Article

HST/COS Observations of the Ly alpha Forest toward the BL Lac Object 1ES1553+113

05/2010;
Source: arXiv
ABSTRACT
We present new far-ultraviolet spectra from the Cosmic Origins Spectrograph (HST/COS) of the BL Lac object 1ES1553+113 covering the wavelength range 1135-1795 A. The data show a smooth continuum with a wealth of narrow absorption features arising in the ISM and IGM. These features include 41 Lya absorbers at 0<z<0.43, fourteen of which are detected in multiple Lyman lines and six in one or more metal lines. We analyze a metal-rich triplet of Lya absorbers at z=0.188 in which OVI, NV, and CIII absorption is detected. Silicon ions (SiIII/IV) are not detected to fairly strong upper limits, and we use the measured SiIII/CIII upper limit to derive an abundance limit [C/Si]>0.6 for the strongest component of the absorber complex. Galaxy redshift surveys show a number of massive galaxies at approximately the same redshift as this absorption complex, suggesting that it arises in a large-scale galaxy filament. As one of the brightest extragalactic X-ray and gamma-ray sources, 1ES1553+113 is of great interest to the high-energy astrophysics community. With no intrinsic emission or absorption features, 1ES1553+113 has no direct redshift determination. We use intervening Lya absorbers to place a direct limit on the redshift: z_em>0.395 based on a confirmed Lya+OVI absorber and z_em>0.433 based on a single-line detection of Lya. COS/FUV data are only sensitive to Lya absorbers at z<0.47, but we present statistical arguments that z_em<0.58 based on the non-detection of any Lyb absorbers at z>0.4. Comment: ApJ submitted: 11 pages, 7 figures

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arXiv:1005.2191v1 [astro-ph.CO] 12 May 2010
ApJ submitted May 11, 2010
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HUBBLE/COS OBSERVATIONS OF THE LY α FORE ST TOWARD THE BL LAC OBJECT 1ES 1553+113
Charles W. Danforth, Brian A. Keeney, John T. Stocke, J. Michael Shull, & Yangsen Yao
CASA, Department of Astrophysical & Planetary Sciences, University of Colorado, 389-UCB, Boulder, CO 80309;
danforth@casa.colorado.edu
ApJ submitted May 11, 2010
ABSTRACT
We present new moderate-reso lution, far-ultraviolet spectra from the Hubble Space Tele-
scope/Cosmic Origins Spectrograph (HST/COS) of the BL Lac object 1ES 1553+113 covering the
wavelength range 1135
˚
A < λ < 1795
˚
A. The data show a smooth continuum with a wealth of
narrow (b < 100 km s
1
) absor ption features arising in the interstellar medium (ISM) and intergalac-
tic medium (IGM). These features include 41 Lyα absorbers at 0 < z
abs
< 0.43 , fourteen of which
are detected in multiple Ly man lines and six of which show absorption in one or more metal lines.
We analyze a metal-rich triplet (∆cz 1000 km s
1
) of Lyα a bsorbers at z
abs
0.188 in which
O VI, N V, and C III absorption is detected. Silicon ions (Si III, Si IV) are not detected to fairly
strong upper limits, and we use the measured Si III/C III upp e r limit to derive an abundance limit
(C/Si) 4 (C/Si)
for the strongest component of the absorber complex. Galaxy re dshift surveys
show a number of massive galaxies at approximately the same redshift as this absorption complex,
suggesting that it arises in a large-scale galaxy filament. As one of the brightest extragalactic X-ray
and γ-ray sources, 1ES 1553+113 is of great interest to the high-energy astrophysics community. With
no intrinsic emission or abs orption features, 1ES 1553+113 has no direct redshift determination. We
use intervening Lyα absorbers to place a direct limit on the r edshift: z
em
> 0.395 based on a confirmed
Lyα+O VI absorber and z
em
> 0.433 based on a single-line detec tion of Lyα. The curr ent COS data
are only sensitive to Lyα absorbers at z < 0.47, but we present statistical a rguments that z
em
. 0.58
(at a 1 σ confidence limit) based on the non- de tec tion of any Lyβ absorbers at z > 0.4.
Subject headings: BL Lacertae objects: individual: 1ES 1553+113, galaxies: active, intergalactic
medium, quasar s: absorption line s, ultraviolet: general
1. INTRODUCTION
The current interpretation of BL La c objects (Ghis-
ellini, Maraschi, & Treves 1985) is that they are ac-
tive galactic nuclei (AGN) with a strongly relativistic jet
pointed toward our line of sight. As such, any line emis-
sion or accretion disk features seen in most other types
of AGN could be masked by the bright jet if present
in BL Lac objects. Their spectr a usually show a fea-
tureless power- law continuum extending from radio to
X-ray wavelengths. This spectral characteristic ma kes
BL Lac objects ideal for observing inter vening absorp-
tion features ar ising in the interstellar medium (ISM)
and intergalactic medium (IGM). Since their continuum
is easily defined, they make excellent ta rgets for studying
weak metal-line sy stems and low-contrast, highly ther-
mally bro adened H I absorbers (e.g., Richter et al. 2004;
Lehner et al. 2007; Danforth, Stocke & Shull 2010).
The BL Lac object 1ES 1553+113 shows the charac-
teristic featurele ss power-law spectrum and is one of the
brightest known sources of extragalac tic high-energy ra-
diation from X-rays up to VHE (TeV) photons (Costa-
mante & Ghisellini 2002). However, the featurele ss
spectrum makes it difficult to determine the redshift of
the object and hence its luminosity. Indirect methods
have given a wide range o f limits for the redshift of
BASED O N OBSERVATIONS M ADE WITH THE NASA/ESA
HUBBLE SPACE TELESCOPE, OBTAINED FROM THE DATA
ARCHIVE AT THE SPACE TELESCOPE SCIENCE INSTI-
TUTE. STSCI IS OPERATED BY THE ASSOCIATION OF UNI-
VERSITIES FOR RESEARCH IN ASTRONOMY, INC. UNDER
NASA CONTRACT NAS 5-26555.
1ES 1553+113; the nondetection of a host galaxy gave
limits from z
em
> 0.09 to z
em
> 0.78 (Hutchings & Neff
1992; Scarpa et al. 2000; Urry et al. 2000; Carangelo et al.
2003; Sbarufatti et al. 2006; Tr e ves, Falomo, & Uslenghi
2007). The shape of the γ-ray spectrum observed by
the Fermi Observatory and ground-based VHE detectors
(HESS, MAGIC) constrains the redshift to values from
z
em
< 0.4 to z
em
< 0.8 (Ahar onian et al. 2006; Albert
et al. 2007; Mazin & Goebel 2007; Abdo et al. 2010)
based on assumptions about the intrinsic spec tral energy
distribution (SED) and pair-productio n interactions with
the cosmic infrar e d background. The only direct redshift
determination (z
em
= 0.36; Miller & Green 1983) was
based on a spurious featur e in low-resolution UV spectra
from the International Ultraviolet Explorer (IUE). The
detection was later retracted (Falo mo & Treves 19 90),
but the erroneous reds hift value lives on.
1ES 1553+113 is of interest as a bright background
continuum source for de tec ting intergalactic absorption
along the sight line. Bright X-ray sources a re especially
valuable for potentially detecting the long-predicted
O VII and O VIII tracers (Bregman 2007) of intergalactic
gas at T = 10
6
10
7
K. Even for a bright X-ray sour ce,
the r e quired integration times would be very long. How-
ever, a sufficiently long IGM pathlength provided by a
bright high-z target would make the required o bserving
time investment more attractive.
In this paper, we present the first medium-resolution
far-UV spectroscopic observations of 1ES 1553 +113 in-
cluding Hubble Space Telescope/Cosmic Origins Spectro-
graph (HST/COS Green et al. 2010; Osterman et al.
Page 1
2 Danforth et al.
Fig. 1.— Overview of the COS spectrum of 1ES 1553+113. The full COS/G130M+G160M dataset is shown, smoothed by 35 pixels ( 5
resolution elements). Error is shown in gray.
2010) obs ervations (λ = 1135 1795
˚
A) as well as
archival data at 905 1187
˚
A fr om the Far Ultraviolet
Spectroscopic Explorer (FUSE; Moos et al. 2000; Sah-
now et al. 2000). We confirm the featureless power-
law nature of the spectrum over this wavelength range.
Absorption is seen in 42 intervening systems including
41 Lyα absorbers and six metal-line systems. The fre-
quency of IGM absorbers is consistent with larger sur-
veys using FUSE and HST/STIS data (Danforth & Shull
2005, 2008; hereafter DS08), and the systems are spread
across the entire redshift range covered by the combined
COS/FUSE dataset (z . 0.47).
The observations and data re duction techniques are
discussed in §2, and we present a preliminary catalog of
absorption lines in §3. Our conclusions are prese nted in
§4.
2. OBSERVATIONS AND DATA ANALYSIS
Far-UV observations of 1ES 1553+113 were carried out
2009 September 22 by HST/COS as part of the COS
Guaranteed Time Obse rvations (PID 11528, PI Green).
Five exposures were made in each of the G130M (1135 <
λ < 1480
˚
A) and G160M (1400 < λ < 1795
˚
A) medium-
resolution gratings (R 18, 000) totalling 3.1 and 3.8
ksec, respectively. Four central wavelength settings at
each grating dithered known instrumental features alo ng
the spectrum and provided continuous spec tral coverage
over 1135 < λ < 1795
˚
A (see Green et al. 2010; Oster man
et al. 2010). After retrieval from the archive, all ten
exposures were reduced loc ally using CalCOS v2.11f.
Flat-fielding, alignment, and coaddition of the pro-
cessed exposures were carried out using IDL routines de-
veloped by the COS GTO team specifically for COS FUV
data
1
. Firs t, the data were corrected for the most egre-
gious instrumental features. While attempts at a true
“flat-fielding” of COS data show promise, the technique
is not yet robust enoug h to improve data of moder ate
S/N. However, we are able to correct the narrow 15%-
opaque features arising from ion repellor grid wires in
1
See http://casa.colorado.edu/danforth/costools.html
for our coaddition and flat-fielding algorithm and additional dis-
cussion.
the detector. A one-dimensional map of gr id-wire opac-
ity for each detector was shifted from detector coordi-
nates into wavelength space and divided from the flux
and error vectors. Exposure time in the location o f grid
wires was decreased to 70%, giving these pixels less
weight in the final coaddition. We also modify the error
and local exposure time at the edges of the detector seg-
ments to de-weight flux contributions from these regions.
With four different central wavelength settings per grat-
ing, any residual instrumental artifacts from grid-wire
shadows and detector segment boundaries should have
negligible effect on the final spectrum.
The exposures are aligned with each other and inter po-
lated onto a common wavelength scale. One ex posure in
each grating/detector was picked as a wavelength refer-
ence, and the remaining exposures were cross-correlated
with it. The wavelength region of cr oss-correlation for
each case was picked to include a strong ISM absorption
feature, and shifts were typically on the order of a reso-
lution element ( 0.07
˚
A) or less. The COS wavelength
solution has no t yet been rigoro usly characterized, and
we see a systematic shift between strong I SM lines and
their expected LSR velocities. The shift is appr oximately
constant across the COS wavelength range, so we apply
a uniform +0.17
˚
A shift to the wavelength vectors ( 40
km s
1
at 1300
˚
A) to bring ISM line c entroids to the
expected v
LSR
0 seen in many ISM absorbers.
Next, the aligned exposures were interpolated onto a
uniform wavelength grid and coadded. The flux at each
position was taken to be the expo sure-weighted mean of
flux in each exposure. Since exposure time was reduce d
in certain wavelength locations, as noted above, pixels
near detector edges and where grid-wire shadows were
removed received les s weight than those in less suspect
locations. The combined data show S/N 20 per 7-
pixel ( 0.07
˚
A) resolution element a nd are sufficient to
detect narrow absorption features down to W
λ
15 m
˚
A
at 4σ sig nificance. Figure 1 shows the entire combined
COS/G130M a nd COS/G160M spectra. Figure 2 shows
a more detailed view of the spectrum with prominent
lines marked.
In additio n to the COS data, we utilize 45 ksec of Far
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