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In the news: Giant gas bubbles, microquasars and unusually large black holes

November 26, 2010

There’s been much excitement recently over the discovery by Fermi/LAT of two giant gas bubbles approximately 25,000 light years across in our galaxy. They span more than half the gamma-ray sky corresponding to a location in the visible sky stretching fron Virgo to Grus.

As with most of these discoveries, there is much rampant (and probably premature) speculation about their origin:

“The bubble emissions are much more energetic than the gamma-ray fog seen elsewhere in the Milky Way, and appear to have defined edges, suggesting it formed as a result of a large and rapid energy release. One possible culprit includes a particle jet from the supermassive black hole in the Galaxy’s centre, a phenomena observed in other galaxies, too. But while there is no evidence for such a jet being active today, the bubble could represent an ancient jet. An alternative theory is that the bubbles were blown out from gas outflow during a burst of star formation, another process also seen in other galaxies.”

If they are the result of a relativistic jet from the black hole at the centre of our galaxy, then this is yet another example of a correspondance between galactic-scale astrophysical proceses and similar local-scale processes.

Bubbles of gas created by the action of relativistic jets from X-ray binaries (XRBs) upon the local interstellar medium (ISM) are actually quite common, for example, the bubble around the microquasar Cygnus X-1, as described by the astronomer Elena Gallo in her 2006 paper (Gallo 2006):

“More recently, a low surface brightness arc of radio emission has been discovered around Cygnus X-1…and interpreted in terms of a shocked compressed hollow sphere of free-free emitting gas driven by an under-luminous synchrotron lobe inflated by the jet of Cygnus X-1”

Unlike the famous ring around SN1987A, this material definitely isn’t the result of a supernova explosion. From the discovery paper by Gallo et al (2005):

“A ring of radio emission – with a diameter of ∼1 million AU – appears northeast of Cygnus X-1…and seems to draw an edge between the tail of the nearby HII nebula Sh2-101 (whose distance is consistent with that to Cygnus X-1) and the direction of the radio jet powered by Cygnus X-1.  Since Cygnus X-1 moves in the sky along a trajectory which is roughly perpendicular to the jet and thus can not possibly be traced back to the ring centre, this rules out that the ring might be the low-luminosity remnant of the natal supernova of the black hole. In analogy with extragalactic jet sources, the ring of Cygnus X-1 could be the result of a strong shock that develops at the location where the collimated jet impacts on the ambient interstellar medium. The jet particles inflate a radio lobe which is over-pressured with respect to the surroundings, thus the lobe expands sideways forming a spherical bubble of shock-compressed ISM, which we observe as a ring because of limb brightening effects.”

Even though Cygnus X-1 is a relatively well-known galactic object, probably the most exotic example of a relativistic-jet driven nebula is that around the Ultraluminous X-Ray source S26 in the galaxy NGC7793, which is located 12 million light-years distant in the constellation of Sculptor:

“A black hole only slightly heavier than our Sun is emitting the most powerful jets of energy ever seen, rivaling that of quasars a million times larger, and creating a bubble of hot gas and fast-moving particles 1000 light-years across…The bubble has a diameter of 1000 light-years and is expanding at about a million kilometers per hour. The black hole, located 12 million light-years away in the outer spiral of galaxy NGC 7793, has been blowing the bubble for about 200,000 years.”

This nebula is the subject of a new paper in this month’s Monthy Notes of the Royal Astronomical Society journal by Roberto Soria and collegues (Soria et al 2010). Here is the abstract in toto:

“We have studied the structure and energetics of the powerful microquasar/shock-ionized nebula S26 in NGC7793, with particular focus on its radio and X-ray properties. Using the Australia Telescope Compact Array, we have resolved for the first time the radio lobe structure and mapped the spectral index of the radio cocoon. The steep spectral index of the radio lobes is consistent with optically-thin synchrotron emission; outside the lobes, the spectral index is flatter, suggesting an additional contribution from free-free emission, and perhaps ongoing ejections near the core. The radio core is not detected, while the X-ray core has a 0.3-8 keV luminosity ~6 × 1036 erg s-1. The size of the radio cocoon matches that seen in the optical emission lines and diffuse soft X-ray emission. The total 5.5-GHz flux of cocoon and lobes is ~2.1 mJy, which at the assumed distance of 3.9 Mpc corresponds to about three times the luminosity of Cas A. The total 9.0-GHz flux is ~1.6 mJy. The X-ray hotspots (combined 0.3-8 keV luminosity ~2 × 1037 erg s-1) are located ~20 pc outwards of the radio hotspots (i.e. downstream along the jet direction), consistent with a different physical origin of X-ray and radio emission (thermal-plasma and synchrotron, respectively). The total particle energy in the bubble is ~1053 erg: from the observed radio flux, we estimate that only approximately a few times 1050 erg is stored in the relativistic electrons; the rest is stored in protons, nuclei and non-relativistic electrons. The X-ray-emitting component of the gas in the hotspots contains ~1051 erg, and ~1052 erg over the whole cocoon. We suggest that S26 provides a clue to understand how the ambient medium is heated by the mechanical power of a black hole near its Eddington accretion rate”.

ULX-driven nebula are an important area of research because their projenitors represent a possible “missing link” between galactic-scale quasars and their smaller stellar cousins, microquasars:

“Recent X-ray observations of galaxies have uncovered a populations of sources that have high X-ray luminosities but are not coincident with the nucleus of the galaxy. They have luminosities which are greater than that possible for a normal black hole to be powering them.

There is a maximum possible luminosity of a black hole as photons carry momentum and so can exert a pressure. This means that if there are enough of them they will blow away in-falling material, which sets this maximum luminosity, and it depends on the mass of the central object. Hence astronomers know that the luminosities are too high for a black hole which has a mass a few times that of the Sun to be powering these sources. These sources are long-lived, and so they cannot be special types of supernovae for example.”

The key debate over the true nature of ULXes currently is whither they are a new type of object – so-called Intermediate Mass Black Holes (IMBH), or simply standard XRBs that appear more luminous due to relativistic beaming (effectively making them, by a similar analogy as quasars and microquasars, microblazars):

“These sources are too luminous for a normal black hole – how about a more massive one? The masses required are in the region of 100 Solar Masses or more. The problem with this explanation is that there is no clear way to create black holes of this mass. Either stars could merge at the centre of clusters, faster than they evolve, so forming a very massive star, which when it dies forms a massive black hole; or black holes could merge, also at the centres of clusters. Both of these scenarios have their disadvantages, merging black holes tend to get catapulted out of the centre of the clusters and heavy stars have a very strong wind, see High Mass X-ray Binaries, and so the final black hole mass would be less than the mass of the merged stars. Surveys of some clusters show that there is some mass present that we do not see, and this could be as a result of these IMBHs.

However, what could occur is that the emission from these objects is not isotropic, but concentrated into beams, see jets. This reduces the necessity for the most massive black holes, and means that ULXs could just be special X-ray Binaries. As to why some X-ray Binaries have emission concentrated into particular directions and others do not is another matter. The other explanation is that the maximum luminosity limit is temporarily over-stepped for a few years or decades.”

The Soria et al paper is important in this context because it demonstrates that whatever its rrue nature, S26 demonstrates properties in common with both its smaller and larger cousins. They observed similar components in the nebula that also occur in certain powerful radio galaxies (called Fanaroff-Riley Type II (FRII) galaxies):

“We showed that its structure is a scaled-down version of powerful FRII radio galaxies, with a core, radio lobes, X-ray hot spots and cocoon. It is the first time that all these elements have been found in a non-nuclear BH”.

Yet this object also shares many characteristics with standard microquasars such as the aforementioned Cygnus X-1, yet on a much larger scale:

“Hα images (PSM10) may suggest an even larger size, ~ 340×170 pc. Thus, the volume-averaged shell radius Rs ≈ 100 pc. Its characteristic size is an order of magnitude larger than the jet driven bubble around Cyg X-1, which has an estimated jet power ~ 1037 erg s−1

However they report an unusual spacial anticorrelation between X-ray and radio emitting regions, which is the reverse of the standard configuration found in microquasars:

“We showed that the radio and X-ray hot spots are not spatially coincident: the X-ray hot spots are ≈ 20 pc further out than the peak of the radio intensity in the lobes.”

They then suggest that this means that X-ray and radio emission come from different populations of radiating particles, with the X-ray emission thermal in origin (like that from accretion disks), and the radio emission originating, like microquasars, via the Sychrotron process.

As mentioned above, the calculated jet-power for objects such as S26 is actually greater than is seemingly possible using standard BH models, and on the assumption that relativistic beaming isn’t involved, they propose, like another recent paper on this object (Pakull et al. 2010), that the extreme luminosity shown in this microquasar is due primarily to thermal emission from non-relativistic protons and nuclei, outstripping any contribution from non-thermal relativistic particles that are normally found in standard microquasars:

“The total particle energy in the bubble is ~ 1053 erg. Based on the measured radio flux and size of the bubble, and using standard equipartition relations for microquasar lobes, we estimated that the energy carried by the synchrotron-emitting relativistic electrons is a few 100 times less than the energy stored in protons, nuclei and non-relativistic electrons; non-relativistic particles provide most of the pressure to inflate the bubble.”

Whatever its true nature though, S26 will continue to be an object of great importance to those scientists engaged in unravelling the astrophysical processes behind relativistic jets in the Universe.

References:

E. Gallo (2006) Radio emission and jets from Galactic microquasars Proceedings of the VI Microquasar Workshop: Microquasars and Beyond. September 18-22, 2006, Como, Italy., p.9.1 (ADS)
E. Gallo et al (2005) Nature 436 819 (ADS)
M. Pakull et al (2010) Nature 466 209 (ADS)
R. Soria et al (2010) MNRAS 409 541 (ADS)

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