LINER and LLAGN Research at the LHEA X-ray Branch
Supermassive Blackholes Lurking in the Nuclei of Galaxies
Typically, AGN have X-ray luminosities on the order of
10^42 to 10^49 erg/s.
Some AGN, known as low-luminosity AGN (LLAGN), have X-ray luminosities of less
than 10^42 ergs/s. Another class of galaxy, know as LINERs (low-ionization
nuclear emission line regions) may also be smaller-scale AGN. LINERs and
LLAGN,
found in more than 1/3 of all nearby galaxies, far outnumber "normal" AGN,
found in < 5% of nearby galaxies. X-ray observations help detect these
supermassive blackholes becuase X-ray penetrate throught the dust and gas
typically found in the nuclei of galaxies.
It is possible that most
galaxies (including our Milky Way) may have supermassive
blackholes in their nuclei, and we do not know it (yet) because the blackholes
are obscured or are currently "quiet".
The LINER M81
While the has been evidence for an AGN in the M81 for some time, including
X-ray observations by Einstein and BBXRT, ASCA has shown that M81 is in fact
a bona-fide AGN. Specifically, ASCA has shown that the X-ray emission is
variable
on short time scales (see the light curve), and that there is complex,
possibly broad Fe K emission at around 6.5 keV present in its X-ray spectrum,
also shown below.
 |
The X-ray light curve observed by one of the CCDs aboard ASCA during an
observation of M81 |
 |
The ratio of data to model for a simple power-law fit to the ASCA M81
SIS (top) and GIS (bottom) spectra, with the Fe-K emission clearly visible at
around 6 keV |
The Sombrero Galaxy
Because of a dust lane running across the Sombrero Galaxy (also known as NGC
4594), making up the "rim" of the sombrero, observations of its nucleus have
been difficult. Dynamical evidence suggests that NGC 4594 might harbor one
of the most massive blackholes known, up to a billion solar masses. Recent
observations with the ROSAT HRI have shown that most of the X-ray emission
is concentrated in the nuclear region of NGC 4594. ASCA, with its ability
to detect photons above 2 keV that are efficient at penetrating large amounts
of gas and dust, has shown that NGC 4594's X-ray spectrum contains a power-law
component reminsiscent of AGN (see the spectrum below, which also shows that
there is a soft component probably due to starburst activity).
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The (unfolded) ASCA spectra of NGC 4594 |
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