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TRANSIENT OPTICAL SIGNAL IN CLOSE PROXIMITY TO M74 X-1 ULX CXOU J013651.1+15454
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TRANSIENT OPTICAL SIGNAL IN CLOSE PROXIMITY TO M74 X-1 ULX CXOU J013651.1+15454
Rudy E. Kokich, Alexandra J. Kokich, Andrea I. Hudson
Using a small TSAPO65Q astrograph, on 19 Dec 2017 we accidentally recorded a transient optical signal (TOS) of apparent magnitude around 17.4 in close proximity to the ultra-luminous x-ray source (ULX) in the nearby spiral galaxy M74. Since its discovery in 2005 by NASA's Chandra X-ray Observatory space telescope, evidence is mounting that this strong x-ray source is generated by an intermediate-mass black hole of approximately 10,000 solar masses. It seems counterintuitive that a telescope of such modest aperture could reveal an optical signal from a black hole in another galaxy 32 million LY distant. But, preliminary calculations show that energy requirements for the detected signal are five hundred times lower than those of a type Ia supernova, and that mass requirements for the generation of that energy involve only a minute fraction of a small planet.
Image 1: Transient Optical Signal after post-processing. See the Appendix for the unprocessed image.
Black holes are regions of space of such extreme mass density that no electromagnetic radiation or particle of matter can escape their intense gravitational fields. In 1783 John Michell, an English clergyman and natural philosopher, was the first to propose the existence of massive "dark stars" with such great gravitational pull that their escape velocity would exceed the speed of light. In 1796 the idea was given credence by French mathematician Pierre-Simon Laplace. In 1915 Einstein's theory of general relativity offered a theoretical framework for explaining how a sufficiently compressed mass might curve spacetime to create a black hole. Later the same year, Karl Scwarzschild offered the first precise solution to Einstein's field equations relating mass, energy, and gravitation in terms of local spacetime curvature.
Scwarzschild's equation for a non-rotating black hole, in which G is the gravitational constant, essentially defines the radius, Rs, of a sphere which must contain mass M in order for the escape velocity to be equal to the speed of light, c.
Rs = 2GM / (c^2)
Rs, named the Schwarzschild radius, specifies the surface of a sphere, or the event horizon of a non-rotating black hole, within which all particles, photons included, must fall into the central body. In the middle of a black hole lies a point, a gravitational singularity, where spacetime curvature, gravitation, and density reach infinity, and physical laws cease to make mathematical sense. At 1.5 times the Schwarzschild radius lies the photon sphere, within which photons, if undisturbed by infalling matter, are contained in circular orbits around the black hole.
The radius, Rs, of the event horizon is linearly proportional to the mass, M, while its volume, V, is proportional to the third power of the radius ( V = (4/3) p Rs^3 ). As the mass doubles, the radius of the event horizon doubles, but its volume increases by a factor of eight. Therefore small black holes have much higher average density ( D = M / V ) than large ones. The most massive black holes have lower average density than main sequence stars. Average density of a black hole is given by:
D = M / ((4/3) p Rs^3)
It is not surprising that initially most scientists, Einstein included, considered this black hole model to be a mathematical curiosity with no equivalent in physical reality. The attidude persisted for decades, especially in the presumed impossibility of observational confirmation. Since the 1950's astronomy research gradually expanded from visible light into the full range of the electromagnetic spectrum, from radio waves to gamma rays. Discovery of new objects, especially in the x-ray range, like active galactic nuclei (AGN), quasars, magnetars, and blazars, suggested that friction and nuclear reactions alone were insufficient to generate temperatures required for the release of such enormous levels of energy. X-rays are a form of electromagenetic radiation of frequency and energy levels a thousand times higher than those of visible light. Whereas visible light is emitted at temperatures around 10,000 K, x-rays are produced when matter is heated to tens and hundreds of millions K. The existence of black holes, where such heat is created by gravitational compression of matter, became ever more plausible as a possible explanation.
The first suspected stellar mass black hole, discovered in 1971, was Cygnus X-1, an x-ray binary star system in which an invisible, compact object appears to be pulling mass from a large, visible companion. Stellar mass black holes are now thought to form by gravitational collapse of heavy stars at the end of their life cycles. In 1974, the detection of a strong x-ray source in the center of our galaxy, Sagittarius A, led to the discovery of a supermassive black hole (SMBH) weighing about four million suns. Evidence now suggests that most, if not all, galaxies have a supermassive black hole in the center, some as massive as billions of suns, which seems to play a cruicial role in the formation of the galaxy. In 2000, in galaxy M82, NASA's Chandra X-ray Observatory space telescope discovered a new type of black hole of intermediate-mass between stellar mass black holes and supermassive black holes. In 2005, in spiral galaxy M74, a similar x-ray source was discovered around an intermediate-mass black hole estimated at 10,000 solar masses. Such objects which radiate 10 - 1,000 more x-ray energy than neutron stars were named Ultra-Luminous X Ray Sources (ULX).
In addition to previously described (1) singularity, (2) event horizon, and (3) photon sphere, the current black hole model includes:
(4) the accretion disk in which matter captured by the black hole forms a disk of superheated plasma whirling at nearly light speed around the event horizon. Surrounding the accretion disk there may be an opaque torus of cooler matter in high orbit around the black hole which gradually spirals into the accretion disk.
(5) polar jets, also called astrophysical jets, relativistic jets, or x-ray jets, which are formed along the black hole's axis of rotation as ionized particles in the accretion disk spiral close to the event horizon, accelerate to relativistic speeds, and escape in the form of collimated particle beams in the direction perpendicular to the accretion disk. The process is not well understood, but is probably driven by extreme magnetic fields and kinetic energy transfer from other particles approaching in the equatorial plane. Such jets, thin and long, stretching over many thousands of light years, were first detected in 1918 on optical images of M87's core, which was later found to contain a supermassive black hole. Smaller jets are found near neutron stars and stellar mass black holes, while far more modest versions appear near some protostars. Surrounding each polar jet is a wide cone of ionized gas clouds radiating the classical emission line spectrum.
Image 2: An accreting black hole model showing the event horizon, polar jets, and the accretion disk
A non-accreting black hole releases no radiation, and can only be detected through gravitational lensing and gravitational wave analysis. In black holes which are actively consuming matter, the accretion disk and polar jets, surrounded by immensely strong magnetic fields, emit intense radiation throughout the spectrum by several mechanisms.
(A) Not all matter which approaches the accretion disk is absorbed. Computer simulations show that in star-disk collisions, when a star is attracted into the accretion disk of a black hole, only about half of its mass is captured. The other half is gravitationaly dispersed into a cloud of hot plasma, and flung out into space resulting in a luminous flare lasting hundreds of days.
Image 3: In a star-disk collision about 50% of the star's mass is catapulted away from the black hole
(B) The matter in the form of stars, gas, and dust which is actually captured by the gravity vortex is superheated to tens of millions of degrees by gravitational compression, friction, and fission reactions whereby atoms and molecules are torn apart into subatomic particles. In a non-rotating black hole about 6% of incoming matter is converted to energy and radiated into surrounding space. In a rapidly rotating black hole of the same mass, the diameter of the event horizon is much smaller, and up to 42 % of incoming matter can be converted to radiation. The accretion disk emits radiation at all wavelengths, but most intensely in the high energy x-ray range.
© Polar jets emit radiation from surrounding ionized gas clouds and from superheated plasma within the jet itself where temperatures may reach billions of degrees. This results in strong radiation at all wavelengths, including optical, but most intense in the high energy end of the spectrum. In optical and x-ray images, polar jets display shock waves of bead-like zones of plasma congestion receding from the black hole at nearly the speed of light.
Image 4: Optical image of M87 X-1 supermassive black hole jet
(D) All methods of energy radiation mentioned so far are thermal processes, depending on particles of matter at extreme densities heated to extreme teperatures. Black holes also radiate energy through a (generally speaking) non-thermal process whereby charged particles passing through strong magnetic fields become accelerated (diverted) into helical paths along magnetic field lines. Radiation emitted by particles moving at relativistic speeds is called synchrotron radiation, or cyclotron radiation for slower particles. In the process of emitting radiation the particles lose energy, and undergo magnetic braking. The vast majority of particles involved are electrons, but protons, heavier ions, and positrons (if any) may also be involved. The process generates a wide range of wavelength signals, from radio waves to optical to x-rays, where the signals do not precisely coincide in location or in timing.
Black hole emissions can be highly variable. Numerous black holes manifest optical long-term variability of several magnitudes over an observation period of years. It is thought that long term variability is due to thermal processes resulting from changes in the quantity of matter flowing into the gravitational vortex. This is primarily determined by the availability of gas, dust, and star material in the black hole's neighborhood. However, the Eddington effect plays an additional role in variability, whereby a burst of energy from inflowing matter generates radiation pressure, temporarily resisting inflow of additional matter. Plumes of superheated plasma ejected from a black hole, either at the periphery of the accretion disk of within a polar jet, also contribute to variability, and may last from hundreds of days to years.
Image 5: Long term variability in Quasi-Stellar Objects (QSO) powered by super-massive black holes
Black holes also manifest short-term variability. For example, in 2002 infrared flux density of Sagittarius A SMBH was measured to change by a factor of 4 in one week, and by a factor of 2 in merely 40 minutes (https://arxiv.org/abs/astro-ph/0309076). M74 X-1 ULX, the subject of this article, also exhibits pronounced short term variability on the order of several thousand seconds (https://arxiv.org/abs/astro-ph/0505260). Since it is not possible for tremendous quantities of matter to heat up and cool down so rapidly, it is thought that short-term light variability is caused by a nonthermal process in which limited populations of charged subatomic particles are injected or drawn into the twisting magnetic fields generating synchrotron radiation.
Since change in luminosity involves photons reaching the observer from the near to the far end of the accretion disk, the maximum occurs when photons from the widest, middle cross-section arrive to the observer. The time period, T, between the minimum and the maximum on the short-term variability curve is an indicator of the radius, R, of the accretion flow. This can then be used to estimate the mass and the size of the event horizon for an accreting, non-rotating black hole.
For the Milky Way SMBH, Sagittarius A:
T = 40 min = 2400 sec. Method (1)
R = c T = 300,000 Km/sec x 2,400 sec = 720x10^6 Km
1 Astronomical Unit = 149.6x10^6 Km
R = 720 / 149.6 = 4.8 AU, or somewhat less than the orbit of Jupiter
Validity of this approach is shown by a study based on the variability of eleven quasars (https://arxiv.org/pdf/1002.4160.pdf), in which the relationship between the accretion disk radius in cm, R, the black hole mass, M, and the solar mass, Ms, is given by the following approximate relation:
log R = 15.8 + 0.8 log ( M / 10^9 Ms) Method (2)
Solving this equation for Sagittarius A with 4 x 10^6 solar masses yields the estimated accretion disk radius of 5.1 AU, fairly consistent with Method (1).
If Method (1) is used to estimate the accretion disk radius, the equation in Method (2) can be solved for the black hole mass. Entering the mass into the Scwarzschild's equation will then give the radius of the event horizon for a non-rotating black hole.
OBSERVATIONS AND ANALYSIS
On 19 Dec 2017, around 03:00 UTC, we captured a transient optical signal (TOS) in close proximity to the intermediate-mass black hole M74 X-1 ULX, CXOU J013651.1+15454, located in the spiral galaxy M74. Using a small TSAPO65Q astrograph, a full spectrum modified Canon T3i camera at iso 6400, and an Astronomik CLS-CCD filter, a series of 24 x 90 second exposures was taken with the intended purpose of evaluating a new equatorial mount. Several days later, a stack of the best 13 frames was enhanced through post-processing, and the resulting image was posted on CloudyNights.com (CN) on 22 Dec 2017
When blinking the enhanced image against the M74 image from the Digital Sky Survey 2 (DSS2 blue), a prominent new optical signal was detected at coordinates (J2000) 01h 36m 51.2s +15* 45' 35''
Using the original, unenhanced image, the transient signal was compared to the following five similar stars which could be identified on the DSS2 survey, suggesting an apparent magnitude (m) around 17.4:
USNOA2 1050-00444150, m = 17.45
USNOA2 1050-00443114, m = 17.5
USNOA2 1050-00439160, m = 17.0
USNOA2 1050-00438887, m = 17.9
USNOA2 1050-00443540, m = 17.1
USNOA2 1050-00445197, m = 17.5
The new light signal was determined not to be an artifact because it appeared on 21 of 24 original images. Three images had been compromised by high altitude atmospheric haze.
The signal was determied not to be of (Milky Way) galactic origin. It was far too faint to be a supernova, while SIMBAD query (http://simbad.u-strasbg.fr/simbad/sim-fcoo) by the TOS coordinates revealed no stars within a 15 arcsec radius.
SIMBAD search revealed close proximity of the new optical signal to M74 X-1 ultra-luminous x-ray source, ULX: CXOU J013651.1+154547, also designated as KKG2005, thought to be arising from an intermediate-mass black hole of about 10,000 solar masses. On 24 Dec 2017, closeups of enhanced and unenhanced images were posted for member review on CN under the title ”Possible Optical Image of M74's ULX".
Over the next two days CN member pablotwa submitted images of M74 taken with an 18 inch telescope (24 Dec 2017) and 14 inch telescope (25 Dec 2017) which approached the limiting magnitude 19.5, but showed no evidence of an optical signal at specified coordinates.
Review of several comprehensive star catalogs with attention to this region revealed some inconsistencies.
SIMBAD query (http://simbad.u-strasbg.fr/simbad/sim-fcoo) by the TLS coordinates of (J2000) 01h 36m 51.2s +15* 45' 35'' within a 15 arcsec radius revealed only the M74 ULX and a nearby HII region, but no starlike objects.
Object A: Stellarium, based on the NOMAD star catalog which includes data on 1.1 billion stars, lists a 14.75 apparent magnitude star with no designation at nearby coordinates (J2000) 01h 36m 51.0s +15*.45' 49.5''
Object B: USNOA2 catalog lists a star, USNOA2-1050-00441770, of apparent magnitude 16.4 at coordinates (J2000) 01h 36m 50.97s +15* 45' 49.6''.
Since these two objects have essentially identical coordinates, significant difference in magnitude notwithstanding, we feel they represent the same photographic feature we will henceforth refer to as Object AB. The object does not coincide with our transient optical signal, does not appear on the SIMBAD query, and does not show on our photographs, although well within the limiting magnitude. DSS2-red images show it as a bright localized object, however DSS2-blue images show a faint feature, most likely an HII region within M74 which was misidentified as a star during digitalization of red sky survey plates. It lies even closer to the ULX x-ray range center than our TOS, and may be a manifestation of black hole's irregularly variable thermal activity.
ESTIMATES OF GENERAL PROPERTIES
Since the coordinates of the TOS closely match those of M74 ULX, and since no persistent galactic objects are revealed by DSS2 or more recent images of the area, the signal is assumed to be extragalactic in origin, within M74, at the distance of 32 million LY, or 9.8 million parsecs.
From the apparent magnitude (m = 17.4) and the distance in parsecs (D = 9.8 x 10^6), absolute magnitude (M) of the transient light source is given by the relationship:
m - M = (5 x log D) - 5 (1)
M = m + 5 - (5 x log D)
M = 17.4 + 5 - (5 x log (9.8 x 10^6))
M = 22.4 - (5 x 7)
M = -12.6
From the distance of 10 parsecs, or 32.6 LY, the transient optical signal would appear as bright as the full moon. By comparison, at the same distance, the Sun with absolute magnitude of 4.83 (Ms = 4.83) would be barely visible as a nearly 5th magnitude star.
Luminosity of the TOS (L) compared to the luminosity of the Sun (Ls) is related to the difference in absolute magnitudes of the TOS (M = -12.6) and of the Sun (Ms = 4.83) as follows:
M - Ms = -2.512 log (L / Ls) (2)
Ms - M = 2.512 log (L/Ls)
log (L / Ls) = (Ms - M) / 2.512 = (4.83 + 12.6) / 2.512 = 6.94
log (L / Ls) = 10^ 6.94
L / Ls = 8.7 x 10^6
The transient light source is approximately 8.7 million times brighter, more luminous or more energetic, than the Sun.
As luminous as it is compared to the Sun, the TOS luminosity (L), or energy output, is hundreds of times weaker than the luminosity of type Ia supernovas (Ln) whose absolute magnitudes are consistently -19.3 (Mn = -19.3)
Mn - M = -2.512 log (Ln / L) (3)
M - Mn = 2.512 log (Ln / L)
log (Ln /L) = (M - Mn) / 2.512 = (-12.6 --19.3) / 2.512 = 2.67
Ln / L = 10^2.67 = 468
The TLS is about 500 times less luminous than a type Ia supernova.
To approximate the quantity of matter which needs to be converted to energy in order to achieve the luminosity of the transient signal (L), we will use the luminosity of the Sun (Ls = 3.8x10^26 J/s) as a reference. Using Einstein's mass-energy equation to relate the change in solar mass (ms) required to generate energy (E = Ls), where the speed of light (c) is 3 x 10^8 m/s:
E = ms c^2 (4)
ms = E / c^2 = Ls / c^2 = 3.8 x 10^26 / (3 x 10^8)^2
ms = 4.2 x 10^9 Kg/s 4.2 x 10^6 t/s
4.2 billion Kg/s, or 4.2 million metric tons per second, is converted to energy to produce one solar luminosity (Ls). In equation (2) we determined that the transient optical signal is 8.7 million times more luminous than the Sun. Therefore, the change in TOS mass (m) required to generate TOS luminosity (L) is given by the relation:
L / Ls = m / ms = 8.7 x 10^6 (5)
m = ms (8.7 x 10^6) = (4.2 x 10^9) (8.7 x 10^6)
m = 3.7 x 10^16 Kg/s = 37 x 10^12 t/s
The required change in the TOS mass is approximately 37 trillion metric tons per second. Earth's mass in metric tons (me) is approximately 6 x 10^21, and would be able to generate the TOS signal for a time period (T) of approximately 5 years.
Tsec = me / m (6)
Tsec = (6 x 10^21) / (37 x 10^12) = 1.62 x 10^8 seconds = 5.14 years.
Rather than a star, Object AB listed in the NOMAD and USNOA2 star catalogs appears to be an ionized hydrogen region in close angular proximity to the ULX. It is quite prominent on infrared, red, and ultraviolet surveys, but not on the blue and optical surveys. See Images 11 and 12 in the appendix. Based on the difference in listed magnitudes, and the current absence on our images, the region may be variable. That would suggest physical proximity to the ULX and a thermal emission phenomenon whereby the region is excited by radiation emanating from the ULX.
The transient optical signal (TOS) was present at estimated apparent magnitude of 17.4 on 19 Dec 2017, and absent by 24 Dec 2017 on images with limiting magnitude around 19.5. Since it is not known when the TOS originally appeared, or precisely when it faded, total period of the signal can not be defined based on a single positive observation.
The TOS does not appear to be an artifact or a (Milky Way) galactic object. It does not coincide with Object AB. Low absolute magnitude and relatively rapid fading in less than 5 days are not consistent with an extragalactic supernova. The possibility remains the signal arose from a distant blazar in M74's background, however this is considered to be statistically unlikely.
Based on its luminosity, rapid fading, and proximity to M74 X-1 ULX, we feel the TOS is a short-term variability phenomenon related to the ULX, and caused by non-thermal emission of synchrotron radiation most likely connected with the polar jet. This ULX is known to exhibit extreme variability and rapid changes in the spectral state.
The small amount of matter required for the generation of observed luminosity, estimated in equation (5), is truly remarkable. 37 trillion metric tons per second is a substantial number in human terms, but minimal on the astronomical scale, comparable to a tiny fraction of a small planet.
We feel this phenomenon is most likely cyclical, and should be monitored on all electromagnetic wavelengths. We believe that any optical signals will have close temporal association with x-ray and possibly radio emissions. Spectroscopy would reveal the precise nature and distance of the signal. In the optical band the object is apparently accessible to very modest amateur equipment.
Image 6: Transient Optical Signal on original TSapo65q image, coadded, unenhanced. FOV ~10' x 5'
Image 7: Wide field location of the Transient Optical Signal on the DSS2 Blue sky survey. Center of M74 ULX x-ray signal range marked in red, TOS marked in green. FOV ~10' x 5'
Image 8: Narrow field location of the TOS on the SDSS9 optical sky survey. FOV ~240'' x 111''
Image 9: Narrow field location of the TOS on the XMM x-ray sky survey. FOV ~240'' x 111''
Image 10: Narrow field location of the TOS on the GALEXGR6-AIS UV survey. FOV ~240'' x 111''.
Image 11: Narrow field location of Object AB on the DSS2 RED survey. FOV ~240'' x 111''
Image 12: Narrow field location of Object AB on the Sitzer IRAC HEALPix IR survey. FOV ~240'' x 111''
- Scott Beith, Mason Dixon, kkt and 2 others like this