Arecibo HI Absorption Measurements of Pulsars and the Electron Density at Intermediate Longitudes in the First Galactic Quadrant

Abstract

We have used the Arecibo telescope to measure the HI absorption spectra of eight pulsars. We show how kinematic distance measurements depend upon the values of the galactic constants R_o and Theta_o, and we select our preferred current values from the literature. We then derive kinematic distances for the low-latitude pulsars in our sample and electron densities along their lines of sight. We combine these measurements with all others in the inner galactic plane visible from Arecibo to study the electron density in this region. The electron density in the interarm range 48 degrees < l < 70 degrees is [0.017 (-0.007,+0.012) (68% c.l.)] cm^(-3). This is 0.75 (-0.22,+0.49) (68% c.l.) of the value calculated by the Cordes & Lazio (2002) galactic electron density model. The model agrees more closely with electron density measurements toward Arecibo pulsars lying closer to the galactic center, at 30 degrees<l<48 degrees. Our analysis leads to the best current estimate of the distance of the relativistic binary pulsar B1913+16: d=(9.0 +/- 3) kpc. We use the high-latitude pulsars to search for small-scale structure in the interstellar hydrogen observed in absorption over multiple epochs. PSR B0301+19 exhibited significant changes in its absorption spectrum over 22 yr, indicating HI structure on a ~500 AU scale. Comment: Accepted by Astrophysical Journal September 2007

Full text

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arXiv:0709.3854v1 [astro-ph] 24 Sep 2007
Accepted by ApJ 2007 September
Arecibo HI Absorption Measurements of Pulsars and the Electron
Density at Intermediate Longitudes in the First Galactic
Quadrant
J. M. Weisberg1, S. Stanimirovi´ c2, K. Xilouris3, A. Hedden1,3, A. de la Fuente1, S. B.
Anderson4, and F. A. Jenet4,5
ABSTRACT
We have used the Arecibo telescope to measure the HI absorption spectra of
eight pulsars. We show how kinematic distance measurements depend upon the
values of the galactic constants Roand Θo, and we select our preferred current val-
ues from the literature. We then derive kinematic distances for the low-latitude
pulsars in our sample and electron densities along their lines of sight. We com-
bine these measurements with all others in the inner galactic plane visible from
Arecibo to study the electron density in this region. The electron density in the
interarm range 48◦< l < 70◦is [0.017 (−0.007,+0.012) (68% c.l.)] cm−3. This
is 0.75 (−0.22,+0.49) (68% c.l.) of the value calculated by the Cordes & Lazio
(2002) galactic electron density model. The model agrees more closely with elec-
tron density measurements toward Arecibo pulsars lying closer to the galactic
center, at 30◦< l < 48◦. Our analysis leads to the best current estimate of the
distance of the relativistic binary pulsar B1913+16: d = (9.0 ± 3) kpc.
We use the high-latitude pulsars to search for small-scale structure in the in-
terstellar hydrogen observed in absorption over multiple epochs. PSR B0301+19
exhibited significant changes in its absorption spectrum over 22 yr, indicating HI
structure on a ∼ 500 AU scale.
Subject headings: Pulsars: distances — Galaxy: fundamental parameters — ISM:
structure — ISM: abundances — ISM: clouds — Radio lines: ISM
1Department of Physics and Astronomy, Carleton College, Northfield, MN 55057
2Department of Astronomy, University of California, Berkeley, CA 94720
3Department of Astronomy, University of Arizona, Tucson, AZ 85721
4Department of Astronomy, California Institute of Technology, MS 105-24, Pasadena CA 91125
5Center for Gravitational Wave Astronomy, University of Texas at Brownsville, TX 78520

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1.Introduction
Neutral hydrogen (HI) absorption measurements of pulsar signals at λ = 21 cm are
important probes of various properties of the interstellar medium such as the small-scale
structure of of cold HI (Dickey et al. 1981; Stanimirovi´ c et al. 2003b) and calibrators of the
pulsar distance scale and electron density models at large galactic distances (Weisberg et al.
1979, 1987, 1995; Frail & Weisberg 1990). These measurements are complementary to inter-
ferometrically determined parallaxes, which can be utilized on nearer sources (Brisken et al.
2002; Chatterjee et al. 2004). We report new Arecibo HI absorption observations of eight
pulsars, and we use these measurements to study the electron density of the interstellar
medium and the small-scale structure of neutral hydrogen clouds.
Sensitivity limitations indicate that few if any additional pulsar HI absorption measure-
ments of electron density in the galactic plane at the intermediate first-quadrant galactic
longitudes accessible to the Arecibo telescope will be made in the next decade or so. Hence
it is an appropriate time to combine the new results with all previous pulsar HI measurements
at these longitudes to globally assess the density in the galactic plane in this region. We will
use this information to estimate the distance to the relativistic binary pulsar B1913+16. In
addition, HI absorption measurements on some of our high-latitude pulsars were originally
measured over twenty years ago, during which interval the pulsars have moved many AU
through the interstellar medium. Hence comparison of the old and new absorption spectra
yields information on the small scale structure of the absorbing neutral hydrogen.
The plan of the paper is as follows: The pulsar HI absorption observational technique is
sketched in the next section (§2). We present HI absorption spectra and kinematic distance
results for low-latitude pulsars in §3. In §4, we use all measured pulsar distances in the
inner galactic plane visible from Arecibo Observatory to analyze the electron density in that
region. In §4.1, we review the latest work on the galactocentric distance of the Sun and the
orbital velocity of the Local Standard of Rest in order to select an optimum model to use in
refining pulsar kinematic distances and galactic electron densities. Then in §4.2, we apply
the results of §4 and §4.1 to determine the distance of the relativistic binary pulsar PSR
B1913+16. We provide absorption spectra of high-latitude pulsars in §5, along with analyses
of time-variability of absorption in those cases where earlier epoch data are available. Finally,
we discuss our conclusions in §6.

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2. Observations
All observations were made with the 305-m Arecibo telescope from 1998 to 2000. The ra-
dio frequency signals near 1420 MHz were mixed to baseband, sampled, and recorded with the
Caltech Baseband Recorder (CBR, Jenet et al. 2001), a 10 MHz bandwidth, fast-sampling
receiver backend. Every 100 ns, the CBR sampled complex voltages with four-level digitiza-
tion from the two orthogonally polarized feed channels, and recorded the samples to tape for
later processing. (For additional details of the observing techniques and equipment, see Sta-
nimirovi´ c et al [2003a,b]). The data were then corrected for quantization (Jenet & Anderson
1998), Fourier transformed and folded modulo the apparent pulsar period with the Super-
computers of Caltech’s Center for Advanced Computing Research, resulting in data cubes
consisting of intensity as a function of pulsar rotational phase (128 phase bins) and radio
frequency (2048 frequency channels, each having 1 km/s width). Subsequent processing at
Carleton College collapsed the data cubes into two spectra: the pulsar-on spectrum is a sum
of the spectra gathered during the pulsar pulse weighted by Ipsr(φ)2, while the sum of those
gathered in the interval between pulses is called the pulsar-off spectrum. Here Ipsr(φ) is the
broadband pulsar intensity in a given pulse phase bin φ.
Two final spectra are formed and displayed for each pulsar. The pulsar absorption
spectrum, which represents the spectrum of the pulsar alone less any absorption caused by
intervening neutral hydrogen, is the normalized difference of the pulsar-on and -off spectra.
In order to maximize sensitivity, multiple integrations are summed with a weight depend-
ing on the square of the pulsar signal strength Ipsr(t) during each integration. In some
cases, additional sensitivity was achieved by Hanning smoothing the final absorption spec-
trum, yielding a resolution of 2 km/s. (Any absorption spectrum that has been Hanning
smoothed is labelled as such when displayed below.) The HI emission spectrum is the time-
integrated, pulsar-off spectrum, calibrated in brightness temperature units by matching its
peak with the Leiden/Argentine/Bonn HI Survey (Hartmann & Burton 1997; Arnal et al.
2000; Bajaja et al. 2005; Kalberla et al. 2005). All spectra were frequency switched and low-
order polynomials were fitted to and removed from them in some cases in order to flatten
intrinsic or scintillation-induced bandpass ripples.
The basic observing parameters are tabulated for low- and high-latitude pulsars in the
early columns of Tables 1 and 3, respectively. The quantities Tsys,off−lineand στ,off−line, the
system temperature and measured 1-σ noise in optical depth away from the HI line, are
both given. The former value is the sum of an estimated 40 K receiver contribution, plus a
sky background determined by extrapolating the 408 MHz sky temperature (Haslam et al.
1982) at the pulsar position to 1420 MHz with a spectral index of -2.6. The HI emission line
itself contributes significantly to the system temperature (and hence to the noise) at those

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velocities where it is present. To determine the expected optical depth noise at any velocity
v, one may use στ(v) = στ,off−line× [THI(v) + Tsys,off−line]/Tsys,off−line, where THI(v) is the
measured brightness temperature of HI at that velocity.
3. Kinematic Distance Analyses and Results on Low-Latitude Pulsars
A low-latitude pulsar HI absorption spectrum can be used to set limits on the pulsar’s
distance, using the “kinematic” technique. A pulsar is farther than an HI cloud that absorbs
its signal, and closer than one that does not. The latter “no absorption” criterion is impossi-
ble to ascertain in real spectra because of the inevitable presence of noise which could mask
weak absorption. However it was shown by Weisberg et al. (1979) that emission features
with Tb≥ 35 K almost always exhibit significant absorption of radiation from background
objects. Therefore subsequent investigators have assigned an upper distance limit only at the
velocity where one finds both no significant absorption, and an emission feature of Tb≥ 35
K. We use a flat Fich et al. (1989) galactic rotation model and the IAU galactic constants of
Ro= 8.5 kpc and Θo= 220 km/s (Kerr & Lynden-Bell 1986) to convert the radial velocities
to distance. The resulting model radial velocities are shown in a panel under each absorp-
tion/emission spectrum pair. We also add and subtract velocities of 7 km/s to our nominal
velocities, in order to derive estimates of the uncertainties in distance limits due to stream-
ing and random gas motions in the Galaxy. These procedures are identical to those used
in the critical evaluation of all such measurements then extant by Frail & Weisberg (1990),
and by all subsequent pulsar HI absorption experimenters. Hence our results are directly
intercomparable with these earlier ones. Our derived distance limits are discussed below for
each pulsar, and are summarized in Table 1. See §4.1 for a discussion of modifications to
our derived distances for different values of the Galactic constants.
3.1.PSR J1909+0254=B1907+02 (l=37.◦6; b=-2.◦7), Fig. 1
The farthest observed absorption feature is at v = 60 km/s, well before the tangent
point. Hence the pulsar lies beyond a lower distance limit of 3.8 ± 0.5 kpc. Unfortunately
the HI emission has Tb? 10 K at all velocities where one might test for an upper distance
limit via lack of absorption, because the b = 2.◦7 line of sight rapidly leaves the hydrogen
layer. Since significant absorption could not in any case be guaranteed at the velocity of
such weak emission (Weisberg et al. 1979), no upper distance limit can be set.

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3.2.PSR J1922+2110=B1920+21 (l=55.◦3; b=2.◦9), Fig. 2
Weisberg et al. (1987) observed this pulsar. Their HI spectrum was contaminated by
interstellar scintillation and was rather noisy, prompting us to reobserve it with greater
sensitivity. The highest velocity absorption that Weisberg et al. (1987) could confidently
detect was at v = 26 km/s, leading to their limit of (1.9 ± 0.7 kpc ) ? d.
observations clearly exhibit absorption at v = 41 km/s, near the tangent point. Hence we
revise the lower distance limit significantly upwards: (4.8±1.8 kpc ) ? d. The Weisberg et al.
(1987) spectrum also showed an absorption feature at this velocity but it was not sufficiently
above the noise to serve as a reliable kinematic distance indicator. The dip in the current
absorption spectrum at v = −48 km/s is probably noise and will not be used to set a distance
limit. (It appears much more prominently in one of the two circular polarization channels
than in the other, and is not visible in the Weisberg et al. (1987) absorption spectrum.
Furthermore, the nearby source G55.6+2.3=B1923+210 does not exhibit reliable far side
absorption until v ? −60 km/s [Dickey et al 1983; Colgan et al 1988].) Conversely, the lack
of absorption in the Tb= 41 K emission feature at v = −65 km/s sets the upper distance
limit of d ? (16.2 ± 1.0) pc.
Our new
3.3.PSR J1926+1648=B1924+16 (l=51.◦9; b=0.◦1), Fig. 3
While the Fich et al. model predicts a tangent point velocity of 48 km/s in this direction,
we detect significant emission and absorption well beyond this velocity, with the last major
feature centered near v = 61 km/s. These features are due to the Sagittarius arm (Burton
1970). Garwood & Dickey (1989) and Colgan et al. (1988) also observe HI absorption out
to these velocities in the nearby sources G50.625-0.031 and G51.4-0.0, respectively. We
choose the tangent point as our lower distance limit, so (5.2 ± 1.8) kpc ? d. The lack of
absorption in the Tb∼ 42 K emission feature at v ∼ −47 km/s yields the upper distance
limit: d ? 14.9 ± 0.8 kpc
4.Analysis of the Electron Density in the Inner Galactic Plane Visible from
Arecibo
The currently reported HI absorption distance measurements are probably among the
last to be determined at Arecibo in the foreseeable future, as all known pulsars having suf-
ficient flux density to achieve adequate S/N in a reasonable time have now been measured
as well as is possible with this procedure. Unfortunately galactic HI emission is now the