Term Paper (2)

Term Paper for the Course ‘Science-1’ (ISC201) Review of the Research Article titled FORMATION OF STRONGLY MAGNETISED NEUTRON STARS: IMPLICATIONS FOR GAMMA RAY BURSTS Reviewer: Name: Shashank Sharma Roll No: 200802034 Details about the Research Article: Formation of strongly magnetised neutron stars: Implications for Gamma Ray Bursts

Robert C. Duncan Department of Astronomy and McDonald Observatory, University of Texas, Austin TX-78712.

And

Christopher Thompson Canadian Institute for Theoretical Astrophysics, University of Toronto, 60St. George Street, Toronto, Ontario, Canada M5S1A1

Publisher: The Astrophysical Journal Volume: 392 Recieved 1991 December 23; Accepted 1992 March 2

Date of Publication: 1992 June 10 Pages: L9-L13

Introduction: Neutron stars are the stars with mass 8 to 20 times as that of the Sun. These massive stars die in type II supernova explosion, as the stellar core implodes into a dense ball of subatomic particles. There are two types of Neutron Stars MAGNETAR • If the new born neutron star spins fast enough, it generates an intense magnetic • field. Field Lines inside the star gets twisted. • The Magnetar settles into neat layers, with twisted field lines inside and smooth • lines outside. It might emit a narrow radio beam • After 10,000 years Magnetar is cooled • off, and much of its magnetism is decayed away. It emits very little energy

PULSAR If the new born neutron stars spins slowly, it’s magnetic field though strong by everyday standards, does not reach Magnetar levels. The mature Pulsar is cooler than a Magnetar of equal age. It emits a broad radio beam, which radio telescopes can readily detect. After 10 million years the Pulsar is cooled off and no longer emits a radio beam.

On March 5, 1979, Two Soviet spaceraft, Venera 11and 12, in solar system. The radiation readings on board both probes was around a nominal 100 counts per second. But at 10:51 A.M. EST, a pulse of gamma radiation hit them. Within a fraction of a millisecond, the radiation level shot above 200,000 counts per second and quickly went off scale[1]. Soon after this many subsequent Soft Gamma bursts known as SGR(Soft Gamma Repeaters) These Gamma rays were found to be comming from supernova remnant N49 in the LMC. They were all having the same properties and so were expected coming from a Magnetar. Megnetars Candidates in supernova remanant N 49 LMC a small galaxy near our own milky way. Source: E. L. WRIGHT University of California at Los Angeles,THE COBE PROJECT, DIRBE AND NASA.

A magnetar after its birth has very high speed of rotating ~ 1ms. The hot ionized gases rise up and the cold one gets condensed to the surface. Hot ionized gases are good conductors and so carry along the magnetic field lines with it. This amplifies its magnetic field. Bdipole ~1015 Gauss. This phenomenon is known as dynamo. Soon after this the magnetar loses its rotational energy to the magnetic braking, giving a large amount of energy to the associated supernova explosion. Such magnetic fields result in magnetically-induced anisotropic neutrino emission, core rotational instability, and fragmentation, and/or anisotropic magnetic winds. These phenomenons might result in emission from the magnetar causing it to recoil with large (~1000km s-1) velocity. Magnetic torques continue to spin down the star after convection cases, releasing the remaining rotational energy on the spin down time scale τSD≈0.6B15-2(Pi/1 ms) 2 hr. where B15=BDipole/1015. Magnetars get their rotation period longer than the radio pulsars of same age P≥10s after only 104B15-2yr. They evolve to “death line” beyond which their magnetospheres become charge-starved on a time scale ~ 6X105B15-1yr, when their periods are P≥70B15½s [2]. Causes for recoil: 1. Star may undergo anisotropic mass loss. For example a young magnetar rotates close to the critical angular velocity for nonaxisymmetrical gravitation radiation instabilities [3] . a mass 1X10-2M ejected at an escape velocity from 20 Km radius would impart a ~103 Km s-1 recoil to the star. 2. Second class of recoil mechanism is anisotropic neutrino emission, which can be induced in a number of ways by a strong magnetic field. A fractional anisotropy in the radiated scalar momentum,

𝛿𝛿𝛿𝛿 𝑝𝑝

~0.03 would produce a ~103Km s-1 recoil.

Observations & Result: 1. 1979 March 5 burst coincided with the supernova remnant N49 in the LMC. The burst was modulated over the period of 8±0.05 s for over 20 cycles [4] , and its spectrum contained an emission line at 440MeV. The periodic modulation was probably due to rotation. This was known as soft gamma repeater (SGR). 2. Equating the spin down age t =

𝑃𝑃

2Ṗ

with the age of SN remnant, t4 x 104yr where 0.6 < t4 < 1.6 [5],

yields Ṗ ≈ 1.27 x 10-12t4-1. Approximating the spindown torque as being due to magnetic dipole radiation [6] [7] implies for a 8s rotation period, a surface dipole field B≈6X10 14t4-1/2G, in the magnetar regime. There has been many other such SGRs suggesting a population of ≥10 4 object in the Galactic halo within a distance ~ 100kpc. If these stars continue to burst with roughly the same luminosity over 107 -108 yr, they constitute a halo population of GRB sources.

Discussion In this letter they have outlined a physical scenario for the formation of highly magnetized neutron stars (“Magnetars”). They have indicated several mechanisms which could impart large recoils to these stars, sufficient to escape from Galactic disk. They have explained the SGRs that occur due to the phenomenon of Magnetars which radiate magnetic energy and emit gamma radiation. The Introduction part of the paper was quite difficult to understand. But after reading the observation part things seemed understandable.

bibliography references 1. Scientific American. special edition “secret life of stars” 2004. 2. Ruderman 1987. 3. Chandrashekhar 1970. 4. cline 1982.

5. shull 1983 6. Pacini 1967. 7. Gunn & Ostricker 1969.

http://articles.adsabs.harvard.edu Title: Formation of very strongly magnetized neutron stars - Implications for gamma-ray bursts Authors: Duncan, R. C. & Thompson, C. Journal: Astrophysical Journal, Part 2 - Letters (ISSN 0004-637X), vol. 392, no. 1, June 10, 1992, p. L9-L13. Research supported by NSERC. Bibliographic Code: 1992ApJ...392L...9D G: Gauss (1T=104G) LMC: Large Magellanic Cloud. kpc= 103Parsec (1 kpc = 3.08568025 × 1019 meters)