An Update on the X-ray transient Narrow-Line Seyfert 1 galaxy WPVS 007: Swift observations of UV variability and persistence of X-ray faintness

0.28 MB·English

by  Dirk Grupe

An Update on the X-ray transient Narrow-Line Seyfert 1 galaxy
WPVS 007: Swift observations of UV variability and persistence of
X-ray faintness
Dirk Grupe1
7 [e-mail]
0 , Patricia Schady1,2, Karen M. Leighly3, Stefanie Komossa4, Paul T. O’Brien5, John A.
0
Nousek1
2

n
a
J

9
ABSTRACT
1

We report on the detection of UV variability and the persistence of X-ray faintness of the
1
X-ray transient Narrow-Line Seyfert 1 galaxy WPVS 007 based on the first year of monitoring
v
4 this AGN with Swift between 2005 October and 2007 January. WPVS 007 has been an unusual
6 source. While being X-ray brightduring the ROSAT All-Sky Survey it has been extremely faint
5 inallfollowingX-rayobservations. Swift alsofinds this NLS1 to be X-rayfaint andnotdetected
1 in the Swift X-Ray Telescope at an 3σ upper limit of 2.6×10−17 W m−2 in the 0.3-10.0 keV
0
band and confirms that the AGN is still in a low state. During the 2006 July and December
7
observations with Swift’s UV-Optical Telescope (UVOT) the AGN became fainter by about 0.2
0
/ mag in the UV filters and by about 0.1 mag in V, B, and U comparedwith the 2005 October to
h
2006Januaryand2006September/Octoberobservationsfollowedby a rebrightening inthe 2007
p
- Januaryobservation. This variabilitycanbe causedeither by achangeinthe absorptioncolumn
o densityandthereforethereddeningintheUV,orbyfluxvariationsofthecentralengine. Wealso
r
t noticed that the flux in the UVOT filters agree with earlier measurements by the International
s
UltravioletExplorertakenbetween 1993-1995,but spectra takenby the Hubble Space Telescope
a
: Faint Object Spectrograph show that WPVS 007 was fainter in the UV by a factor of at least
v
2 in 1996. The flat optical/UV spectrum suggests that some UV extinction is present in the
i
X spectrum, but that alone cannot at all account for the dramatic fading in the X-ray flux. Most
r likelyweseeapartialcoveringabsorberinX-rays. Alternatively,thecurrentX-rayemissionseen
a
from WPVS 007 may also be the emission from the host galaxy.
Subject headings: galaxies: active, galaxies: individual (WPVS 007)
1. Introduction
1DepartmentofAstronomyandAstrophysics,Pennsyl-
vania State University, 525 Davey Lab, University Park, When observed at optical or lower energies,
PA16802 radio-quiet AGN appear to be rather stable and
2Mullard Space Science Laboratory, Holmbury nothighlyvariable. However,thispicturechanges
St. Mary, Dorking, Surrey RH5 6NT, U.K.; email:
dramatically when AGN are observed in X-rays.
[e-mail]
3Homer L. Dodge Department of Physics and Astron- Flux variability by factors 2-3 on timescales of
omy,UniversityofOklahoma,440WestBrooksStreet,Nor- days to months is quite common among low- and
man,OK73019;email: [e-mail] high-luminosity AGN. An increasing number that
4Max-Planck-Institut fu¨r extraterrestrische Physik, vary by factors 10-30 has emerged in the last
Giessenbachstr., D-85748 Garching, Germany; email: sko- decade, including in intermediate-type Seyferts
[e-mail]
5DepartmentofPhysics&Astronomy,UniversityofLe-
icester,Leicester,LE17R,UK,email: [e-mail]
1
and Narrow-Line Seyfert 1 galaxies (NLS1s), and emergence of a BAL flow (Leighly et al. 2005;
there is good evidence that a substantial part of Leighly et al., in preparation 2007). A discovery
that variability is caused by (cold) absorption, in of a re-brightening and following rise in the X-
terms of complete or partial covering. Interest- ray luminosity, and in spectral changes, will set
ingly, the highest amplitudes of variability have tight constraints on the movement and location
been detected from the cores of non-active galax- of the absorber, and on the nature of the absorp-
iesintermsoftransientflares,interpretedastidal tion. The high amplitude of the variability makes
disruptions of stars by the black holes at the cen- WPVS 007exceptionalamongthe knowncasesof
ters of these galaxies (e.g. Komossa et al. 2004, absorption variability. In order to detect a possi-
and references therein). blere-brighteningandthereforethedisappearance
NLS1s are objects with extreme properties: of the absorber, we began a monitoring campaign
they show the steepest X-ray spectra, strongest using Swift in 2005 October.
optical FeII emission and weakest [OIII] emission The Swift mission (Gehrels et al. 2004) was
(e.g. Boller et al. 1996; Boroson& Green 1992; launched on 2004 November 20th. The main pur-
Laor et al. 1997;Grupe et al. 2004a). Themost pose is to hunt and observe Gamma-Ray Bursts
common explanation for their extreme properties (GRBs). However, part of the observing time is
is that they have relatively small black hole mass used for fill-in targets and targets-of-opportunity
andahighEddingtonratioL/L (e.g. Boroson when no GRB can be observed. Due to its multi-
edd
2002; Grupe et al. 2004a; Sulentic et al. 2000). wavelength capacities and its flexible scheduling,
NLS1s are also known to be objects with very Swift is an ideal observatory of all types of AGN,
strong X-ray variability (e.g. Leighly 1999). asdemonstratedbye.gGrupe et al. (2006)onthe
TheNLS1WPVS007(1RXSJ003916.6−511701, NLS1 RX J0148.3–2758 and Markwardt et al.
RBS0088;α =00h39m15.s8,δ =−51◦17′03′.′0, (2005) on the search for obscured AGN in the
2000 2000
z=0.029; Grupe et al. 1995) was discoveredin a BAT survey. Swift is equipped with three tele-
surveyoffaintsoutherngalaxieswithHαemission scopes: at the high energy end the Burst Alert
byWamsteker et al. (1985)andisauniqueX-ray Telescope (BAT, Barthelmy 2005) operating in
transient AGN. While X-ray transience is typi- the 15-150 keV energy range, the X-Ray Tele-
cally associated with an X-ray outburst caused scope (XRT, Burrows et al. 2005), which cov-
by a dramatic increase in the accretion rate or ers the soft X-ray range between 0.3-10.0 keV,
the very onset of accretion, the situation is very and at the long wavelength end, the UV-Optical
different in WPVS 007. During the ROSAT All- Telescope (UVOT, Roming et al. 2005). The
Sky Survey (RASS, Voges et al. 1999), when XRT uses a CCD detector identical to the EPIC
the source was X-ray bright, the optical-to-X-ray MOS on-board XMM (Turner et al. 2001). The
flux ratio was in a normal range for AGN (e.g. UVOT covers the range between 1700-6500˚Aand
Beuermann et al. 1999; Maccacaro et al. 1988). is a sister instrument of XMM’s Optical Monitor
However,allfollow-upX-rayobservationsbetween (OM, Mason et al. 2001). TheUVOThasasim-
1994 and 2002 using ROSAT and Chandra found ilar set of filters as the OM (Mason et al. 2001;
it to have almost vanished from the X-ray sky Roming et al. 2005). However, the UVOT UV
(Grupe et al. 2001; Vaughan et al. 2004). throughput is a factor of about 10 higher than in
the OM.
Until recently the cause for the transient
behavior of WPVS 007 had been a mystery. The outline of this paper is as follows: in §2
Grupe et al. (1995) suggested that this tran- we describe the Swift observations and the data
sience could be due to a temperature change in reduction, in §3 we present the results of the
the accretion disk that would shift the soft X- Swift XRT andUVOT data analysisandcompare
ray spectrum out of the ROSAT PSPC energy the UVOT data with earlier IUE and HST spec-
observing window. However, in recent years it tra, and in §4 we discuss the results. Through-
became clear that the cause of the transience is out the paper spectral indexes are denoted as en-
absorption. In1996July,WPVS007wasobserved ergy spectral indexes with Fν ∝ ν−α. Luminosi-
by HST (Goodrich 2000; Constantin & Shields ties are calculated assuming a ΛCDM cosmology
2003); a 2003 FUSE observation revealed the with ΩM=0.27, ΩΛ=0.73 and a Hubble constant
2
of H =75 km s−1 Mpc−1 using a luminosity dis- with the absorption column density at the Galac-
0
tances D=118 Mpc given by Hogg (1999). All tic value (2.84 × 1020 cm−2, Dickey & Lockman
errors are 1σ unless stated otherwise. 1990) and an energy spectra slope α =3.0. Note,
X
however, that this is just an estimate, since we
2. Observations and data reduction do not know what the low-state X-ray spectrum
of WPVS 007 really looks like. For all observa-
WPVS 007 has been monitored by Swift be-
tions the counts were corrected due to the expo-
tween 2005 October and 2007 January. Table1
sure maps.
lists the Swift XRT observations, including the
As listed in Table2 UVOT observations were
startandendtimes,thetotalexposuretimes,and
performed in all filters except during the 2005
the 3σ upper limits. Note that we do not in-
October and December, and 2006 January ob-
cludethesegment005data(2006September06)in
servations only the UV filters were used. Pho-
the XRT analysis because during the time of that
tometry on all UVOT individual and coadded
observation the XRT detector was rather warm
exposures was performed with the tool uvot-
resulting in an enhanced detector background. maghist version 0.1. A 6′′ and 12′′ radius ex-
However, the UVOT data from that time period
traction regions were used centered on WPVS
were not affected. The Swift UVOT observations
007 for the optical (V, B, and U) and UV filters
are summarized in Table2. Also note that seg-
(UV W1, UV M2, and UV W2), respectively and
ment 008 does not exist. WPVS 007 was orig-
the background count rate was measured with a
inally scheduled for 2006 December 03, but the ′′
20 radius aperture in a nearby source free re-
observations were superseded by the detections of
gion. All reported magnitudes have been cor-
GRBs 061201 and 061202 (Marshall et al. 2006;
rected for Galactic extinction, where the redden-
Sakamoto et al. 2006, respectively) before the
ingintheline-of-sighttotheobjectisE(B−V)=
start of the WPVS 007 observations. Segment
0.012mag(Schlegel et al.1998). Alldatawereas-
numbers, however, can only be used once by the
pect corrected and coadded before measuring the
Swift scheduling tool. The segment numbers in
magnitudes and fluxes. The UVOT uses Vega-
bothtablerefertothedaysSwiftobservedWPVS
based magnitudes with the following zeropoints:
007 (See the description in Grupe et al. 2006).
V=17.88±0.09, B=19.16±0.12, U=18.38±0.23,
In the first observation of 2006 December (seg-
UVW1=17.69±0.20, UVM2=17.29±0.23, and
ment 009) we noticed that the AGN became sig-
UVW2=17.77±0.20(Brown et al. 2007).
nificantly fainter in the UV by 0.2 mag. In order
Prior the Swift observations WPVS 007 was
toinvestigatethisbehaviorandtogetabetter es-
observed in the UV by IUE four times between
timate ofthe time scalewe initiatedanadditional
1993 and 1995, HST in 1996 July, and FUSE in
ToOfortwopointingsof2kseachwhichwereexe-
2003 November. In this paper we make use of
cuted on 2006December 12 and21 (segments 010
the IUE and HST data. Table3 lists these ob-
and 011). Also note that the 2007 January obser-
servations. The FUSE data will be presented by
vationwassplitintotwosegmentsdueto schedul-
Leighly et al., in preparation (2007) which will
ing constrains, even though the observations were
also contain a spectral analysis of the miniBALs
performed in consecutive orbits.
present in the HST data.
The XRT was operating in photon counting
mode(Hill et al. 2004)andthedatawerereduced
3. Results
bythetaskxrtpipelineversion0.10.4.,whichisin-
cluded in the HEASOFT package 6.1. Photons 3.1. X-rays
were collected in the 0.3-10.0 keV energy range.
We do not detect WPVS 007 in X-rays in any
The upper limits were determined from the back-
ofthemonitoringobservationsperformedbySwift
ground in the XRT. The photons were extracted
so far as listed in Table1. To determine 3σ up-
with XSELECT version 2.4. In order to compare
per limits we applied the method by Kraft et al.
theobservationsfromdifferentmissionsweusethe
HEASARC tool PIMMS version3.8. For the con- (1991). This method determines the confidence
levelsforlownumbersofcountsusingtheBayesian
versionweassumedanabsorbedpowerlawmodel
method for Poisson-distributed data. The back-
3
ground in all three observations was measured in UV W1, UV M2, and UV W2 with statistical er-
′′
a circularregionwith r=235 aroundthe position rors. Figure3 displays where these stars are lo-
of WPVS 007. For the source itself we assumed cated relative to WPVS 007. Note that during
′′
anextractionradiusof23.5 . The3σupperlimits some of the observations not all the 4 stars were
arelistedinTable1. WecoaddedallXRTobserva- in the field of view of the UVOT. U magnitudes
tions together, except for the 2006 September 06 couldonlybegivenforStars#3and4,because#1
observation (segment 005) when the background and2aretoobrightintheUfilterthattheysuffer
was too high. From these coadded data with a significantlyfromcoincidentlosses. Alsonotethat
total exposure time of 23.2 ks we measured an star#1and#2werenotinthefieldofviewduring
upper limit of 1.04×10−3 counts s−1 in the Swift- someoftheobservations. AsshowninTable4,the
XRT.Assumingapowerlawmodelspectrumwith variance in the field stars is small compared with
α =3.0 and N at the Galactic value this upper the variability observedin WPVS007. Therefore,
X H
limit count rate converts to an upper limit in un- we consider the UV variability found in WPVS
absorbedfluxinthe0.3-10keVbandof2.6×10−17 007 to be real. The change by 0.2 mag in the UV
W m−2. filters observed during the 2006 July and Decem-
Figure1 displays the long term light curve of ber observations is larger than the uncertainties
WPVS 007. The ROSAT values were taken from between the measurements in the field stars.
Grupe et al. (2001) and the Chandra data point WPVS 007 has shown variability on timescales
from Vaughan et al. (2004). The light curve ofmonthsbeforebetweentheIUEandHSTobser-
showsthatWPVS007isstillinalowstateandthe vations between 1993 and 1996. Figure4 displays
upper limit of the coadded Swift observations is the Swift UVOT measurements from 2006 Jan-
consistentwiththeROSATPSPCandHRIdetec- uary, the HST spectrum from 1996 (Goodrich
tions. NotethatFigure1usesPSPCcountswhich 2000; Constantin & Shields 2003), and the IUE
were converted by PIMMS assuming a power law spectrum averaging three spectra taken between
spectrum with α =3.0 and N at the Galactic 1993 and 1995. We excluded the 1995 November
X H
value. observation due to strong cosmic ray events dur-
ing that observation. The data shown in Figure4
3.2. UVOT Photometry are the observed data, uncorrected for redden-
ing. The IUE spectra and the Swift-UVOT data
The magnitudes in the UVOT filters are given
seem to agree. However, the HST spectra show
in Table2 and previous UV observations by IUE
a significantly lower flux than the IUE and Swift
and HST are listed in Table3. The errors quoted
data. Dunn et al. (2006) presented an Internet
in Table2 are statistical errors. As listed in Ta-
database1 ofUVcontinuumlightcurvesofSeyfert
ble2 and shown in Figure2 WPVS 007 became
galaxies which also showed that WPVS 007 dis-
fainter in the optical filters by about 0.1 mag and
playssignificantvariabilityintheUVbetweenthe
by about 0.2 mag in the UV filters in the 2006
IUE and HST observations. One possibility of a
July and December observations compared with
lower flux in the HST data is a mis-alignment of
the observations obtained in 2005 and 2006 Jan-
′′
the source in the 1 aperture in the HST FOS.
uaryand2006SeptemberandOctober. TheAGN
However, as listed in Table3, the AGN was ob-
became brighter again by about 0.2 mag in the
served during several orbits and the fluxes in all
2007 January observation which was about three
′′
these spectra agree with each other. A 1 aper-
weeksafterthelastobservationin2006December.
′′
ture is also ratherlargecomparedto the 0.1 reso-
From the current light curve as shown in Figure2
lution. Therefore we exclude a mis-allignment as
the AGN seems to be variable in the optical/UV
the cause of the lower flux in WPVS 007 during
on timescales of a few weeks to several months.
the HST observation in 1996 July. Note that
In order to determine whether this variability
Winkler et al. (1992) reports V=15.28±0.03,
is real or just within the uncertainties we picked
B=15.77±0.03, and U=15.15±0.03. While the
4 field stars of similar magnitude as WPVS 007
small differences in V and B can be explained by
as reference stars and compared their magnitudes
segment by segment. Table4 lists these 4 stars
1http://www.chara.gsu.edu/PEGA/IUE
with the coordinates and their magnitudes in U,
4
thedifferentcentralwavelengthbetweenthefilters s−1, which converts to an unabsorbed flux in the
usedbyWinkler et al. (1992)andtheUVOT,the 0.3-10keVbandof2.6×10−17Wm−2. Thisupper
differenceinUsuggeststhatWPVS007isvariable limit is at a similar level to the ROSAT pointed
in U. PSPC and HRI observations. Note that Chan-
dra, with its superb point spread function, was
3.3. Spectral Energy Distribution able to detect WPVS 007 at an even lower level
(Vaughan et al. 2004) within an exposure time
Figure5 displays the spectral energy distribu-
of 10 ks. Even though we could not detect a re-
tion of WPVS 007 using 2MASS NIR data de-
brightening of WPVS 007 in X-rays, it is still an
rived from the Nasa Extragalacticdatabase, opti-
exciting AGN. Note that the purpose of the Swift
cal/UV data from Swift’s UVOT from 2006 July
observationsisnottoobtainadeepdetection,but
andthe X-raydata fromthe Chandra observation
to monitor the AGN in order to detect when it
from 2002 (Vaughan et al. 2004). The Chandra
re-brightens again.
data are displayed as an un-absorbed power law
OurUVOTobservationssuggestthatthe AGN
model with α =3.0. The optical-to-Xray slope
X
α 2 measured from this plot is α =5.4. This is is variable in the UV bands by about 0.2 mag
ox ox
within timescales of a few months. UV variabil-
anextremevalueforanAGNwhichtypicallyhave
ity, however, is not uncommon in AGN and has
values around α =1.5 (e.g. Yuan et al. 1998;
ox
beenreportedforvariousAGNsuchasNGC 4151
Strateva et al. 2005). Assuming an α =1.5 we
ox
wouldexpectafluxat2keVF =6×10−16 W (Edelson et al. 1996; Crenshaw, et al. 1996),
2keV
m−2, or a luminosity at 2 keV L = 1×1035 NGC 5548 (Clavel et al. 1991; Korista et al.
2keV
1995),Akn564(Collier et al. 2001),and3C390.3
W. However, as shown in Figure5, this is not
(O’Brien et al. 1998),butonlyafewNLS1shave
the case. During the RASS observation, assum-
ing an UV spectrum like during the Swift obser- repeated UV coverage as good as WPVS 007. In
the case of WPVS 007 we can speculate that the
vations, the α was in the order of α =5.0. The
ox ox
UV variability is likely to be caused by a change
flattening of the UV spectrum, as shown by the
in the absorption column density and therefore
UVOT photometry data, suggests some intrinsic
the reddening in the UV. Using the change in
reddening of the AGN. A NLS1 typically has a
the UV W1 magnitude by 0.2 mag we can derive
very blue optical/UV spectrum, as shown e.g in
Grupe et al. (2006) for the NLS1 RX J0148.3- an additional EB−V=0.032. This would cause an
additional reddening by 0.10 mag in V, 0.17 in
2758. Assuming that the intrinsic optical UV
U, and 0.28 in UV W2 which is consistent with
spectrum of WPVS 007 is similar to that of RX
the changes we observe during the 2006 July and
J0148.3-2758,we can estimate a reddening by 0.6
December observations. However, a change in lu-
mag in the UV W2 filter. This results in a lower
minosity of the central engine cannot completely
limitoftheintrinsicreddeningEB−V=0.073. Note
been excluded based on the current data set.
thatthisisaroughestimate. BasedontheHα/Hβ
fluxratio,Winkler et al. (1992)estimatedtheex- The variability we observedin the UV between
tinction to A ≈1.0 which is significantly higher the IUE, HST and Swift-UVOT observations was
V
than our estimate. previouslyalsonoticedbyDunn et al. (2006)be-
tween the IUE and HST observations. This is
4. Discussion not a calibration issue. There are no problems
reported on the HST FOS and IUE observations
Our main results are that WPVS 007 is still either. Also, looking at the light curves presented
in a low state in X-rays and that it shows signifi- in the UV continuum light curve database by
cantvariabilityintheUVontimescalesofmonths. Dunn et al. (2006) suggest that the UV flux is
Adding all Swift observationstogether (except for already decaying between the IUE observation of
the segment 005 data of 2006 September 06) we 1994 to 1995 and that the 1996 HST observation
determine a 3σ upper limit = 1.04×10−3 counts is consistent with this decay.
WPVS 007 may be one of the most extreme
2TheX-rayloudnessisdefinedbyTananbaum etal. (1979)
cases of X-ray weak NLS1s such as those found
asαox=–0.384log(f2keV/f2500˚A). by Williams et al. (2002, 2004). The reddened
5
optical/UV spectrum also suggests that the X- theIUEandHSTdataforanyproblems,Matthias
ray weakness of WPVS 007 is caused by absorp- DietrichforvariousdiscussionsaboutUVvariabil-
tion. However, of the 10 photons detected at the ity in AGN, and the anonymous referee for useful
position of WPVS 007 in the Chandra ACIS-S comments and suggestions to improve the paper.
data (Vaughan et al. 2004), 8 have energies be- This research has made use of the NASA/IPAC
low1 keV.Asimple coldabsorberwouldhaveab- Extragalactic Database (NED) which is operated
sorbed all these photons. The solution could be by the Jet Propulsion Laboratory, Caltech, under
apartialcoveringabsorberthatwouldallowsome contractwiththeNationalAeronauticsandSpace
of the soft X-ray photons to escape. Such par- Administration. This research was supported by
tial covering absorbershave been found in several NASA contract NAS5-00136 (D.G., & J.N.).
NLS1s, such as Mkn 1239 Grupe et al. (2004b)
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Fig. 1.—Long-termlightcurveofWPVS007containingtheROSATAll-SkySurveyandpointedPSPCand
HRI observations, the Chandra, and the upper limits derived from the Swift XRT observations. The count
rateswereconvertedbyassuminganabsorbedpowerlawmodelwithN =2.84×1020cm−2 andα =3.0. The
H X
large red downwardarrow displays the upper limit of the coadded exposures of the Swift-XRT observations
(23.2 ks).
8
Fig. 2.— Swift UVOT light curves of WPVS 007. The values are given in Table2.
Fig. 3.—SwiftUVOTW2imageofthe2006JanuaryobservationwiththereferencestarsaslistedinTable4
and WPVS 007.
9
Fig. 4.— UV observations of WPVS 007. The red points are the Swift UVOT observations from 2006
January, the black spectrum is the HST observation from 1996, and the green spectrum the average of the
IUE observation from 1993, 1994,and 1995 December.
10