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Supernovae, Neutron Stars, Pulsars, Magnetar, and GRB

Crab Nebula When stars with mass greater than 5 Msun exhaust their nuclear fuel, they collapse suddenly in a process called supernova explosion, which flings off huge amount of heavy elements into interstellar space. Supernovae can be classified into two types. Their characteristics are listed in Table 08-04. Figures 08-13a shows images of the Crab Nebula taken at different wavelengths. The Crab Nebula is a supernova remnant after an explosion at 1054 AD.

Figure 08-13a Crab Nebula [view large image]

In the optical image, red colour comes from electron recombination to form neutral hydrogen (producing emission lines), while blue colour is generated by synchrotron radiation. The X-ray image shows an enlarged view of the central region with rings of high-energy particles flinging
outward near the speed of light and powerful jets rushing off from the poles (see more explanation on the structure in Figure 08-13a). Figure 08-13b further classifies the supernovae into sub-types according to the spectral signature.

Characteristic Type I Type II
Initial Mass < 8 Msun 8 - 50 Msun
Light Curve Smooth Decline Decline + Plateau
Maximum Luminosity 10 billion Lsun ~ 1 billion Lsun
Mode of Energy Generation Nuclear Gravitational
Stellar Type Old Population II Young Population I
H Absorption Lines No Yes
Binary System Usually No
MilkyWay Frequency ~ 1/36 year ~ 1/44 year
Occurrence in
Elliptical Galaxy
Yes No
Supernova, Types of

Table 08-04 Types of Supernova

Figure 08-13b Supernova, and the Spectral Types of [view large image]

In general the supernova also emits gamma-ray burst that lasts more than two seconds leaving behind a black hole (shorter burst happens when an old neutron star spirals into a preexisting black hole or another neutron star). The supernova explosion disperses heavy chemical elements produced during the star's life time into the interstellar space. They have become the building materials so essential to life.

SN200GY GRB080319B The brightest supernova on record so far (2007) is the SN2006GY (Figure 08-14), which was the dying explosion of a star 150 times the mass of the Sun. In such an exceptionally massive star, it is suspected that an instability (produced by high temperature and pressure) could convert light into matter-antimatter pairs. This would cause a pressure drop, making the star contract and igniting a runaway nuclear reaction to obliterate the stellar core. Thus, unlike other massive star supernovae, neither neutron star, nor black hole, would remain.

Figure 08-14 SN200GY
[view large image]

Figure 08-15a GRB080319B [view large image]

The record was broken by the detection of a more powerful supernova cataloged as GRB080319B in March 2008. It occurred at a distance of 7.5 billion light years from Earth with a luminosity of 2.5 million times more than the brightest known supernova.
GRB090423 The fading afterglow was captured in both X-rays and Ultraviolet light (Figure 08-15a). The farthest gamma-ray burst (GRB090423) was detected by the Swift Observatory in April 2009. At redshift z = 8.2, it occurred only 630 million years after the Big Bang and could be one of those first stars turning into a black hole. The infrared afterglow was captured by the Gemini Observatory as shown in Figure 08-15b. Observation on another cosmic blast GRB090102 indicates that there is a considerable polarization of the order 10% in the optical radiation (unusually high for astrophysical sources). The most logical - but not the only - explanation for the high degree of polarization is that the emitting source is permeated by

Figure 08-15b GRB090423 [view large image]

large-scale, ordered magnetic fields, and that the emission is non-thermal synchrotron emission from electrons as they spiral around the magnetic fields at relativistic speeds.

GRB120323A GRB Model Meanwhile, preliminary data for the energy spectrum of gamma-ray burst 120323A (Figure 08-15c, also see Nature News), discovered in March 2012 by the Fermi telescope, shows a bump that is likely to come from thermal emissions - casting doubt on a long-held view that synchrotron emissions alone could explain the bursts. The thermal emission could originate from the surface of a fireball as a spinning star collapses to form a black hole and explodes in a supernova as shown in Figure 08-15d. Since the GRB is much brighter than the quasars, it is now used to find out the chemical composition

Figure 08-15c GRB120323A

Figure 08-15d GRB Model [view large image]

and evolution in the early universe at the epoch of a few hundred million years after the Big Bang.

Failed Supernova It is estimated that the observed number of supernovae is only about half of the prediction from theoretical calculation. One explanation proposes that some supernovae fail to explode because the shock wave has been stalled by the material rushing into the core (Figure 08-15e). The failed supernovae should produce huge amount of very energetic electron-neutrinos. If the next generation of neutrino detectors are able to detect the diffuse supernova neutrino background, then by comparing

Figure 08-15e Failed Supernova [view large image]

the spread of energies to those seen in the individual supernovae bursts, researchers will be able to work out the proportion of successful to failed supernovae.

Inverse Beta Neutron Star When the core of the star collapses to a density of about 1014 gm/cm3 (the same order of magnitude in the nuclei) it causes the atomic electrons to combine with the nuclear protons in the electron capture reaction as shown in Figure 08-16. This is the point where gravitational forces have won out over the pressure supplied by nuclear matter. Figure 08-17a shows the structure of a neutron star in several layers over a depth of ~ 10 km:

Figure 08-16 Electron Capture
[view large image]

Figure 08-17a Neutron Star, Structure

In 2010, two independent research teams have found that the core of the neutron star in CasA (Figure 08-17b) is cooling at the rate of 4% over a 10-year period by analyzing data from NASA's Chandra X-ray Observatory. This rapid cooling (by astronomical standard) can be
Superfluid in CasA explained with the formation of superfluid in the core. The process releases neutrinos, which carry away a lot of energy. The charged protons there also make the core superconducting. The onset of superfluidity in materials on Earth occurs at extremely low temperatures near absolute zero, while it happens in neutron star at temperature near a billion degrees. The difference can be explained by the fact that the low temperature variety involves the very weak force between Cooper pair, while the interaction is via the strong nuclear force between nucleons in neutron star. This information may show us how to achieve superfluidity and superconductor at room temperature on Earth. The X-ray image in Figure 08-17b is colored in red (iron), green (silicon) and blue (synchrotron), optical data is in gold color.

Figure 08-17b Superfluid in CasA [view large image]

Figure 08-18 summarizes the relationship between the microscopic matters (such as electrons and neutrons) and the macroscopic properties (such as density and pressure). The drawing plots the matter's density horizontally. The vertical axis is the resistance to
Equation compression (the percentage increase in pressure that accompanies a 1% increase in density). The boxes attached to the curve show what is happening to the microscopic matter as it is compressed from low densities to high.

At normal densities, cold, dead matter is composed of iron. As the iron is squeezed from its normal density of 7.6 gm/cm3 up toward 100, then 1000 gm/cm3, the iron resists by the same means as a rock resists compression - the degeneracy-like motions of electrons. When the density has reached 100000 gm/cm3, the electron's degeneracy pressure completely overwhelm the electric forces with which the nuclei pull on the electrons. The electrons no longer congregate around the iron nuclei; they completely ignore the nuclei and form the electron gas moving around freely. At a density of about 107 gm/cm3 the motion of the electrons become relativistic (near the speed of light).

Figure 08-18 Equation of State [view large image]

Gravitational energy released during the star's collapse represents almost 10% of its rest mass energy. For comparison, the fusion of hydrogen to helium releases less than 1% of the rest mass energy of the particles involved. This efficient release of gravitational energy rises the temperature at the center of a newborn neutron star to 500x109 oK. The neutron star loses most of this energy in a matter of minutes, as neutrinos race from the star's core. The remaining energy radiates slowly from the star's surface. Even after a million years, the surface temperature of a neutron star can be several 105 oK emitting soft X-rays and some visible light as well. So neutron stars can be thought of as glowing embers, slowly dissipating the heat generated when they formed.

All stars are rotating. If a star was to collapse down to the size of a neutron star while conserving its angular momentum, it would end up spinning very rapidly. In practice, the observed periods of the thousand or so known pulsars range from 4 sec to 1.6x10-6 sec. Only a tiny,
Pulsar Signal Pulsar Model dense neutron star could spin this fast; larger stars (including white dwarfs) would be torn to shreds by centrifugal force. The magnetic field at the surface of a collapsing star grows in strength as the surface area of the star decreases (decreases in radius / increases in magnetic field strength ~ 1 / 105). The magnetic field strengths at the surfaces of neutron stars are likely to be between 108 and 1013 gauss. In some extreme examples (known as magnetars) they may be as high as 1015 gauss. Figure 08-19a shows the pulsar signal from the neutron star inside the Crab Nebula with a period of 0.03 sec in the X-ray range.

Figure 08-19a Pulsar Signal
[view large image]

Figure 08-19b Pulsar Model [large image]

As shown in Figure 08-19b, charged particles accelerated by the field follow helical paths around the magnetic lines of force and emit radiation along the direction in which they are moving. Because the field lines bunch together at the magnetic poles, the emitted radiation is concentrated into two narrow beams directed along the magnetic axis. If the magnetic axis is tilted at an angle to the rotation axis, then the star's rotation will cause the beams to sweep around just like the beam of a lighthouse, thereby giving rise to the pulsar phenomenon. Depending on the energies of the charged particles and the strength of the field, this process can give rise to pulses over a wide range of wavelengths from gamma ray to radio.

All pulsars appear very gradually to be slowing down by the interaction with their surroundings. A pulsar, however, will occasionally
Magnetar Magnetar Image undergo a sudden small increase in rotation rate. These events, called glitches, are believed to be caused by "starquakes" that occur when the outer crust (a solid crystalline layer of heavy nuclei) slips to the fluid interior or by some abruptly adjustments. Extremely violent starquakes are believed to be induced when the intense magnetic fields of magnetars fracture their crusts. The energy released in such events may be responsible for producing the intense bursts of gamma rays that characterize objects called soft gamma-ray repeaters. Figure 08-19c shows the evolution of a neutron star to either a pulsar or magnetar. Figure 08-19d is an artistís impression of a magnetar. A powerful

Figure 08-19c Magnetar Pathway
[view large image]

Figure 08-19d Magnetar [view large image]

explosion just beneath a magnetar's surface has been detected by the XMM-Newton orbiting X-ray observatory in 2007.

Magnetohydrodynamical Simulation in 2019 shows that the merger of two massive stars could produce another massive star with strong magnetic field. It may evolve to a magnetar at the terminal phase of its evolution (Figure 08-19e). Here's the evolutionary sequence :

  • Phase (1) - The initial configuration involves a 9-Myr-0ld binary of 8 and 9 Msun. Such choice is to imitate the observed 17 Msun magnetic "blue straggler star" -Sco, which could be the progenitor of the magnetar relating to the enigmatic fast radio burst (FRB) (See Phase (4) in Figure 08-19e, which depicts the evolutionary path of the merger).

  • Phase (2) - In about 10 days, the merged product develops a strong magnetic field up to 108 G and a torus of 3 Msun. The rotational speed of the solid core and the torus is indicated by color code in terms of the critical Keplerian velocity vcrit, which is the angular velocity at a point when the centrifugal acceleration overcomes the sum of the gravitational and radiative accelerations (Figure 08-19e).

  • Magnetar Evolution
  • Phase (3) - Most of the torus is accreted rapidly into a 16.9 Msun star with the rest 0.1 Msun forming a disk. The star is out of equilibrium during the process. Thermal relaxation settles the star back to equilibrium in ~ 1000 years. Meanwhile, the star reaches a maxiunum radius of about 200 Rsun. It becomes very luminous and is shedding mass because of the very rapid rotation approaching vrot/vcrit ~ 1. (see Figure 08-19e).

  • Phase (4) - It is now a main star with mass = 16.9 Msun and a strong surface magnetic field similar to -Sco as indicated by the rectangle in Figure 08-19e.

  • Phase (5) - The merged star evolves similar to a genuine main sequence star (orange line). It terminates in a supernova explosion which may form a magnetar.
  • Figure 08-19e HR Diagram of Merging Stars to Magnetar

    See original article in "Stellar Merger as the Origin of Magnetic Massive Stars".
    By 2010 there is enough observations to sort the pulsars into subclasses according to the strength of the magnetic field, and electromagnetic radiation as shown in Table 08-05 and Figure 08-19f.

    Subclass Magnetic Field (Gauss) Electromagnetic Radiation Pulsation Frequency
    Magnetar 1013 - 1015 X-ray, Gamma Ray 1-0.1/sec
    High-Magnetic-Field Radio Pulsar 1013 - 1014 X-ray, Gamma Ray 1-0.1/sec
    Isolated Neutron Star (INS) 5x1012 X-ray 10/sec
    Radio Pulsar 108 - 1013 Radio 0.1-105/sec
    Rotating Radio Transient (RRAT) 1010 Intermittent Radio  
    Compact Central Object (CCO) 109 - 1010 X-ray  
    Millisecond Pulsar 109 Radio, X-ray, Gamma Ray 100-1000/sec

    Table 08-05 Subclass of Pulsars

    Figure 08-19f shows the various subclasses of pulsars in terms of the strength of the magnetic field and their age together with other characteristics. Of the nearly 2000 pulsars observed so far, about 90% are radio (rotation-powered) pulsars with wide range of magnetic fields, ages, and rotational rates (the yellow patch in Figure 08-19f). The RRATs are similar to radio pulsars but the emission of radio waves is only intermittent and in short burst. The INSs are radio-quiet but emit X-ray pulses with a visible-light counterpart. The magnetars has a rapid rate of spin-down and possesses the highest known magnetic field in the universe.
    Subclass of Pulsars In one theory, both the High-Magnetic-Field radio pulsar and INSs are the progenies of magnetars. The millisecond pulsars had been in the terminal phase of the pulsar evolution. They are now at the stage of spin-up, because they happen to be in a binary system, which enables their rejuvenation by accretion of matter from the other star. CCOs are the central object at the centers of supernova remnants. They pulsate irregularly only in X-ray. These objects may be born with slow spinning rate and low magnetic field. Many pulsars are surrounded by nebula with their intensely magnetized wind of highly energetic

    Figure 08-19f Subclass of Pulsars [view large image]

    particles resulting in dramatic morphologies such as the "Hand of God". Astronomers detect this "pulsar wind nebulae" only in the most powerful pulsars.

    One theory suggests that radio pulsars, RRATs, INSs, and magnetars have such disparate properties simply because of their different magnetic fields at birth combined with their present ages, which manifest the consequence of field decay. The scarcely magnetized CCOs fit in as the neutron stars with the lowest magnetic field at birth; they are X-ray hot only because of their young age.

    GRBs are thought to be triggered by the collapse or merger of stars to form black holes at the end within a minutes or hours. Recently in 2010, observations from Fermi Gamma-ray Space Telescope, and the Swift satellite have allowed astronomers to learn more about the details of the explosions. It is reported that at least for one GRB an intermittent phase is detected. It lasted for hundreds of seconds and has the signature of a magnetar, the rapid rotation (with a rate of more

    Figure 08-19g GRB Light Curve [view large image]

    Figure 08-19h GRB 130427a [view large image]

    than one thousand per second) of which delays the eventual collapse to a black hold as shown in Figure 08-19g.

    GRB130427A Spectral Lags On April 27, 2013 a gamma-ray burst (GRB130427) was captured at its moment of eruption. The burst lasted for hours, and remained detectable for the better part of a day. One gamma-ray was recorded with an energy of 1011 Gev. Figure 08-19h shows images of the sky (North Galactic Pole at the center) before and after the eruption. The burst subsequently was detected in optical, infrared, and radio wavelengths by ground-based observatories. The view of the supernova will arrive by middle of May, 2013.

    Figure 08-19i GRB130427A Spectral Lags [view large image]

    A paper was promptly published on May 12, 2013 about the GRB130427A event. It is claimed that the data support a scenario of quantum foam at Planck scale. According to one of the theories photons with higher energy would lag behind the lower energy variety (Figure 08-19i). However, it is not ruled out the possibility of a process producing two different bursts with different energies at two different time.

    Figure 08-19i shows the very high energy burst (I), which is preceded by a relatively long time interval of "silence" (II), and some high energy photons farther back in time (III). The sequence seems to indicate two separate events. A single event with spectral lags would have a pattern of continuous curve in photon energy.

    Using the formula for estimating the Planck structure "turn on" energy EQG,1 :
    t = (D/c) (E/EQG,1),
    with GRB130427A red shift z ~ 0.34 (corresponding to D ~ 4.6x1027cm), t ~ 300 sec, E ~ 100 Gev (from Figure 08-19i), we obtain EQG,1 ~ 0.04 EPlanck, which seems to be too low for the granular structure of space to show up.

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