Extreme soft X-ray emission from the broad-line quasar REJ2248-511

1
Extreme soft X-ray emission from the broad-line
quasar REJ2248-511 R.L.C. Starling1, E.M.
Puchnarewicz1, K.O. Mason1 E.
Romero-Colmenero2 1 Mullard Space Science
Laboratory, University College London, Dorking,
Surrey RH5 6NT, UK. 2
South African Astronomical Observatory, PO Box 9,
Observatory, 7935 South Africa. e-mail
rlcs_at_mssl.ucl.ac.uk
Most ultrasoft AGN and quasars have both a
large excess of flux at soft X-ray energies and
low-velocity optical emission lines from the
Broad Line Region (FWHMlt2000 km s-1) and are
known as Narrow-line Seyfert 1 galaxies. It has
been suggested that the coexistence of these two
properties is due to a low mass black hole and a
high accretion rate. However, REJ2248-511 is one
of just a handful of ultrasoft AGN which do have
high-velocity optical Broad lines. Here we
present the preliminary findings from the first
of three XMM-Newton monitoring observations to
examine the relationship between the soft X-ray
spectrum and the velocities of the Broad line
Clouds in this object. The soft excess has varied
dramatically between previous observations and
these monitoring observations should provide a
deeper insight into the behaviour and the source
of this variable X-ray emission.
Comparison with previous data Between the ROSAT
(1992 Puchnarewicz et al. 1995) and ASCA (1997
Breeveld et al. 2001) observations the ultrasoft
component completely disappeared and with XMM we
find that it has returned though less strongly
(Fig 1). In contrast, the hard X-ray power law
remains constant throughout all three
observations. Previous optical spectra (Grupe et
al. 1998 Mason et al. 1995) showed that the
continuum slope decreased dramatically becoming
much redder in a one year period, with no change
in the BLR line strengths and velocities. The
large amplitude variability in the Big Blue Bump
feature suggests that this object is alternating
between a high and low state, indicated by the
flux of soft X-rays, a behaviour seen in galactic
black hole candidates.
The XMM-Newton X-ray spectrum REJ2248-511 was
observed with the EPIC and RGS instruments
on-board XMM on 26 October 2000 for 40ks. The
0.3-10.0 keV X-ray continuum consists of a soft
excess below 2.0 keV superimposed on a hard power
law (G1.870.08) (Fig 2). The soft X-ray
spectrum cannot be fit with a single component.
The 0.3-2.0 keV spectrum can be modelled as 2
blackbodies (kT120810 eV kT2964 eV, soft
component flux2.5573x10-12 ergs cm-2 s-1). It is
often thought that the ultrasoft excess is the
high energy tail of the accretion disk (with high
accretion rates), but a disk blackbody is a very
poor fit to the soft end of this spectrum. There
is no evidence for an Iron line, however, the
luminosity of this object is such that it may be
classed as a quasar in which Iron lines are much
less common. We do not observe any temporal or
spectral variability during the observation.
Fig 1. Comparison of the XMM pn (blue), XMM
MOS1/MOS2 (greens), ROSAT PSPC (black) and ASCA
GIS2/GIS3 (reds) spectra over the range 0.7-10.0
keV with the ASCA single power law fit. The lower
panel shows the delta chi-squared. The ROSAT and
ASCA data were supplied and reduced by A.A.
Breeveld (MSSL).
The RGS spectrum has poor S/N although there may
be some detectable features and a more detailed
analysis of the RGS spectrum is in progress. Here
(Fig 3) we fit a power law continuum with the
parameters which best-fit the pn continuum in the
0.6-2.1 keV range.
Fig2. Above The EPIC pn spectrum (binned to
have a minimum of 20 counts per bin) and ratio to
the best-fit model (log scale). Left the
unfolded best-fit model and pn spectrum.
Fig 3. The combined RGS1 and RGS2 spectrum (bins
of 10 channels) and best-fit power law continuum
model.
Future work The full analysis of the data
presented here is underway (Starling et al., in
prep.). This XMM observation was just the first
in a series of 3 designed to monitor the
variability of the soft X-ray spectrum. The final
observation was made in late October 2001 and we
expect to have the data soon. We have also
obtained optical spectrophotometry
quasi-simultaneous with two of the XMM
observations. With the largest BLR velocities
measured among ultrasoft AGN (Fig 3). REJ2248-511
is an ideal test case with which to examine the
relationship between the BLR and the incident
ionizing continuum, modelling various scenarios
with the photoionization code Cloudy (Ferland et
al. 1997). With the coming data we will also be
able to model the Big Blue Bump and probe the
nature of this emission. We can then calculate
the central black hole mass and accretion rate to
discover the physical difference between this
ultrasoft source and the NLS1s..