 |
Fig. 1 - Visual observation of the „twin quasar" Q 0957+561 by the author (C-14, 266x). Inset: The two images on DSS (16.7 mag / 17.0 mag, 6").
On False and True Double Quasars
Wolfgang Steinicke
Quasars are often seen as exotic objects for amateurs, but many of them are visual targets, even for 8". I started observing them visually in the early 80's with the aid of my „Catalogue of Bright Quasars and BL Lacertae Objects" listing 222 objects. Up to now I glimpsed nearly 100 QSOs. Since 1997 there is also a quasar programm organized by the „German Deep-Sky Association". Over 200 objects were observed with different apertures; often more than once, to monitor their light curves. A few QSOs are bright (PHL 1811 in Capricornus is the latest example with 14mag), but many are variable. Some violent objects show peaks around 13-14 mag, but they can also dim to very low levels, unreachable for visual observing.
A special - and even more exotic - subclass are double or multiple quasars. Let's start with the „false" cases: QSOs split into different images by gravitational lensing. The effect was predicted by Einsteins General Relativity and there was great excitement, when Walsh found the first case in 1979, the „twin quasar" in Ursa Major Q 0957+561. The spectra of the two images are nearly identical. My first visual observation dates November 3, 1984 (Fig. 1). The combined magnitude of 16.1 mag was in the reach of my C-14, but I could not resolve the images only 6" separated. Ronald Stoyan, using a 14" Newton in 1995, thought to see an elongated image or even „two nuclei". The object was clearly resolved later by Stefan Karge with a 24" Hypergraph at 400x under optimal seeing.
|
Fig. 1 - Visual observation of the „twin quasar" Q 0957+561 by the author (C-14, 266x). Inset: The two images on DSS (16.7 mag / 17.0 mag, 6").
Up to now over 40 gravitational lenses are known [1], among them the „triple quasar" PG 1115+080 in Leo (combined magnitude 15.8, separations 2.1" and 2.7"), which I saw in 1984, or the famous "clover leaf quasar" H1413+117 in Bootes with four images. Variable objects show synchronous brightness variations.
What's about true pairs? Surprisingly the more exotic case of lensing came first and we have to wait for ordinary pairs nearly 10 years. But they were long time expected. QSOs are extreme luminous cores of galaxies, and as there exist double galaxies, we might find true double quasars too! They could be either created at once or later merged by gravitonal forces, as seen in rich clusters of galaxies.
 |
Fig. 2 - DSS-image of the first true pair: OM-076 (18.7 mag / 21.5 mag, 4.2").
How do we distinguish between false and true pairs? In a true pair the components should show different spectra (not identical copies) at the same redshift. Having in mind, that the brightness increases if the central black hole is feeded, we should detect different, unsynchronous light variations for both objects.The first true double quasar was OM-076 in Crater (Fig. 2), found by Djorgovsky in 1987. The 11 known cases [2] are listed in Tab. 1. All show nearly identical redshifts (there are no „optical doubles"). The latest example CT 344 in Sculptor (Fig. 3), showing also the closest separation of 0.3", was found in 2001 by Junkkarinen.
 |
Fig. 3 - The closest case: CT 344 with 17.8 mag and 0.4" (not resolved on DSS-image).
Most objects are beyond the limit of amateur telescopes. Only for the brightest, HS 1216+5032 in Canes Venatici, visual observations are recorded using 20" telescopes (Fig. 4). But CCD imaging might be easy; already with small telescopes you can reach about 20mag. As an example Fig. 5 shows the quasar APM08279+5255 in Ursa Major. With 7 Billion ly (redshift z=3,87) this is the most distant object, which can be seen visually! I observed the 16.5mag quasar in spring with my new 20" Dobson. Due to its brightness and distance, it was first thought to be the „most luminous object" in the universe. But a detailed analysis showed two lensed images of equal magnitude in 0.4" separation. Gravitational lensing amplifies the brightness, so the true luminosity of the very object is lower.
 |
Fig. 4 - Visual observation of the brightest quasar pair HS 1216+5032 by the author in 2001 (20" Dobson, 500x). Inset: The two images on DSS (17.2 mag / 18.6 mag, 8.9").
 |
Fig. 5 - First amateur image of APM08279+5255, a gravitational lense 7 Bill. ly distant, by W. Düskau (2000) with 5" APO refractor f/8 and ST-7E.
No matter if double quasars are true or false, any observation is a challenge: we reach the limit of our equipment, but even more that of nature, physics and cosmic importance.
Tab. 1 - All known true double quasars (Con = constellation, V = visual magnitude, z = redshift, d = separation in arcsec).
| Object | Position (2000) | Con | V | z | d | Names | ||
| 1 | Q 0023+171 | A
B |
00 25 37.2 +17 28 00
00 25 36.9 +17 28 04 |
Psc | 21.9
23.1 |
0.944
0.946 |
4.8 | MG1 J002538+1728 |
| 2 | CT 344 | A
B |
01 05 34.7 -27 36 59 | Scl | 17.8 | 0.848 | 0.3 | LBQS 0103-2753 |
| 3 | PHL 1222 | A
B |
01 53 53.9 +05 02 59 | Psc | 17.6
21.5 |
1.904 | 3.3 | UM 144, Q 0151+048 |
| 4 | CTQ 839 | A
B |
02 52 57.9 -32 49 09 | For | 18.3
20.8 |
2.240 | 2.1 | |
| 5 | OM-076 | A
B |
11 47 51.6 -07 24 41
11 47 53.6 -07 24 44 |
Crt | 18.7
21.5 |
1.342
1.341 |
4.2 | PKS 1145-071, Q 1145-071 |
| 6 | HS 1216+5032 | A
B |
12 18 41.0 +50 15 35
12 18 40.3 +50 15 41 |
CVn | 17.2
18.6 |
1.450
1.451 |
8.9 | |
| 7 | Q 1343+266 | A
B |
13 45 43.8 +26 25 07
13 45 44.5 +26 25 06 |
Boo | 20.4
20.0 |
2.029
2.031 |
9.5 | RRS IV 26
RRS IV 27 |
| 8 | RIXOS F212_032 | A
B |
16 29 02.3 +37 24 34
16 29 02.6 +37 24 29 |
Her | 18.6
18.8 |
0.923
0.923 |
4.3 | 1WGA J1629.0+3724 |
| 9 | FIRST J1643+3156 | A
B |
16 43 11.4 +31 56 19
16 43 11.4 +31 56 21 |
Her | 18.4
18.4 |
0.586
0.587 |
2.3 | 87GB 164118.4+320129 |
| 10 | LBQS 2153-2056 | A
B |
21 55 53.5 -20 41 46 | Cap | 17.9
21.3 |
1.845
1.845 |
7.8 | Q 2153-209 |
| 11 | MGC 2214+3550 | A
B |
22 14 57.0 +35 51 25
22 14 57.7 +35 51 28 |
Lac | 18.8
19.3 |
0.879
0.876 |
3.0 | B2 2212+35 |
Literature
[1] Veron-Cetty, M.-P., Veron, P., A Catalogue of Quasars and Active Galactic Nuclei, 10th edition, ESO 2001