Discovery of a Jet-Like Structure at the High Redshift QSO CXOMP J084128.3+131107

Abstract

The Chandra Multiwavelength Project (ChaMP) has discovered a jet-like structure associated with a newly recognized QSO at redshift z=1.866. The system was 9.4 arcmin off-axis during an observation of 3C 207. Although significantly distorted by the mirror PSF, we use both a raytrace and a nearby bright point source to show that the X-ray image must arise from some combination of point and extended sources, or else from a minimum of three distinct point sources. We favor the former situation, as three unrelated sources would have a small probability of occurring by chance in such a close alignment. We show that interpretation as a jet emitting X-rays via inverse Compton (IC) scattering on the cosmic microwave background (CMB) is plausible. This would be a surprising and unique discovery of a radio-quiet QSO with an X-ray jet, since we have obtained upper limits of 100 microJy on the QSO emission at 8.46 GHz, and limits of 200 microJy for emission from the putative jet. Comment: 12 pages including 4 figures. Accepted for publication by ApJ Letters

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arXiv:astro-ph/0403250v1 10 Mar 2004
Discovery of a Jet-Like Structure at the High Redshift QSO
CXOMP J084128.3+131107
D.A. Schwartz, J. Silverman, M. Birkinshaw1, M. Karovska, T. Aldcroft, W. Barkhouse, P.
Green, D.-W. Kim, B. J. Wilkes, D. M. Worrall1
Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138
das@head.cfa.harvard.edu
ABSTRACT
The Chandra Multiwavelength Project (ChaMP) has discovered a jet-like
structure associated with a newly recognized QSO at redshift z=1.866. The
system was 9.4′off-axis during an observation of 3C 207. Although significantly
distorted by the mirror PSF, we use both a raytrace and a nearby bright point
source to show that the X-ray image must arise from some combination of point
and extended sources, or else from a minimum of three distinct point sources.
We favor the former situation, as three unrelated sources would have a small
probability of occurring by chance in such a close alignment. We show that
interpretation as a jet emitting X-rays via inverse Compton (IC) scattering on the
cosmic microwave background (CMB) is plausible. This would be a surprising
and unique discovery of a radio-quiet QSO with an X-ray jet, since we have
obtained upper limits of 100 µJy on the QSO emission at 8.46 GHz, and limits
of 200 µJy for emission from the putative jet.
Subject headings: quasars: general — galaxies: jets — X-rays: galaxies
1. INTRODUCTION
The objectives of the Chandra Multiwavelength Project (ChaMP) include identification
and categorization of a complete, well-defined sample of serendipitous sources (Kim et al.
2004; Green et al. 2004). The results will be of use, e.g., to study luminosity functions and
their evolution, to quantify the newly resolved source(s) of the hard diffuse X-ray background,
and to study cosmic structure and clustering of AGN and galaxies. The wide angle nature
1also, University of Bristol

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of this survey also makes it ideal to discover rare and unusual objects suitable for detailed
study; e.g., lensed QSOs and X-ray jets.
Schwartz (2002a,b) has pointed out that if the jets observed in X-rays on scales of tens
to hundreds of kpc are emitting via IC scattering of the CMB as suggested by Tavecchio
et al. (2000) and Celotti et al. (2001), then they will maintain the same apparent surface
brightness independent of redshift, and therefore can be detected to arbitrarily large redshifts,
up to the epoch at which they form. The Chandra observations of such large scale jets in
QSOs and powerful FR II radio sources are typically interpreted as IC/CMB emission,
(Schwartz et al. 2000; Harris and Krawczynski 2002; Marshall et al. 2001; Sambruna et al.
2001; Siemiginowska et al. 2002). All such interpretations require the assumption that the
jet is either relativistically beamed with Doppler factors of order δ ∼ 3 to 15, or that the
energy density in relativistic electrons grossly exceeds the magnetic field energy density by
at least two orders of magnitude. Detection of the X-ray “beacons” predicted by Schwartz
(2002a,b) would provide additional evidence that the above assumptions are well founded.
We report the discovery of a candidate for such a system: CXOMP J084128.3+131107,
(hereafter called J0841). The X-ray image shows an elongated structure. Despite the broad
point response function (PSF) of the Chandra telescope at this 9.4′off-axis angle, we show
that at least three point sources would be required to simulate the observed extent. We favor
an interpretation of emission from the jet of an optically identified QSO which is close to the
peak X-ray intensity. We also mention alternate interpretations. Due to the small probability
for three unrelated sources to occur by chance in this configuration, such interpretations may
be even more unusual.
2. OBSERVATIONS OF J0841
The serendipitous detection of J0841 on the ACIS-I2 chip occurred using the data from
obsid 2130, an observation of 3C 207 with ACIS-S3 (Brunetti et al. 2002). Figure 1 shows
the X-ray contours superposed on a red-band image. The strongest X-ray peak is coincident
within 1.′′5 with an r′=20.9 mag object. A spectrum of this object (Fig. 2) was obtained
in a 10 minute exposure on Magellan using LDSS-2, and clearly shows a broad emission
line QSO. The optical data have about 13˚ A resolution. The spectrum was cross-correlated
against the composite SDSS QSO spectrum (Vanden Berk et al. 2001) to give a redshift
1.866.

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Fig. 1.— X-ray contours (0.5 to 7 keV) in the region of J0841, superposed on a red-band
image. Contour levels are 0.25, 0.50, 0.75, 1.0, and 1.25 counts per 0.′′98×0.′′98 pixel. Back-
ground is 0.03 counts per pixel. The r′=20.9 object in the eastern contour peak is a QSO at
redshift z=1.866. The position difference between the X-ray peak and the optical source is
1.′′5, consistent with the Chandra PSF distortion at this large off-axis angle.

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Fig. 2.— Ten minute Magellan exposure of J0841. The broad emission lines give a redshift
1.8661 ± 0.0005

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Although the contours in Fig. 1 seem to indicate an extended X-ray structure, one must
be careful due to the distorted telescope response at this large off-axis angle. Figure 3 shows
the X-ray data in the region of the QSO, together with data around the nearby Einstein
medium survey point source MS0838.6+1325 (Maccacaro et al. (1991), a z=0.723 QSO, also
called EMSS 0841+131), which happens to lie in the same Chandra field at a similar off-axis
angle, 9.′3, and at the nearby azimuth of 247◦vs. 265◦for J0841. Each is compared with a
high fidelity raytrace1of a 1.5 keV point source at this off-axis angle and the same azimuth
as J0841.2Both QSOs are expected to have relatively hard spectra, for which 1.5 keV is a
good mean energy, so we do not expect significant effects due to spectral differences. J0841
is clearly not a single point source.
We now show that two point sources could not produce the observed X-ray structure.
Specifically, in the top panel of Fig. 3, taking point sources at the QSO position and at the
center of the ellipse marked B, we show that region A contains a significant excess of counts
over background plus those counts which could be attributed to the QSO, plus those counts
which could be attributed to the source B. The expected counts in box A are based on the
measured ratio of counts in the ellipse marked QSO to the counts in a box marked A to
the west of the QSO, or a similar box to the east of the QSO (not shown). We derive this
predicted ratio both from real data, EMSS 0841+1314, and from a raytrace, and in both
cases we predict ≤10 counts in box A, (including the non-X-ray background). However, we
observe 21 counts in box A, and the probability of this is less than 0.1
We present the expected number of counts in box A in more detail for both methods:
based on the raytrace image (middle panel of Fig. 3), and based on the observation of
EMSS 0841+131 (bottom panel of Fig. 3). The raytrace contains 1567 counts in the box
A, and 29945 in the QSO ellipse, for a measured ratio of 0.052. For EMSS 0841+131,
after background subtraction, those numbers are 28.3 and 669.6, for a ratio of 0.042±0.008,
consistent with the raytrace prediction. From the observed 78.6 net counts inside the J0841
QSO ellipse (top panel of Fig. 3), after background subtraction, we use the raytrace result
of 0.052 to predict 4.1 counts from the quasar would fall in box A. We do a similar analysis,
but with the raytrace or EMSS 0841+131 source centered in the B region. We predict 0.113
and 0.091+/-0.012, respectively for the raytrace and for the EMSS 0841+131 data, for the
fraction of counts inside the B region which would appear in the box A. From the net 32.6
counts observed inside region B, (top panel of Fig. 3), after background subtraction, the
1http://cxc.harvard.edu/chart
2Note that the Chandra point response function is azimuthally asymmetric,
see http://cxc.harvard.edu/ccw.02