X-ray transient AGN and galaxies, and why we need new soft X-ray surveys

Abstract

X-ray transience is the most extreme form of variability observed in AGN or normal non-active galaxies. While factors of 2-3 on timescales of days to years are quite common among AGN, X-ray transients appear only once and vanish from the X-ray sky years later. The ROSAT All-Sky Survey with its sensitivity to energies down to 0.1 keV was the an ideal tool to discover these sources. X-ray transience in AGN or galaxies can be caused by dramatic changes in the accretion rate of the central black hole or by changes of the properties of the accretion disk. So far only a handful of sources are known. In order to estimate how often such an event occurs in a galaxy, a new soft X-ray survey is needed. In these proceedings I describe the currently known X-ray transient AGN and galaxies and will argue for a new soft X-ray survey in order to discover more of these extreme X-ray sources.

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arXiv:astro-ph/0201048v1 4 Jan 2002
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X-RAY TRANSIENT AGN AND GALAXIES, AND WHY WE NEED NEW SOFT X-RAY
SURVEYS
D.Grupe1
Max-Planck-Institut f¨ ur extraterrestrische Physik, Giessenbachstr. D-85748 Garching, Germany
Abstract
X-ray transience is the most extreme form of variabil-
ity observed in AGN or normal non-active galaxies. While
factors of 2-3 on timescales of days to years are quite com-
mon among AGN, X-ray transients appear only once and
vanish from the X-ray sky years later. The ROSAT All-
Sky Survey with its sensitivity to energies down to 0.1
keV was the an ideal tool to discover these sources. X-ray
transience in AGN or galaxies can be caused by dramatic
changes in the accretion rate of the central black hole or
by changes of the properties of the accretion disk. So far
only a handful of sources are known. In order to estimate
how often such an event occurs in a galaxy, a new soft X-
ray survey is needed. In these proceedings I describe the
currently known X-ray transient AGN and galaxies and
will argue for a new soft X-ray survey in order to discover
more of these extreme X-ray sources.
Key words: X-ray surveys, AGN, X-ray transients
1. Introduction
The ROSAT All-Sky Survey (RASS, Voges et al. 1999) has
established a new phenomenon among AGN and galaxies
- X-ray transience. These X-ray transient AGN and galax-
ies are very bright once and appear to be fainter or even
vanish in X-rays when observed years later(e.g. Grupe et
al. 2001a, Komossa & Bade 1999). X-ray transience can
appear in Active Galactic Nuclei as well as in normal non-
active galaxies. While the X-ray transience in AGN might
be caused by changes of the accretion disk properties, in
non-active galaxies it might be caused by an X-ray out-
burst. These X-ray outbursts can be caused by an accre-
tion event, either by instabilities in the accretion disk or
by the disruption of a star by the central black hole. Disk
instabilities may cause an X-ray outburst in AGN while
the tidal disruption model is favoured to explain the out-
bursts in non-active galaxies which do not show any signs
of nuclear activity.
So far only in one source a response in the optical emis-
sion lines has been observed, IC 3599. Future soft X-ray
surveys will allow us to search for this type of sources.
Fast follow-up observations in the optical and in X-rays
will provide us with a powerful tool to map the inner re-
gion of an AGN in order to locate the line emitting regions
and to describe what the geometry of the whole system is.
2. The Sample
Our sample of soft X-ray AGN contains 113 sources se-
lected from the RASS by the PSPC count rates CR > 0.5
cts s−1and the hardness ratio HR <0.0 (Grupe et al. 1998,
Grupe et al. 2001a, Thomas et al. 1999). Pointed PSPC
and HRI observations are available for 60 and 50 sources,
respectively. All in all, for more than 80 sources at least
one pointed observation is available (Grupe et al. 2001a).
In this way we have a tool to search for long-term large
amplitude variations. Fig. 1 displays the RASS vs. pointed
observation count rates. HRI count rates have been con-
verted into PSPC count rates assuming no spectral change
between both observations. The solid line marks no change,
the short-dashed line a change by a factor of 10 and the
long-dashed line by a factor of 100 between the RASS
and the pointed observation. Four sources turned out to
vary by factors of almost 100 or even more: IC 3599,
WPVS007, RX J1624.9+7554, and RX J2217.9–
5941. The first three are X-ray transients while RX J2217.9–
5941 is an X-ray transient candidate.
3. X-ray transient AGN and galaxies
3.1. WPVS007
The Narrow-line Seyfert 1 galaxy (NLS1) WPVS 007 was
the softest AGN observed during the RASS (Grupe et al.
1995b) and had a mean count rate of about 1 cts s−1.
In later pointings the source shows only the flux expected
from a normal non-active galaxy (Grupe et al. 2001a). The
flux measured during the RASS observation was what was
expected from an average fX/foptratio of an AGN. The
question that has to be asked for WPVS 007 is not why
it was bright during the RASS it is more why it is off now
and what caused the dramatic switch off of this source in
less than three years. A possible scenario to explain this
dramatic turn-off is a temperature change in the Comp-
tonization layer of the accretion disk that shifts the soft
X-ray photons out of the ROSAT energy window. Because
no optical variability has been detected in this source, it
can be expected that the AGN engine is still running and
Proc. Symposium ‘New Visions of the X-ray Universe in the XMM-Newton and Chandra Era’ 26–30 November 2001,

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Figure1. RASS vs. pointed observation count rates (taken
from Grupe et al. 2001a)
the source should appear in the X-ray sky again. There-
fore a monitoring of this source by XMM or Chandra is
needed.
3.2. IC 3599
The Seyfert 2 galaxy IC 3599 has shown an X-ray outburst
during the RASS (see Fig. 2). During the RASS observa-
tion it was one of the brightest AGN in the X-ray sky
(4.90 PSPC cts s−1) and has been even seen in ROSAT’s
Wide Field Camera (Pounds et al. 1993; Edelson et al.
1999). In later pointed PSPC and HRI observations the
source showed a dramatic decrease in its count rate over
the years (Grupe et al. 2001a). The X-ray outburst during
the RASS was followed by a response in its optical emis-
sion lines. While in the May 1991 spectrum (Fig. 3; Brandt
et al. 1995) the Hβ line as well as highly ionized Fe line
(e.g. [FeX]) appear to be extremely strong, in the spectra
taken years later (Fig. 3; Grupe et al. 1995a) these lines
have become much weaker, but other lines (e.g. FeVII)
showed up instead. A possible explanation of this X-ray
outburst is an accretion event either caused by an instabil-
ity of the accretion disk or by a tidal disruption of a star
orbiting around the central black hole. Such tidal disrup-
tion events have been proposed by e.g. Rees 1990. They
are a consequence if every galaxy habours a supermassive
black hole in its center (see Rees 1989).
3.3. RX J1624.9+7554
RX J1624.9+7554 has shown a dramatic decrease of its X-
ray flux by at least a factor of more than 200 between the
Figure2.
Grupe et al. 2001a)
Long-term lightcurve of IC 3599 (taken from
RASS and a pointed observation 1.5 years later (Grupe et
al. 1999). Optical spectroscopy performed 8 years after the
RASS, identified this source as a normal non-active galaxy.
Because of the lack of any signs of nuclear activity in this
galaxy, the most plausible explanation for this X-ray event
is the tidal disruption of a star by the central black hole.
With a 0.2-2.0 keV luminosity LX=1.6 1037W during the
outburst it was the brightest non-active transient observed
so far.
3.4. RX J2217.9–5941
The NLS1 RX J2217.9–5941 is a possible X-ray transient
candidate. It is highly variable on time scales of days as
well as years (Grupe et al. 2001a). During its two-day
RASS observation the count rate decrease by a factor
of 15. Observed several times in pointed observations by
ROSAT and ASCA the source has become fainter over the
years. It is not clear yet if this source will become a tran-
sient. However, due to the black hole mass of ≈ 108M⊙the
timescales are larger than in e.g. IC 3599 (MBH≈ 106M⊙).
RX J2217.9–5941 might be a bigger brother of WPVS007
and we observe RX J2217.9–5941 slowly become fainter
over the years. They both have in common to be NLS1s
and have steep X-ray spectra at least during their RASS
observation.
3.5. Other X-ray transient galaxies
Besides RX J1624.9+7554 a few more non-active galaxies
have been reported of in the literature: NGC 5905, RX
J1242.6–1119, and RX J1420.4+5334 (Komossa & Bade
1999; Komossa & Greiner 1999; Greiner et al. 2000). While
NGC 5905 was observed in an outburst state during the
RASS and a later turn-off, the two other sources are the
only examples of a ‘turn-on’. Both where brighter when
observed in pointed observations after the RASS. All the
three sources have in common not to show any signs of
nuclear activity. The most plausible explanation for the

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Figure3. Optical spectra of IC 3599. The left panel displays the spectrum taken half a year after the X-ray outburst
during the RASS (taken from Brandt et al. 1995) and the right panel shows the spectra taken years later (Grupe et al.
1995a). The spectra of 1993, 1994, and 1995 are displayed with an offset.
outburst is like in RX J1624.9+7554 a tidal disruption of
a star by the central black hole.
4. Discussion
The ROSAT All-Sky Survey has shown the potential of
soft X-ray surveys to discover X-ray transient AGN and
galaxies. There are in principle two types of transience in
AGN and galaxies: a) a sudden decrease of the X-ray flux
in normally bright X-ray sources, and b) an X-ray outburst
caused by either an accretion disk instability or by a tidal
disruption of a star orbiting the central black hole. The
first case, which might have been seen in WPVS 007 and
RX J2217.9–5941, might be the transition between a high
into a low state. Such behaviour is known from galactic
black hole candidates. The second case, an X-ray outburst,
was observed in IC 3599, NGC 5905, RX J1624.9+7554,
RX J1242.6–1119, and RX J1420.4+5334. The follow-up
optical observations of IC 3599 showed how the light front
moved through the inner region of the AGN/galaxy and
caused different emission lines to show up over time. While
reverberation mapping is a powerful tool in bright, ‘nor-
mal’ AGN to map the inner region, X-ray transient AGN
and galaxies show much larger and dramatic changes in
their optical spectra. The only problem is, such X-ray out-
burst events are rare. In order to find those extraordinary
sources, soft X-ray surveys are needed.
New soft X-ray surveys will provide us with a statisti-
cally relevant data set that can clarify how common the
chance between low and high states is in AGN. Repeat-
ing soft X-ray surveys will also give us an estimate how
often outburst events appear in non-active galaxies, or in
other words, how many years we have to wait until such
an event happens in a galaxy. This result is also of interest
for our own galaxy, which black hole mass of 2.6 106M⊙is
comparable to the ones found in the galaxies in which an
outburst occurred.
The requirements for future soft X-ray surveys are be-
ing sensitive at energies <0.3 keV and using an imaging
X-ray telescope with a high effective area and a large field
of view. A sensitivity in soft X-rays (<0.3 keV, at least)
is needed, because X-ray outbursts have a very soft X-ray
spectrum, due to the high accretion rate during the event.
A high effective area is needed for getting enough photons
to derive spectra and lightcurves on short timescales. And
last but not least, a large field of view is needed to get a
long coverage of the source when passing through the field
of view of the telescope.
Currently, three X-ray surveys are planned, ROSITA,
Lobster Eye, and MAXI. They all have in common to work
at higher energies. While ROSITA may have enough spa-
tial resolution to identify the optical counterpart of an
X-ray transient, Lobster Eye is an all-sky monitor which
does not have enough spatial resolving power. MAXI will
not have enough spatial resolution either, plus will not be
sensitive to low energy photons. ROSITA will be a power-
ful tool to discover obscured hard AGN, but inefficient for
the search of short living soft X-ray events such as X-ray
outburst. The other problem is that the number of pho-
tons collected per scan is too small to derive lightcurves
and spectra. Therefore, none of the currently planned X-
ray survey missions fullfill the requirements for a search for
soft X-ray transient AGN and galaxies. Therefore a new
soft X-ray survey mission is needed, performed in a similar
way as the RASS was, except it should be repeated sev-
eral times in order to a) get long-term light curves, b) see
the same number of ‘turn-ons’ as ‘turn-offs’. Such a sur-
vey will not be only important for AGN/galaxy research,
it is also needed for the discovery and study of e.g. new
cataclysmic variables and super-soft X-ray sources.

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Acknowledgements
I would like to thank Dr. Stefanie Komossa for carefully read-
ing the manuscript. The ROSAT project was supported by the
Bundesministerium f¨ ur Bildung und Forschung (BMBF/DLR)
and the Max-Planck-Society (MPG).
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