Physics with the ASCA satellite

[Documents with full size images]

Our research subjects with the Japanese X-ray satellite ASCA are;

Some highlights are;

  1. The Past Activities of Our Galactic Center ?
  2. Possible origin of the cosmic ray --- evidence for the Fermi acceleration in the shell of supernova remnants
    1. SN1006
    2. G347.5-0.5
  3. Discovery of hard X-rays from protostars embedded in the dense cloud
    1. Coronet Cluster
    2. Orion star forming region (other documents; in Japanese; for press release (2000 April))

The Past Activities of Our Galactic Center ?

[ASCA image of the Galactic Center]
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The center of our Galaxy (the Milky Way Galaxy) has drawn much attention, in connection with possible presence of a massive black hole. The radio and infrared observations have revealed highly complex features in the close vicinity of the Galactic center, and indicated a large mass concentration of the order of million solar masses within less than 1 light year from the dynamical center. However, in spite of extensive observations, direct evidence for high-energy activities of the Galactic center, such as observed in active galactic nuclei ( which are believed to exhibit a massive black hole), is still lacking.

Astrophysicists from Japan have presented pieces of evidence for the past activities of our Galactic center. Using the data of the Japanese X-ray astronomical satellite ASCA, we have found X-rays from highly ionized ions of various elements, which are dominated by high-temperature plasma of several million degree with a distinctly bright oval-shaped region around the Galactic center (Sgr A in Figure). The high temperature plasma extends along the Galactic plane (solid lines in Figure) as far as 300 light year on either side of the center and over 150 light year across the plane, but with much reduced X-ray brightness. The oval-shaped plasma contains the energy of a few supernova explosions, while the largely extended plasma has the energy of one thousand supernova explosions. This requires unrealistically high rate of supernova explosions. Possibly, more energetic outbursts at the Galactic center generated the hot plasma.

Another hint of the past nuclear activity comes from the new discovery of strong fluorescent X-rays from cold iron atoms. The authors found two bright regions in the fluorescent X-rays; one coincides to the giant molecular cloud (Sgr B), and the other lies near the molecular cloud at the Radio Arc. The large fluorescent X-ray flux is only expected from X-ray illuminated neutral clouds. However no bright X- ray source responsible to the fluorescent line is found. The most probable solution is that the Galactic center was about ten thousand times more luminous about 300 years ago, the light travel time to the molecular cloud, than it is today.

The observed results indicate that the Galactic center has exhibited intermittent activities, and that it had been luminous in X-rays until the recent past. The oval- shaped plasma and fluorescent X-rays are likely due to the most recent activity. Thus the ASCA results suggest the presence of an active galactic nucleus in our Galaxy, and, together with the evidence for a large mass concentration at the Galactic center, they further support the idea that there is a massive black hole undergoing transient activities.

Figure: (right) Distribution of high temperature plasma with the peak at the Galactic Center (Sgr A). (left) Distribution of the fluorescent line; the northern bright spot (upper-left) is located near the giant molecular cloud (Sgr B), and the other (middle) near the Radio Arc appears to be associated with other molecular clouds. The solid lines represent the Galactic plane.

We also prepare postscript versions (regular(6.8Mbyte) or gzipped(350Kbyte)), and GIF version(109Kbyte)) of the figure.

The origin of cosmic ray? --- The evidence of shock acceleration around the shell of supernova remnants


[ASCA image of SN1006] [ASCA spectra of SN1006]
Click them, and you can get larger images(47Kbyte,13Kbyte,respectively)

The remnant of the brightest historical supernova exploded in AD1006, now provides us with the first conclusive evidence for a lineless, power-low component of X-ray emission from a shell supernova remnant, and thus represents the nonthermal X-rays from shell SNRs. This implies that electrons emitting synchrotron radiation at these X-ray energies must have energies of order 100 TeV, among the highest ever observed or directly inferred in any context, astrophysical or otherwise. They are certainly the highest energies seen in the absence of collapsed objects such as neutron stars of black holes.

Since we are observing 8 keV synchrotron photons. The electrons producing them must have energies of 2.e5 (B/10 microGauss)^-1/2 GeV, or 200 TeV for this magnetic field. This energy is close to that of the 'knee' in the cosmic-ray spectrum, and provides strong evidence that SNR shock waves can accelerate electrons to these energies. The process of shock acceleration, while well-studied theoretically, has lacked direct evidence of operation to the extremes of energies for which it has been invoked, and SN1006 makes an important contribution to providing such evidence.

Figure 1. The ASCA X-ray image of SN1006. X-ray spectra were made from the two elliptical regions.
Figure 2. The X-ray spectra from the rim and central regions; the former is non- thermal spectrum, while the latter is that of thin thermal plasma typical to shell-like SNRs.

We also prepare postscript versions of the figure (left (regular(237Kbyte) or gzipped(25Kbyte)) and right(34Kbyte)).

The origin of cosmic ray? --- The evidence of shock acceleration around the shell of supernova remnants


[ASCA image of G347.5-0.5]
Click it, and you can get a larger image(85Kbyte)

(From: IAU CD-ROM Images) Mosaic X-ray images on the Galactic plane in the Scorpius region observed with the GIS detector on board the X-ray satellite ``ASCA''. The images were obtained by sum of four observations with exposure-correction and smoothing with Gaussian filter of sigma = 0.4'. The correction for vignetting was not performed.

We also prepare postscript versions (regular(2.2Mbyte) or gzipped(565Kbyte)) of the figure.

Discovery of hard X-rays from protostars embedded in the dense cloud

Coronet Cluster

[ASCA image of Coronet Cluster]
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The evolution of Low mass stars start from dynamical gas infalling phase to the Hayashi track called protostar phase, then enter to a quasi static compression, called T-Tauri phase. Then nuclear reaction starts, hence is called the main sequence .

The internal structure of protostars are fully convective, the convection zone shrink as the star evolve, become surface convection at the main sequence. Current consensus is that X-rays are originated from the energy release by the reconnection of the magnetic field. The magnetic filed is amplified by the dynamo process, which is a coupled process of differential rotation and convection. Therefore key parameters of X-ray emission would be convection scale and rotation speed of the star. Convection scale become smaller as the star evolves. As for the rotation speed, T-Tauri rotates faster than main sequence, however whether protostar rotates faster than T-Tauri or not is unclear.

X-rays from main-sequence and T-Tauri have been observed since the Einstein Observatory, however those from protostar has not been until the ASCA satellite.

It is convenient to see the star evolution by the infra-red spectra. Main sequence and late phase of T-Tauri have no circumstellar gas, hence shows black body radiation from the star surface. We call this as the class III spectra. Classical T-Tauri has accretion disk, and the spectrum is the sum of the star surface and the accreting gas. This is the class II spectrum. Protostars have class I spectra, which are dominated by the infalling gas. Therefore radiation from the stellar surface is behind the stronger radiation for the circumstellar gas.

Absorption column increases as we go back to the evolution history. In the protostar phase, the column density is generally exceed E22-23 H/cm**2. Therefore soft X-rays, if any, can not arrive to us. Accordingly hard X-ray imaging is essentially important to observe the protostar itself.

The ASCA results of a star forming region the RCrA dark cloud shown in figure is very demonstrative. Contours show the column of the molecular gas. In the soft X-ray band below 2 keV, the center of the cloud is dark; as literally the dark cloud. Moving to the hard X-ray band, the center becomes bright. It shows complex X-ray structure. The positions of class I objects are given by the blue circles. Good correlation of the hard X-ray image with the position of the protostars, hence many of the protostars in the RCrA cloud are found to be X-ray emitters.

The protostars exhibit typical flare similar to T-Tauri's. The X-ray spectra of pre-flare phase is harder and highly absorbed compared to the T-Tauri. Also clear emission line from iron K-shell emission is found. In the flare phase, the spectrum becomes very strange, with two emission lines, separated about 0.5 keV, suggesting existence of sub-relativistic jet even in the protostars.

Discovery of hard X-rays from protostars embedded in the dense cloud

Orion star forming region

[Optical/Radio/X-ray images of Orion nebula]
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See the document (in Japanese; for press release (2000 April))

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