surroundings.

The image here [http://sci.esa.int/science-e-media/img/35/Cassini_Saturn-Vortex_VLT.jpg] is from the Very Large Telescope in Chile, and was taken in the infrared, where the heat in the vortex is fairly obvious. Mind you, it’s not like it was a firestorm: the maximum temperature was still an extremely cold -150° C (123.15K), but compared to Saturn’s usual -220 or so degrees the spike is quite an increase.

The rise in temperature was unexpected. A 20K rise is about the usual fare for these things, but then, this wasn’t a usual storm. Apparently, this hot spot started as two separate vortices, spawned by the storm seen in visible light, and moving around the planet at slightly different speeds. They eventually merged, forming this one ginormous vortex, which at its biggest was over 62,000 km (38,000 miles) across. Interestingly, it grew to this size around the time the visible storm had faded away.

Still the enormous swing in temperatures may have changed the kind of chemistry taking place, allowing the methane present to evolve into ethylene in a way extremely atypical for Saturn. Cassini and ground-based observations have shown that the temperature and the chemistry of hydrocarbons have been perturbed in the vortex. Researchers expected the beacons to break up and cool down, but by early 2011 they had instead merged into one enormous vortex that for a brief period was larger than of Jupiter’s enormous Great Red Spot.

Suggested Further Reading:

• Besman, B.E.; et al. (2012) Ethylene Emission In The Aftermath of Saturn’s 2010 Northern Storm. American Astronomical Society, DPS meeting #44, #403.06.

• Fletcher, L.N.; et al. (2012) The Evolution & Fate Of Saturn’s Stratospheric Vortex: Infrared Spectroscopy From Cassini. American Astronomical Society, DPS meeting #44, #403.01.

• Momary, T.; et al. (2011) Saturn’s Great White Storm (2010): Correlations Between Clouds & Thermal Fields? American Geophysical Union, Fall Meeting 2011 #P13C-1683

Hanny’s Voorwerp & The Sudden Death Of The Quasar IC 2497 (SDSS J094103.80+344334.2)

The story of IC 2497 begins with the then unknown Dutch school teacher Hanny van Arkel. Whilst participating in the Galazy Zoo “citizen-science” [galaxyzoo.org] project she observed an amazing blue object (blob)* within its vicinity. The object, now known as Hanny’s Voorwerp, is brightest in the g band due to unusually strong [OIII] λ4959, λ5007 emission lines (Lintott et al. 2009) giving the object it’s strong blue appearance. Lintott et al. (2009) later reported the discovery of this object near IC 2497. The Voorwerp is a large (11×16 kpc) cloud of ionized gas approximately 45,000-70,000 lightyears away from the nucleus of the galaxy IC 2497, embedded in a larger reservoir of atomic hydrogen (Józsa et al. 2009). The galaxy of IC2497 has a measured redshift of z = 0.0502213† (Fisher et al. 1995).

N.B. *For more information on Hanny’s voorwerp: [www.astr.ua.edu/keel/.../voorwerp.html]. †Assuming, Gyr [Komatsu et al. 2009];  [Jarosik et al. 2011]) this redshift corresponds to a luminosity distance of 220.4 Mpc and a scale of 969 pc arcsec^-1.

Fig. 1: The above montage shows linear intensity displays of the SDSS guriz images at fairly high contrast, rotated to cardinal directions in orientation. (Credit: SDSS guriz; William C. Peel).

The appearance of Hanny’s voorwerp is perhaps due to one main effect: it consists of remnants of a small galaxy showing the impact of radiation from a bright quasar event that occurred in the center of a nearby galaxy, IC 2497 about 100,000 years previously. In other words, this is caused by IC 2497 previously being a quasar. So, what is so special about this object?

The current paradigm of galaxy formation places huge significance on the central supermassive black hole and its activity. In essence, this paradigm suggests that galaxy formation is significantly controlled by the energy output from supermassive black hole, which grows in highly efficient luminous quasar phases. As outlined by Lintott et al. (2009), the timescale on which black holes transition into and out of such phases is, however, unknown as they are believed to happen over millions of years as predicted by hierarchical models of cosmology and galaxy evolution. Hence, IC 2497, coupled to Hanny’s voorwerp, is special for the possibility that it now represents a quasar which has “died” or switched-off within the last 100,000 years (a relatively short about of time cosmologically speaking).

The presence of other emission lines with high ionization potentials, such as [He II] and [Ne V], together with the narrowness of the emission lines suggests that the Voorwerp is being photoionised by the hard continuum of an active galactic nucleus (AGN; an accreting supermassive black hole), rather than by other processes such as star formation or  shocks (such as might be induced by a jet; Lintott et al. 2009). Radio observations of IC 2497 reveal a nuclear source and a jet hotspot in the nucleus, and a large kiloparsec-scale structure that may be a jet (Józsa et al. 2009; Rampadarath et al. 2010). The voorwerp lieswhere this jet meets the HI reservoir and coincides with a local decrement in atomic hydrogen presumably due to photoionisation. Schawinksi et al. (2011) also conclude that the low X-ray of IC 2497 luminosity definitively rules out the presence of a currently active quasar. Shawinksi et al. (2011) go on to conclude that IC 2497′s central engine has decreased its radiative output by perhaps 2 to 4 orders of magnitude since being in a much more luminous phase within the last 70,000 years.

The sudden death of the quasar in IC 2497 might be due to a sharp decrease in the fuel supply or a change of accretion state (Schawinkski et al. 2011). Rapid changes between “high” (radiatively efficient) and “low-hard” (radiatively inefficient) states most likely driven by instabilities in the accretion disk have been seen in Galactic x-ray binaries (XRBs) which typically have black holes ~ (Nayakshin et al. 2000; Fender & Belloni 2004; Fender et al. 2004; Prat et al. 2010). It has been suggested that supermassive black holes are essentially scaled-up versions of XRBs with similar black hole accretion physics (e.g., Maccarone et al. 2003; McHardy et al. 2006; Körding et al. 2006). IC 2497 may be the first quasar where such a rapid transition between accretion states is observed, of the kind routinely seen in XRBs.

Journal References:

• Lintott, C.J et al. (2009) Galaxy Zoo: `Hanny’s Voorwerp’, A Quasar Light Echo? Monthly Notices of the Royal Astronomical Society, 399 (1): pp. 129-140.

• Rampadarath, H.; Józsa, G. I. G. et al. (2010) Hanny’s Voorwerp . Evidence Of AGN Activity & A Nuclear Starburst In The Central Regions Of IC 2497. Astronomy & Astrophysics, 517 Article I.D.: L8.

• Schawinksi, K.; Lintott, C.J.; van Arkel, H. et al. (2011) The Sudden Death Of The Nearest Quasar. The Astrophysical Journal Letters, 724 (1): pp. L30-L33.

Constraints On The Compact Object In The Eclipsing HMXB XMMU J013236.7+303228

Lurking within Messier 33 (a.k.a the Triangulum Galaxy or NGC 0598) lies the high-mass X-ray binary (HMXB) XMMU J013236.7+303228. As eluded to in previous posts, most stars are actually a member of a douplet: a binary system. Some binary systems are strong in X rays, and in which the normal stellar component is a massive star: usually an O or B star, perhaps sometimes a Be star or a blue supergiant. The compact, X-ray emitting component has been theoretically established to the result of a neutron star, or possibly a black hole, where a considerable fraction of the stellar wind of the massive normal star is captured by the compact object emitting photons in the X-ray energy band as it falls onto the compact object. XMMU J013236.7+303228 is one such HMXB system where a neutron star is the cause of the observed X-ray emission from the system (Pietsch et al. 2004).

There are many differing factors which effect potential neutron star masses, such as the initial mass of the progenitor’s stellar core, the details of the explosion (in particular mass accretion as the explosion develops), subsequent mass accretion from a binary companion and the pressure-density relation, or equation of state (EOS), of the neutron star matter. Such factors, differing as much as they are numerous, are extremely interesting in stellar evolutionary studies.

On the low-mass end, producing a neutron star requires the progenitor’s core to exceed the Chandrasekhar mass, which depends on the uncertain electron fraction. Theoretical models place this minimum mass in the range ≳ 0.9 – 1.3 (Timmes et al. 1996). The largest possible neutron star mass depends on the unknown physics determining the EOS – for example whether kaon condensates or strange matter can from in the interior (e.g. Lattimer & Prakash 2005). The highest-mass neutron star to-date, the millisecond pulsar (MSP) PSR J1614–2230, “weighs” in at 1.97 ± 0.04 (Demorest et al. 2010), which already rules out the presence of exotic hadronic matter at the nuclear saturation density (Demorest et al. 2010; Lattimer et al. 2010).

If one wishes to test the predictions made by a supernova model or a binary evolution model it is necessary to determine neutron star masses at the extreme end of the high-mass spectrum in a variety of systems with different evolutionary schemes and progenitor masses. One such scheme, where a neutron star accompanied by either a high-mass star or another neutron star is believed to have accreted little to no matter over its evolutionary lifetime.

Recently, Bhalerao et al. (2012) presented optical spectroscopic measurements of the eclipsing High Mass X-ray Binary XMMUJ013236.7+303228 in M33. Based on spectra taken at multiple epochs of the 1.73 day binary orbital period they determined physical as well as orbital parameters for the donor star: a B1.5IV sub-giant with effective temperature is T = 22000 − 23000K. From the luminosity, temperature and known distance to M33 (~800 kpc) Bhalerao et al. (2012) derived a radius of .

From the radial–velocity measurements, Bhalerao et al. (2012) determined a velocity semi-amplitude of , whilst from the the physical properties of the B-star determined from the optical spectrum, they were also estimate the star’s mass to be . Based on the X-ray spectrum, the compact companion is likely a neutron star, although no pulsations have yet been detected. Using the spectroscopically derived B-star mass the neutron star mass to be ; which is a result consistent with the neutron star mass in the HMXB Vela X-1, but heavier than the canonical value of 1.4 found for many millisecond pulsars.

Bhalerao et al. (2012) attempted to use as an additional constraint that the B star radius, inferred from temperature, flux and distance, should equate the Roche radius, since the system accretes by Roche lobe overflow. This leads to substantially larger masses, but from trying to apply the technique to known systems the masses are consistently overestimated. Attempting to account for that in our uncertainties, the most accurate derivation for the mass of the neutron star was  and for the B-type companion star . Bhalerao et al. (2012) conclude that precise constraints require detailed modeling of the shape of the Roche surface.

References:

• Pietsch, W.; Misanovic, Z.; Haberl, F.; Hatzidimitriou, D.; Ehle, M.; Trinchieri, G. (2004) XMM-Newton Survey Of The Local Group Galaxy M 33. Astronomy & Astrophysics, 426: pp.11-24.

• van Kerkwijk, M. H.; Breton, R. P.; Kulkarni, S. R. (2011) Evidence For A Massive Neutron Star From A Radial-Velocity Study Of The Companion To The Black-Widow Pulsar PSR B1957+20. The Astrophysical Journal, 728 (2): Article I.D.# 95.

• Bhalerao, V.B.; van Kerkwijk, M.H.; Harrison, F.A. (2012) Constraints On The Compact Object Mass In The Eclipsing High-Mass X-Ray Binary XMMU J013236.7+303228 In M 33. The Astrophysical Journal, 757 (1): Article I.D.# 10.

Suggested Further Reading:

• Liu, Q. Z.; van Paradijs, J.; van den Heuvel, E. P. J. (2000) A Catalogue Of High-Mass X-Ray Binaries (HMXBs). Astronomy & Astrophysics Supplement, 147: pp.25-49.

• Seward, F.D.; Charles, P.A. (2010) Exploring The X-Ray Universe. Cambridge University Press, 2nd Edition. Cambridge, United Kingdom. 

The Erratic Nature Of The Black Hole GRS J191511.6+105644 (V1487 Aquilae)

Since its discovery in 1994, V1487 Aquilae (also known as GRS 1915+105) has been the subject of intense study. Using the ISAAC instrument on the VLT’s 8.2-metre ANTU telescope at the ESO Paranal Observatory and NASA’s Chandra X-ray Observatory, countless scores of astronomers peered into a remote area of the Milky Way to probe the binary system GRS 1915+105: a system which can be conclusively described as a black hole X-ray binary with a low mass optical counterpart (Castro-Tirado, Brandt & Lund 1992; Mirabel et al. 1994; Belloni et al. 1997). Massing in at 14, this is the heaviest known stellar-sized black hole in our Galaxy.

Fig.1: Digitized Sky Survey image showing the crowded field around the microquasar GRS 1915+105 located within the Aquila constellation located near the plane of our Galaxy: RA 19h15m11.6s | Dec +10°56’44.00”. (Credit: X-ray (NASA/CXC/Harvard/J.Neilsen); Optical & IR (Palomar DSS2)).

A few objects, for example SS433, within our own Galaxy look very much like miniature versions of the most energetic quasi-stellar objects (QSO’s), observed to emanate from he centres of an interesting class of galaxies known as active galaxies (AGNs) via processes of accretion onto a central black hole. The only difference between the QSO class and these smaller versions, known as microquasars, is rather obviously their size of the black hole onto which material is being accreted (Fender et al. 1999).

Close up, microquasars are binary stellar systems in our Galaxy in which a more or less normal star, such as our Sun, orbits an intensely gravitationally compact object, such as a neutron star or a black hole (Sunyaev et al. 1996). Those microquasars also show energetic outflows and signs of accretion of matter onto the compact object familiar with QSOs (Abell & Margon 1979; Bregman et al. 1981; Begelman et al. 2006). Not unexpectedly, it appears that the most enigmatic of these systems are the ones that contain a black hole.

GRS 1915+105, located in the Aquilae constellation, is one of a handful of such microquasars known in our Galaxy that contains a black hole compact source. First discovered in 1994 by the GRANAT X-ray satellite, GRS 1915+105 was revealed to have an interesting, if highly erratic, series of highly luminous outbursts in the X-ray wave bands.

Fig.2: The constellation of Aquila. Insert box is the field of view as used for the Palomar Digital Sky Survey 2 optical/infra-red image. (Credit: International Astronomical Union).

The variable X-ray radiation has been interpreted an infall of matter onto a spinning Kerr black hole from the inner region of a surrounding accretion disk (Greiner, McCaughrean & Cuby 2001), providing evidence for the erratic and sporadic nature of GRS 1915+105. This enigmatic source was also observed to eject clouds of hot gas at velocities very close to the speed of light. GRS 1915+105 is thus a prototype microquasar and has become a main target for the study of accretion onto a black hole of stellar mass (Fender et al. 1999). This variability has allowed a precise determination of the mass of the compact object in GRS 1915+105: 14.19±0.13.

Knowing the mass of the black hole in GRS 1915+105 now poses challenges to several fields in astrophysics. First of all, it is not easy to understand how such a massive black hole can be formed in a binary stellar system. It is well known that the most massive stars lose significant fractions of their mass through violent stellar winds at the end of their lives. Interaction among the two stars in a binary system can further increase the mass loss by the massive star. Therefore, how any star can retain enough mass to eventually end up forming a black hole as heavy as 14?

Secondly, the spin of the black hole object in GRS 1915+105 is suspected to provide the unusually high accretion disc temperature. If a black hole spins in the same direction as the matter orbiting it within the disc then it can spread inwards, which results in a much hotter disc.

Two X-ray binaries are known to be very hot, GRS 1915+105 and Nova Scorpii (Zhang et al. 1994; Shahbaz et al. 1999; Mason et al. 2010), and it was therefore believed that these two contain black holes that must spin rapidly. A completely different line of evidence for black hole rotation comes from the quasi-periodic oscillations often seen in X-ray binaries. Those oscillations are generally interpreted as due to effects of the spinning black hole on the surrounding accretion disk, although the exact mechanism is a matter of debate.

However, the new mass determination for the black hole in GRS 1915+105 indicates that the picture may not be as simple as that. In fact, if GRS 1915+105 and Nova Scorpii both have rapidly spinning black holes, none of the current theories for the quasi-periodic oscillations seem to work. And so, as is often the case in science, new information also brings new puzzles.

Journal References:

• Castro-Tirado, A.J. et al. (1994) Discovery & Observations By Watch Of The X-Ray Transient GRS 1915+105. The Astrophysical Journal: Supplement Series, 92 (2) pp. 469-472.

• Mirabel I. F., Rodríguez L. F. (1994) A Superluminal Source In The Galaxy. Nature, 371 pp.46-48.

• Fender, R.P. et al. (1999) e-MERLIN Observations Of Relativistic Ejections From GRS 1915+105. Monthly Notices Royal Astronomical Society, 304 (1) pp.865-876.

• Vierdayanti, K.; Mineshige, S.; Ueda, Y. (2010) Probing The Peculiar Behavior Of GRS 1915+105 At Near-Eddington Luminosity. Japan Astronomical Society Publications, 62 (2) pp.239–253.

Suggested Further Reading:

• Seward, F.D.; Charles, P.A. (2010) Exploring The X-Ray Universe. Cambridge University Press,2nd Edition. Cambridge, United Kingdom. 

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

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 Hole. The 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 243-49. The 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 Transitions. The Astrophysical Journal, 743 (1): Article I.D. #6.