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The hunt for black holes are never easy, that is because nothing can escape from a black hole, not even light. That makes direct observations of this natural phenomenon impossible. In this essay, I am going to discuss on the various techniques used by scientist to detect the presence of black holes. I will also discuss on some of the stellar-mass black hole candidates within the Milky Way and the connection between X-ray sources and black holes.

Techniques of detecting a black hole

Accretion Disk and Shooting Gas Jets

We cannot have visual observations of black holes since not even light can escape it. To detect its presence we have to study it indirect by observing the behavior stars, gases and matters within its vicinity. Therefore, a good candidate of finding a black hole is to look at close binary star systems (Refer to Table-1 for a list of stellar-mass black hole candidates in binary star systems within the Milky Way). If one of the companions in a close binary system is a black hole and is orbiting close enough, tidal forces can draw material from its companion star. If the black hole is rotating, the materials falling into the black hole will form an accretion disk. Materials in the accretion disk also plays another role in the black hole’s spin, the formation of two narrow but powerful jets of gas that appears shooting out of some black holes.

Although accretion disk does not definitively represents a presence of a black hole, it does point scientist to its possibility of having one. Accretion disk can be explained by the presence of a white dwarf or a neutron star. Therefore we have to eliminate that possibility by studying the gravitational influence of the black hole candidate. As the duo orbit around their centre of mass, the visible star would wobble. By observing the Doppler shift of the companion star spectral lines, we can conclude the mass of the candidate. If for example it is calculated to be 7 solar masses or more, is too large for to be a white dwarf or a neutron star, we can assume that it is a good black hole candidate.

As the in-falling materials spirals into the hole it is heated up by friction. These extremely heated gases emit x-rays that fluctuates rapidly ^{2, 3}. We shall discuss further upon X-rays later in this essay.

Figure 1 – Illustration of a black hole in a binary star system.

Gravitational Lensing

Apart from observing binary star systems to look for black holes, we can also infer a black holes presence from gravitational lensing. Since the gravitational pull of a black hole is so strong, a black hole passing in front of a light source will cause the light wave from the light source to bend around it 4. (Refer to Table-2 for a list of stellar-mass black hole candidates found by gravitational lensing within the Milky Way).

Figure 2 – Illustration of a black hole causing gravitational lensing.

Gravitational Waves

Predicted by Einstein’s General Theory of Relativity, gravitational waves are formed by the fluctuation of the curvature of spacetime by the presence and oscillations of massive objects. The greater the object is, the greater the curvature. If the object oscillates in the right way, ripples in spacetime will spread like ripples in a body of water. This is called gravitational waves or gravitational radiation. Detecting such signals is extremely difficult because gravitational radiation is a lot weaker then electromagnetic radiation. Indirect evidence has come from the observation of a binary system of 2 neutron stars, the Hulse-Taylor binary (PSR B1913+16). ⁵

To directly observe this, scientists around the world have built sensitive detectors to detect gravitational waves. There are two types of such detectors, through laser interferometer and resonant detectors. Here is a listing of these facilities: ⁶

• Interferometric Detectors

• LIGO (USA) – 2 Detectors
• LISA (USA/EU) – 2 Detectors
• GEO 600 (Germany)
• TAMA (Japan)
• VIRGO (France)
• AIGO (Australia)

• Resonant Detectors

• ALLEGRO (USA)
• AURIGA (Italy)
• EXPLORER (Italy)
• NIOBE (Australia)
• miniGRAIL (Netherlands)
• GRAVITON (Brazil)

Gamma Ray Burst

Gamma ray burst (GBR) are one time occurrences that are the most luminous events in the universe. The common consensus amongst the scientific community is that GBRs occur with the gravitational collapse of a massive star. It can be the collision of between two orbiting neutron stars, which can possibly form a black hole. Or it can even be the a neutron star colliding into a black hole. Gamma ray detecting satellites such as the now defunct Compton Gamma-Ray Observatory detects on average one GBR per day. However, all of the detect GBR are located beyond our Milky Way ⁷.

Stellar-Mass Black Hole Candidates within the Milky Way

Stellar-Mass black holes in binary star systems [Table -1]

Detectable due to interactions with the companion star (black hole mass, 3-20 solar masses) ⁸

Isolated stellar-mass black holes [Table – 2]

Detectable due to gravitational lensing of background light (black hole mass, 3-20 solar masses) ⁸

Cygnus X-1

Located in the constellation of Cygnus, the swan, it was discovered in 1970s by the Uhuru X-ray satellite. Since it was the first X-ray source to be discovered in the constellation of Cygnus, it was given the designation of Cygnus X-1. A binary star system, its visible orbiting companion is a B0 supergiant known as HDE 226868 with a surface temperature of 3.1 x 104K. Its spectral line indicates that HDE 226868 has an orbital period of 5.6 days. The unseen companion however, is too dim to yield it’s own set of spectral lines. From what we understand of supergiant stars, HDE 226868 is estimated to have a mass of 30 solar masses. We can therefore infer that the mass of its unseen companion to be about 7 solar masses. Because 7 solar masses it dim far too for it to be a white dwarf or a neutron and also from the fact that it does not emit any visible light, it is likely that the unseen companion could be a black hole. We are not totally sure if it could be a black hole because the mass of HDE 226868 is only estimated to be at 30 solar masses. For its spectral class, it could have a lower mass, which would mean that its unseen companion might be a neutron star ⁹.

V404 Cygni

This is probably the best candidate as a stellar-mass black hole. The binary star system yields a visible orbiting companion is estimated to be a main-sequence F0-K2 star. Doppler shift measurements indicated an orbit of 6.47 days, which puts its unseen companion an estimated mass of 6.26 solar masses. Much bigger then a white dwarf or a neutron star, we can conclude with a high degree of certainty that the unseen companion is a black hole ⁹.

A0620-00

At a distance of a mere 2,700 light years, this binary star system black hole candidate is our closest known black hole. Flaring twice in the last century, 1917 and in 1975, its second flare was detected by the orbiting British Ariel 5 satellite hence it’s A reference. Visual observations of its visible companion shows that it is an orange K5 main-sequence star with an orbit of 7.75 days. Since its visible star is relatively faint, scientist were able to observe the spectral lines of visible companion and its X-ray source. Thus, it is estimated that the mass of the X-ray source is between 3.2 solar mass and 9 solar mass ⁹.

X-ray sources from a black hole

As surrounding materials and gases from a companion star goes into the accretion disk around a black hole, friction from collisions between the particles and gases heats up as angular momentum causes it spirals into the black hole to a temperature reaching 2 x 106K. This produces X-rays that flicker or vary in intensity within a second after which they disappear beyond the event horizon. Satellites such as the Chandra X-Ray Observatory detect these X-rays. The rate of X-ray emitted is highly variable and irregular up to one-hundredth of a second. Since nothing can travel faster then the speed of light, the X-rays that are emitted around the object cannot be larger then 3000km across ¹⁰.

In Conclusion

In this essay, I have discussed on the various methods used at present to aid in the detection of black holes through indirect observations. This includes observing a black hole candidates effects on its surroundings in the form of accretion disks and gas jets. We have also discussed on observations of gravitational lensing and the use of sensitive detectors like laser interferometer and resonant detectors to detect gravitational waves. I briefly discussed on stellar-mass black holes within our Milky Way that scientist have identified so far, either from observations of binary star systems or by gravitational lensing. And how the interaction of particles and gases in accretion disks emits X-rays, which we can detect.

References

1. Thorne, Black Holes and Time Warps, pp 349 ~ 350
2. Freedman & Kaufman, Universe 7th Edition, pp 531, 546
3. Don Nardo, Black Holes, pp 46 ~ 47
4. Freedman & Kaufman, Universe 7th Edition, pp 600 ~ 601
5. Freedman & Kaufman, Universe 7th Edition, pp 538 ~ 539
6. The Gravitational Wave Community
7. Gamma-Ray Bursts: Introduction to a Mystery
8. List of black hole candidates
9. Freedman & Kaufman, Universe 7th Edition, pp 540, 541
10. Freedman & Kaufman, Universe 7th Edition,, pp 540

Illustrations in this essay are works of the author. HET 603 Essay.