天文觀測 I Crab & Pulsar

Crab – Chinese Astronomer ’ s Record The supernova was noted on July 4, 1054 A.D. by Chinese astronomers. It was about four times brighter than Venus, or about mag -6. it was visible in daylight for 23 days, and 653 days to the naked eye in the night sky 宋史天文志有關天關客星的記載

Supernova Explosion

Crab Nebula -- Discovery The nebulous remnant was discovered by John Bevis in 1731, who added it to his sky atlas. Charles Messier independently found it on August 28, 1758, when he was looking for comet Halley on its first predicted return, and first thought it was a comet. Of course, he soon recognized that it had no apparent proper motion, and cataloged (M1) it on September 12, Crab Nebula in Bevis ’ catalog

Crab Nebula = SNR of SN 1054 J.C. Duncan of Mt. Wilson Observatory compared photographic plates taken 11.5 years apart, and found that the Crab Nebula was expanding at an average of about 0.2" per year; backtracing of this motion showed that this expansion must have begun about 900 years ago (Duncan 1921). Also the same year, Knut Lundmark noted the proximity of the nebula to the 1054 supernova

Radio Band: Discovery and Pulsar In 1948, the Crab nebula was identified as a strong source of radio radiation, named and listed as Taurus A and later as 3C 144. On November 9, 1968, a pulsating radio source, the Crab Pulsar (also cataloged as NP0532, "NP" for NRAO Pulsar, or PSR ), was discovered in M1 by astronomers of the Arecibo Observatory 300-meter radio telescope in Puerto Rico.

X-ray Band: Discovery and Position X-rays from this object were detected in April 1963 with a high-altitude rocket of type Aerobee with an X-ray detector developed at the Naval Research Laboratory. the X-ray source was named Taurus X-1. Measurements during lunar occultations of the Crab Nebula on July 5, 1964, and repeated in 1974 and 1975, demonstrated that the X-rays come from a region at least 2 arc minutes in size.

Crab: Luminosity Emitted in X-rays by the Crab nebula is about 100 times more than that emitted in the visual light. Its apparent brightness corresponds to an absolute magnitude of about -3.2, or more than 1000 solar luminosities. Its overall luminosity in all spectral ranges was estimated at 100,000 solar luminosities or 5x10 38 erg/s !

What is Crab ? Supernova Supernova Remnant Nebula Pulsar (young) Variable star

Aliases of Crab Messier Object X-ray old name X-ray name Radio: Cambridge Catalog NGC γ-ray name Pulsar: J name Infrared name Pulsar: B name Supernova name SNR name Radio: NRAO catalog Common name

Crab: Multi-wavelength Observation Crab is the one of few astrophysical objects being able to be seen in all the EM wave bands (from radio to γ – ray) X-ray: ChandraUV: ASTRO 1Optical: VLANear IR: 2MASS Mid IR: IRASFar IR: IRASRadio: NRAO Composed

Crab: Pulsar, Multi-wavelength Not only the emission but also pulsar …… Optical X-ray γ-ray Optical Radio Infrared

Pulsar: Discovery Pulsars were discovered by Jocelyn Bell and Antony Hewish in 1967 while they were using a radio array to study the scintillation of quasars. They found a very regular signal, consisting of pulses of radiation with period of 1.34 sec The original name for the object was "LGM", Little Green Men, thinking of it as a beacon made by some extraterrestrial intelligence. Thomas Gold analyzed the results of these observations, determining that the only natural object that could be responsible was a neutron star, a type of stellar object up to then only hypothesized.

Why Pulsar Must Come from Neutron Star ?

Breaking Period MassRadiusBreaking period Sun1 M  7x10 10 cm2.8 hours White dwarf1 M  6x10 8 cm8 sec Neutron star1.4 M  1x10 6 cm0.46 ms

Pulsars: Pulsar period : 1.6 ms to ~300 sec Types of pulsars Radio pulsar X-ray pulsar Millisecond pulsar Isolated pulsar Binary pulsar Rotational-powered pulsar Accretion-powered pulsar Nuclear-powered pulsar Anomalous X-ray pulsar

Sound of Pulsar PSR B P= s f=1.4 Hz

Sound of Pulsar Vela pulsar P=89 ms f=11 Hz

Sound of Pulsar Crab pulsar P=33 ms f=30 Hz

Sound of Pulsar PSR J P=5.747 ms f=174 Hz

Sound of Pulsar PSR B P= ms f=642 Hz

Properties of Neutron Star Lower limit of mass : 1.4 M  Upper of mass: limit of mass : < 3.2 M  Radius: ~10-15 km Mean density: 1x10 14 g/cm 3 Rotational period: 1.6 msec to ~300 sec (from pulsar_ Magnetic field: 10 8 to Gauss

Why Neutron Star Rotates so Fast ?

Pulsar: Light House Effect Consider the neutron star as a magnetic dipole. The poles has strongest magnetic filed and also the large radiation as the result of strong interaction on them. However, if the magnetic dipole is aligned on the rotational axis, no pulsation can be detected

Pulsar: Light House Effect If the magnetic dipole misaligned to the rotational axis, the strong emission from the pole (or poles) may be shining to the observer with periodically with rotational period, which is somewhat like a light house on sea. Thus, it is named “ light house effect ”.

Crab Pulsar: X-ray Band

Isolated Pulsar: Rotation-powered Since dipole is a vector, changing the direction is also changing the dipole moment. Our EM knowledge tells us that when a magnetic dipole moment change with the time, there are EM wave emission. The energy supply for the radiation is the rotational energy of the neutron star for the isolated pulsar.

Pulsar: Braking Index

Pulsar: Age

Pulsar: a Precise Clock Astronomers found that the (isolated) pulsars are precise clocks, which can compete to atom clock. Pulsar period (frequency) ephemeris:

Crab: Age, Magnetic Field and Flux

X-ray Pulsar Most of the pulsars are detected in radio band but of a few of them whose pulsations can be detected in X-ray band. The X-ray pulsation can be originate: Magnetospheric Emission Cooling Neutron Stars Accretion

X-ray Pulsar: Magnetospheric Emission: X-ray pulsars can be produced when high-energy electrons interact in the magnetic field regions above the neutron star magnetic poles. Pulsars seen this way, whether in the radio, optical, X-ray, or gamma-ray, are often referred to as "spin-powered pulsars," because the ultimate source of energy comes from the gradual slowing down of the neutron star rotation.

X-ray Pulsar: Cooling Neutron Stars When a neutron star is first formed in a supernova, its surface is extremely hot (more than 10 9 degrees). Over time, the surface cools. While the surface is still hot enough, it can be seen with X- ray telescopes. If some parts of the neutron star are hotter than others (such as the magnetic poles), then pulses of thermal X-rays from the neutron star surface can be seen as the hot spots pass through our line of sight. Some pulsars, including Geminga, show both thermal and magnetospheric pulses.

X-ray Pulsar: Accretion Power If a neutron star is in a binary system with a normal star, the powerful gravitational field of the neutron star can pull material from the surface of the normal star. As this material spirals around the neutron star, it is funneled by the magnetic field toward the neutron star magnetic poles. In the process, the material is heated until it becomes hot enough to radiate X-rays. As the neutron star spins, these hot regions pass through the line of sight from the Earth and X-ray telescopes see these as X-ray pulsars. Because the gravitational pull on the material is the basic source of energy for this emission, these are often called "accretion powered pulsars."

Pulsar Evolution Basically, normal pulsars have intrinsically precise periods modulated only by slow monotonic increase (spin down) due to graduate loss (radiated away) of rotational energy.

Millisecond Pulsar However, there are very fast-rotating pulsar whose periods are less or equal to ~10 ms. The new category of pulsars is called millisecond pulsars or “ recycled ” pulsars. The extreme small period change rates (much smaller than normal pulsars) imply that the millisecond pulsars have weak magnetic fields (~10 9 G) and their characteristic ages are about 10 9 years. The millisecond pulsars can be either isolated or in binary systems with low mass companion.

P-B Diagram Millisecond pulsars Young pulsars Evolution

Model The millisecond pulsar started as an normal pulsars which lost most of its magnet field and was “ spun up ” to millisecond pulsars by accreting matter from their companion star in an X- ray binary. Two kinds of X-ray binaries – High Mass X-ray Binary (HMXB) and Low Mass X-ray Binary (LMXB) HMXB is hardly to be the candidate because: (1) Nearly spherical accretion provides little angular momentum to spin-up the neutron star. (2)To spin-up normal pulsar to millisecond pulsar requires ~0.1 solar mass. Even at Eddington accretion rate (10 -8 M ⊙ per year), it would take ~10 7 years, which much larger than the mass transfer phase of most HMXBs.

HMXB and LMXB HMXB LMXB

LMXB and Millisecond Pulsar The companion transfers large amount angular momentum to neutron star through accretion. Millisecond pulsar in low mass secondary binary system – The binary was once an accreting LMXB but terminate mass transfer due to the secondary sink into Roche lobe. Isolated millisecond pulsar – The accretion process eventually consumed the all the matter in secondary and left an fast rotating neutron star. This model implies that there should be coherent millisecond X- ray pulsars in ~150 LMXBs. People tried to search but failed to find any millisecond pulsar in LMXBs until April (The fastest pulsar in LMXB discovered before 1995 was A (P=69 ms)).

Millisecond Pulsar in LMXB ???? Although the recycle model was reasonable and wildly believed, there was no direct evidence that the LMXB is the progenitor of millisecond pulsars DO WE SEE MILLISECOND PULSAR IN LMXB ?????