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IS RX J1856.5#3754 A QUARK STAR?
Jeremy J. Drake, 1 Herman L. Marshall, 2 Stefan Dreizler, 3 Peter E. Freeman, 1 Antonella Fruscione, 1
Michael Juda, 1 Vinay Kashyap, 1 Fabrizio Nicastro, 1 Deron O. Pease, 1
Bradford J. Wargelin, 1 and Klaus Werner 3
Received 2001 December 15; accepted 2002 February 21
ABSTRACT
Deep Chandra Low Energy Transmission Grating and High Resolution Camera spectroscopic observa­
tions of the isolated neutron star candidate RX J1856.5#3754 have been analyzed to search for metallic and
resonance cyclotron spectral features and for pulsation behavior. As found from earlier observations, the
X­ray spectrum is well represented by an #60 eV (7 # 10 5 K) blackbody. No unequivocal evidence of spectral
line or edge features has been found, arguing against metal­dominated models. The data contain no evidence
for pulsation, and we place a 99% confidence upper limit of 2.7% on the unaccelerated pulse fraction over a
wide frequency range from 10 #4 to 100 Hz. We argue that the derived interstellar medium neutral hydrogen
column density of 8 # 10 19 cm #2 # NH # 1:1 # 10 20 cm #2 favors the larger distance from two recent Hubble
Space Telescope parallax analyses, placing RX J1856.5#3754 at #140 pc instead of #60 pc and in the out­
skirts of the R CrA dark molecular cloud. That such a comparatively rare region of high interstellar matter
(ISM) density is precisely where an isolated neutron star reheated by accretion of ISM would be expected is
either entirely coincidental or current theoretical arguments excluding this scenario for RX J1856.5#3754
are premature. Taken at face value, the combined observational evidence---a lack of spectral and temporal
features and an implied radius of R1 ¼ 3:8--8.2 km that is too small for current neutron star models---points
to a more compact object, such as allowed for quark matter equations of state.
Subject headings: stars: individual (RX J1856.5#3754) --- stars: neutron --- X­rays: stars
1. INTRODUCTION
The structure and evolution of neutron stars depends on
the properties of matter at nuclear and supranuclear den­
sities. Such conditions are not achievable in terrestrial labo­
ratories and the theoretical description of such superdense
matter remains uncertain. It has therefore been hoped that
neutron stars, in particular those that are isolated and not
complicated by strong accretion or magnetospheric signa­
tures, might provide some empirical insights: observations
of their masses, radii, and cooling characteristics could, in
principle, provide useful constraints for the equation of
state (EOS) of dense matter (e.g., Lattimer & Prakash 2001
and references therein).
In a relatively brief period of 10 6 --10 7 yr, a hot, isolated
neutron star (INS) born in a supernova explosion can cool,
cease pulsar activity, and become essentially inactive (see,
e.g., the review of Treves et al. 2000). Of the estimated 10 8 --
10 9 isolated neutron stars thought to inhabit the Galaxy,
only a tiny fraction are therefore expected to be su#ciently
young to remain hot and visible in X­rays. One possible
mechanism capable of sustaining thermal X­ray emission in
an older INS is accretion of material from the interstellar
matter (ISM). To date, only a handful of these older INS
candidates have been found.
The soft X­ray source RX J1856.5#3754 discovered by
Walter, Wolk, & Neuha ˜user (1996) is the brightest and
probably the closest (Kaplan, van Kerkwijk, & Anderson
2002) of the INS candidates. It was identified with a very
faint (V ' 25:6) optical counterpart by Walter & Matthews
(1997), was found to have an optical flux about a factor of
2--3 higher than that predicted by the Rayleigh­Jeans tail of
the #55 eV blackbody spectrum that represents the low res­
olution ROSAT Position Sensitive Proportional Counter
(PSPC) spectrum (e.g., Walter et al. 1996; Campana, Mere­
ghetti, & Sidoli 1997; Pons et al. 2002), and lies in the line of
sight toward the dark molecular cloud RCrA. However, the
exact nature of RX J1856.5#3754 remains unknown---
whether it is a fairly young, cooling INS, perhaps unde­
tected as a pulsar because of unfortunate beam alignment,
or an older object reheated by ISM accretion.
Very Large Telescope (VLT) observations have recently
revealed an H# nebula around RX J1856.5#3754 and a
blackbody spectrum through the UV­optical range (van
Kerkwijk & Kulkarni 2001a, 2001b). Hubble Space Tele­
scope (HST) astrometry was used by Walter (2001) and
Kaplan et al. (2002) to estimate a parallax and proper
motion, but with conflicting results (16:5 # 2:3 mas vs. 7 # 2
mas). Walter (2001) argued that the proper motion points to
the Sco­Cen OB association and an age of #10 6 yr. How­
ever, Pons et al. (2002) failed to detect the expected pulsa­
tion signature in ROSAT and ASCA data. Modeling X­ray,
EUV, UV, and optical spectra using the Walter (2001) par­
allax and assuming a metal­dominated atmosphere resulted
in stellar radii too small for the current EOS and smaller
than the Schwarzschild radius for a canonical 1.4 M # star,
leading them to conclude that the surface temperature dis­
tribution could be inhomogeneous. In contrast, the same at­
mospheric analysis using the larger distance of Kaplan et al.
(2002) would yield a radius consistent with current theory.
If the atmosphere is indeed metal­dominated, then line and
edge features should be visible in high­resolution X­ray
spectra.
1 Smithsonian Astrophysical Observatory, MS 3, 60 Garden Street,
Cambridge, MA 02138.
2 Massachusetts Institute of Technology Center for Space Research, 77
Massachusetts Avenue, Cambridge, MA 02139.
3 Institut fu ˜ r Astronomie und Astrophysik, Universita ˜t Tu ˜ bingen, Sand
1, 72076 Tu ˜ bingen, Germany.
The Astrophysical Journal, 572:996--1001, 2002 June 20
# 2002. The American Astronomical Society. All rights reserved. Printed in U.S.A.
996

RX J1856.5#3754 was observed in 2000 March for 55 ks
by the Chandra X­Ray Observatory Low Energy Transmis­
sion Grating (LETG) and the High Resolution Camera
spectroscopic (HRC­S) microchannel plate detector array
to look for spectral features and pulsar activity. Within rela­
tively large statistical uncertainties, Burwitz et al. (2001)
failed to detect either significant departures from a black­
body spectrum or pulsations in these data. The prospect
of important scientific gains from a longer observation
prompted, at the request of di#erent researchers, the under­
taking of a very recent set of Chandra observations of
RX J1856.5#3754 under the director's discretionary time
using LETG+HRC­S. A period search using these data by
Ransom, Gaensler, & Slane (2002) has already placed an
upper limit on a pulse fraction of 4.5%. This paper presents
spectroscopic and independent timing analyses of these
data.
2. OBSERVATIONS AND DATA REDUCTION
The observations analyzed in this paper are summarized
in Table 1. Pipeline­processed (CXC software version 6.3.1)
photon event lists were reduced and analysed using the
CIAO software package version 2.2 and independently
using custom IDL 4 software. Processing included extra
pulse­height filtering to reduce background (B. J. Wargelin
et al. 2002, in preparation) and barycentric correction of
event times. Dispersed photon events were extracted using
the now standard `` bow tie '' window, and a small circular
region (31 pixel radius 5 ) was used to extract the zeroth­order
events. We note that an extraneous bright feature (likely a
ghost image of a bright o#­axis source) appeared on the +1
order outer HRC­S plate, which introduces spurious fea­
tures into the extracted spectrum and background near
110 A š . We applied corrections to extracted event times to
ameliorate an HRC electronics problem that assigns time
tags for each event to the event that immediately follows. If
every event were telemetered to the ground, correct times
could be easily reassigned, but because of the high HRC
background rate and a telemetry limit of 184 events s #1 ,
only valid events, which make up approximately 45% of the
total events in these observations, are recorded in data
received on the ground. We have reassigned the time of
every valid event to the preceding valid event during ground
processing; the event times will then be correct about
N valid =N total of the time. One can place an upper limit of #t
on the time errors associated with this process, however,
simply by excluding events with a time shift of more than #t;
the average timing error of such events will, of course, be
less than #t. However, if #t is too small, too many events get
excluded to perform sensitive timing studies. We have
adopted a #t of 2 ms, retaining 20% of the original counts in
the corrected data set.
3. COUNT RATES AND TIMING ANALYSIS
Count rates were derived for each observation segment
from event lists filtered to exclude times of high background
and telemetry saturation, when the telemetered valid event
rate exceeded 184 count s #1 , and to exclude high pulse­
height events that arise entirely from background. These
rates corresponding to zeroth order only are listed in
Table 1.
The count rates for the di#erent observation segments are
statistically consistent, though the 2000 March observation
(observation identification [ObsID] 113) rate of 0:2195 #
0:0020 count s #1 lies 1.9% and 1.9 # above the rate for the
combined 2001 October series rate of 0:2155 # 0:0008. Such
deviations will be obtained by chance about 3% of the time
when the count rates do not vary; it is thus more likely
attributable to quantum e#ciency (QE) variations in the
detector on small scales. Indeed, Pons et al. (2002) note two
ROSAT HRI observations obtained 3 years apart that are
consistent to 1%. The fluxes corresponding to HRI and
PSPC count rates are higher than that obtained from our
Chandra data by 20% and 30%, respectively. As remarked
by Burwitz et al. (2001), these di#erences are likely attribut­
able to absolute calibration uncertainties. We have also
examined the ASCA Solid State Imaging Spectrometer
(SIS) observation described by Pons et al. (2002) and find
fluxes 30%--40% lower than those obtained by Chandra for
the 20--30 A š range, but in agreement with Chandra short­
ward of 20 A š . The Extreme Ultraviolet Explorer Deep Sur­
vey count rate of Pons et al. (2002) is also consistent with
the Chandra observation within allowed uncertainties.
Our search for pulsations used three di#erent techniques,
none of which found any evidence for significant variability.
In contrast to Ransom et al. (2002), we did not include a
deceleration term; this will be discussed below.
4 Interactive Data Language, Research Systems Inc.
5 Our 31 pixel radius extraction region for the zeroth­order source corre­
sponds to an encircled energy fraction of about 92% # 3%, which combines
with an average deadtime of 0.59% to yield a correction factor for a
2# sr aperture of 1.09.
TABLE 1
Summary of Chandra LETG+HRC­S Observations
UT Net Events a
ObsID Start End
Exposure
(s) Zeroth Zeroth+First
Zeroth Rate b
(Hz)
113 ....... 2000 Mar 10 07:55:12 2000 Mar 10 23:37:24 55121 12202 . . . 0:2195 # 0:0020
3382 ..... 2001 Oct 8 08:18:49 2001 Oct 9 03:01:50 101172 20949 86516 0:2166 # 0:0016
3380 ..... 2001 Oct 10 05:06:28 2001 Oct 12 04:00:48 166325 35097 135230 0:2154 # 0:0013
3381 ..... 2001 Oct 12 19:19:26 2001 Oct 14 09:14:28 169956 36011 141349 0:2157 # 0:0013
3399 ..... 2001 Oct 15 11:47:06 2001 Oct 15 14:42:59 9282 1962 7136 0:2126 # 0:0051
a The net event numbers include background events; while including the first­order events increases the net source events, the
number of background events also increases.
b Rates were derived from zeroth­order data when valid event rates did not exceed 184 counts s #1 ; this selection criterion
yielded 417,786 s of the 501,856 s total exposure time.
IS RX J1856.5#3754 A QUARK STAR? 997

We applied the Bayesian method of Gregory & Loredo
(1992) to both the time­corrected and uncorrected zeroth­
order event lists. The method tests for variability by com­
paring the fits of periodic stepwise models to the data with
the fit of a constant model. The odds favoring variability
(based on eq. [5.28] of Gregory & Loredo 1992, with a maxi­
mum number of steps mmax ¼ 12 and limiting angular fre­
quencies ! lo and ! hi equal to 10 #4 and 10 3 , respectively)
were found to be 1:45 # 10 #4 for the whole data set and
3:75 # 10 #3 for the data filtered on #t ¼ 2 ms, both to be
compared with an odds value of 10 2 needed for a confident
pulsation or variability detection.
Our second method employed a fast Fourier transform
(FFT) analysis applied to the combined zeroth and first­
order events, followed by a likelihood ratio test (LRT) to
determine limits on the pulse fraction. The FFT power dis­
tribution was consistent with shot noise. For the LRT of
a given period, P, the data were binned into N phase bins,
giving n counts in each. The source model was
y ¼ A × f cosÏ# i × # 0 ÷, where # i is the phase of bin i and # 0
is the phase of the pulse and f =A is the pulse fraction. The
likelihood equations were then solved for A and the process
applied to greater than 500 frequencies where the FFT
power exceeded a critical level in the frequency range of
0.001--50 Hz. By also including the dispersed events, we
improved the signal­to­noise ratio of the result by a factor
of 1.47 compared with using the zeroth order alone, and we
could obtain a pulse fraction limit lower than the value of
4.5% obtained by Ransom et al. (2002) in their unacceler­
ated search. A pulse fraction upper limit (99% confidence)
of 2.7% was derived applying our likelihood ratio method
using all data, including the dispersed events with
1 G < # < 70 A š . Taking only the events limited by
#t < 2 ms, the pulse fraction limit is 10%.
Third, we computed the Lomb­Scargle periodogram for
both the time­corrected and uncorrected photon arrival
time di#erences in the frequency range of 0.01--1 Hz for the
events in ObsID 3380, 3381, 3382, and 3399. Again, no sig­
nificant peaks were present.
The assumption of a negligible deceleration term in our
period search restricts the range of periods and dipole mag­
netic field strengths for which our search is valid. The coher­
ence limit for phase slippage by 10% over the duration of the
2001 October observations implies that our result is valid
for a magnetic field upper limit of B < 2:3 # 10 13 P 3=2 G
for period P s (e.g., Shapiro & Teukolsky 1983). As noted
by Ransom et al. (2002), this range would exclude very
young and energetic neutron stars, such as the Crab and
Vela pulsars, though most of these younger objects are
also conspicuously strong radio pulsars. All anomalous
X­ray pulsars would lie within our sensitivity limit range.
RX J1856.5#3754 is also most unlikely to be an extremely
young object based on its modest temperature and lumi­
nosity, which are consistent with an object of age #10 5 yr
on canonical cooling curves (e.g., Tsuruta 1997).
4. SPECTRAL ANALYSIS AND MODEL
PARAMETER ESTIMATION
Spectral analysis in the form of model parameter estima­
tion was undertaken using the CIAO Sherpa fitting engine
and independently using specially written IDL software.
Cursory inspection of the spectrum leads immediately to the
conclusion that there are no obvious features indicative of
absorption lines or edges. We found that blackbody models
represent the high­resolution spectra well, in agreement
with earlier studies. We modeled +1 and #1 orders both
separately and simultaneously and added together; results
from these di#erent approaches were statistically indistin­
guishable. Representative results of model fits and residuals
are illustrated in Figure 1. Two sets of best­fit parameters
were obtained from independent analyses that invoked (1)
the existing first­ and higher order CXC calibration 6 and (2)
the same first­order e#ective area with higher orders modi­
fied slightly to improve model fits to sources with power­law
spectra (H. L. Marshall et al. 2002, in preparation). Both
sets are consistent with the results of Burwitz et al. (2001)
based on the 2000 March observation alone. Parameters
and 1 # statistical uncertainties for best­fit models were: (1)
T ¼ 61:2 # 0:3 eV, NH ¼ Ï1:10 # 0:02÷ # 10 20 cm #2 , X­ray
luminosity Ï2:96 # 0:03÷ # 10 31 D 2
100 ergs s #1 , where D 100 is
the distance in units of 100 pc, (2) T ¼ 61:1 # 0:3 eV,
NH ¼ Ï0:81 # 0:02÷ # 10 20 cm #2 , X­ray luminosity
Ï3:16 # 0:03÷ # 10 31 D 2
100 ergs s #1 . Parameters producing
minima in the # 2 test statistic were not sensitive to the exact
binning adopted, though of course the reduced # 2 values
were the following: values ranged from 0.94 for data binned
to a signal­to­noise ratio of S=N ¼ 10 to 1.7 for S=N ¼ 30.
The latter value is dominated by residual e#ective area cali­
bration uncertainties. To investigate the e#ects of these
uncertainties, which are estimated to be about 15% abso­
lutely, a first­order polynomial term was included in the
source model to mimic an e#ective area lower by 15% at
20 A š and higher by 15% at 100 A š and vice versa. Such a
slope skews the blackbody curve and leads systematically to
Fig. 1.---Combined positive and negative order spectra of RX
J1856.5#3754 binned at 0.5 A š intervals, shown with the best­fit blackbody
model with parameters corresponding to method 2 in x 4 and residuals
(observations#model). The deviations from this model are consistent with
Poisson statistics after allowing for calibration uncertainties at the C K
edge and over broader wavelength intervals. The apparent edge at 60 A š
results primarily from one of the HRC­S plate gap boundaries and small
residual QE di#erences between positive­ and relative negative­order outer
plates.
6 Version dated 2000 October 31, available on­line at
http://asc.harvard.edu/cal/Links/Letg/User.
998 DRAKE ET AL. Vol. 572

higher or lower temperature solutions by about 1 eV---
clearly the true temperature uncertainty is driven by this
uncertainty in the e#ective area. Allowing for this uncer­
tainty, we adopt a final temperature of 61:2 # 1:0---in agree­
ment with, but much more tightly constrained than, the
ROSAT temperatures derived by Burwitz et al. (2001;
63 # 3 eV) and Pons et al. (2002; 55:3 # 5:5). We note that a
temperature of 61 eV results in an optical flux about a factor
of 2 lower than that for a 55 eV blackbody, so that the dis­
crepancy between the observed optical flux and that pre­
dicted by the hot blackbody noted by Walter & Matthews
(1997) and Pons et al. (2002) is larger by a similar factor.
However, this optical flux decrement represents only a small
fraction of the total luminosity of RX J1856.5#3754.
Blackbody models were found by both Pons et al. (2002)
and Burwitz et al. (2001) to represent observed spectra bet­
ter than sophisticated model atmospheres, though a uni­
form­temperature blackbody model was formally excluded
in X­ray--EUV--UV--optical modeling in the former work.
However, additional cooler components were found to con­
tribute at most only a few percent to the observed ROSAT
PSPC X­ray flux. We have ruled out the significant presence
of additional thermal and nonthermal emission components
by trial of models combining two blackbodies and models
using a blackbody component with arbitrary power laws: in
both cases the additional components were completely
unconstrained and resulted in no improvement in the good­
ness of the fit. The 3 # upper limit to power­law flux is
5:2 # 10 28 D 2
100 ergs s #1 keV #1 at 1 keV. Our blackbody
models from methods 1 and 2 correspond formally to a
radius over distance ratio (angular size) of R1 =D 100 ¼
4:12 # 0:68 km per 100 pc, where R1 ¼ R= 1#
Ï
2GM=Rc 2
÷ 1=2 is the `` radiation radius '' corresponding to
the true radius R for a star of mass M, and the quoted uncer­
tainty represents the combined temperature determination
uncertainty (#1 eV) and the (dominant) absolute e#ective
area uncertainty of the LETG+HRC­S combination
(#15% ).
In all model comparisons for binning at S=N > 10 the
residual di#erences between the observed counts and the
best­fit model show systematic departures from normal stat­
istical deviations (Fig. 1). Broad deviations are character­
ized by a general overprediction of observed counts for
# < 30 A š and in the region of 75--100 A š , which is dominated
by higher order flux, underprediction by an average of
#10% for the range of 25--38 A š and deviations around the
instrumental C K­edge region of 40--44 A š . The latter results
from a residual calibration error in the HRC­S UV/ion
shield. Other deviations could arise either as a result of
impropriety of a blackbody model for RX J1856.5#3754, or
through calibration errors, which are currently estimated to
be #15% over broad spectral ranges and less over narrower
ranges (J. J. Drake et al. 2002, in preparation). An apparent
edge at 60 A š and flux excess in the range 60--70 A š arises
because of one of the HRC­S plate gap boundaries coincides
with small residual QE di#erences between positive and rel­
ative negative order outer plates.
In the case of narrow line or edge features, in di#erent
combinations of spectral order, data reduction method, and
binning size, we identify possible structure in residuals at
26.5, 27.6, 34.4, 32.4, and 35.9 A š (emission­like features),
and at 28.2, 39.1, and 86.5 A š (absorption­like features), that
might be tempting to attribute to the source. However, we
cannot exclude the possibility that any are chance fluctua­
tions at the 1% level. The issue is complicated by possible
calibration uncertainties on smaller scales, believed to be at
a level of about 5% or less, that should be largely smoothed
out by dither. We have also used a 60 ks observation (ObsID
331) of PKS 2155#304, currently thought to be a featureless
continuum in the spectral range we are concerned with here
(H. L. Marshall et al. 2002, in preparation), as a flat­field
source to aid in feature identification. This spectrum com­
prises about 8 times the number of first­order counts of the
combined RX J1856.5#3754 data. Moreover, we have
imposed a constraint that features must appear in both posi­
tive and negative orders in the RX J1856.5#3754 spectrum.
We examined deviations from the model on scales up to 3 A š
and find only the expected normal distribution of residuals
after allowing for smooth departures resulting from calibra­
tion. The Kolmogorov­Smirnov test applied to deviations
on di#erent scales also revealed no evidence of significant
features at these scales. In summary, all significant devia­
tions that have been found in the residuals can reasonably
be explained by instrumental e#ects. Equivalent width
upper limits were derived by applying counting statistics to
a convolution of the spectrum with a triangular kernel
(Fig. 2).
5. INTERSTELLAR MEDIUM ABSORPTION
The distance of 62 pc derived by Walter (2001) seems at
odds with the neutral H column density, N H , of 10 20 cm #2
derived from the Chandra spectra: measured N H values for
objects at this distance based on di#erent techniques are typ­
ically in the range of 10 18 --10 19 cm #2 (e.g., Fruscione et al.
1994). Walter (2001) indeed remarked on this, citing redden­
ing values E B#V of up to 0.1 derived by Knude &HÜg (1998)
in support of the distance. However, these reddening values
show considerable scatter at low reddening and are based
only on a relatively coarse attribution of spectral type to the
stars considered.
We have estimated N H and the mean local neutral hydro­
gen number density, n H , in the line of sight toward
RX J1856.5#3754 as a function of distance by spatial inter­
polation in the measurements compiled by Fruscione et al.
(1994) and Diplas & Savage (1994) using a technique devel­
oped by P. Jelinsky (1994, unpublished). We illustrate this
Fig. 2.---The 3 # equivalent width upper limit to line features as a func­
tion of wavelength, based on a convolution of the spectrum with a triangu­
lar kernel.
No. 2, 2002 IS RX J1856.5#3754 A QUARK STAR? 999

in Figure 3, together with the allowed distance ranges from
Walter (2001) and Kaplan et al. (2002). The value of N H esti­
mated in this way at a distance of 60 pc is an order of magni­
tude lower than the X­ray measurement, but is in good
agreement with the distance of 140 pc derived by the latter
authors. This distance would place RX J1856.5#3754 on
the outskirts of the R CrA cloud using the cloud distance of
Knude & HÜg (1998) of #170 pc and within the cloud using
the canonical cloud distance of #130 pc. In either case,
RX J1856.5#3754 would likely lie in a region of relatively
high ISM density (#1--10 cm #3 ). While these estimates of
N H are crude and will smooth out any small­scale ISM inho­
mogeneities, the larger distance is easier to reconcile with
the measured column. The cloud distance of Knude & HÜg
(1998) then represents an upper limit to the distance of
RX J1856.5#3754.
6. DISCUSSION
Our results, combined with the recent analysis of Ransom
et al. (2002), demonstrate a lack of pulsed features above a
level of 2.7% (unaccelerated search; 4.5% from the acceler­
ated search of Ransom et al. 2002) and no unequivocal
detection of spectral features. This dearth of indices with
which to restrict parameter space precludes an obvious
answer to the problem of the nature and origin of
RX J1856.5#3754.
The apparent lack of electron or proton resonance cyclo­
tron absorption suggests that magnetic field strengths in the
ranges of Ï1 7÷ # 10 10 and Ï0:2 1:3÷ # 10 14 G are less likely,
as discussed by Paerels et al. (2001) for RX J0720.4#3125
and by Burwitz et al. (2001), but, as emphasized by the lat­
ter, they should not be excluded owing to possible di#cul­
ties in detecting the absorption features. Indeed, neutron
stars with di#erent levels of magnetic field up to 10 15 G have
now been observed with high­resolution X­ray spectrome­
ters and none have so far shown absorption features that
are intrinsic to the stellar photosphere (e.g., RX
J0720.4#3125, Paerels et al. 2001; PSR 0656+14, Marshall
& Schulz 2002; Vela, Pavlov et al. 2001; and 4U 0142+61,
Juett et al. 2001).
Our derived angular size based on modeling of the
LETGS spectra, R1 =D 100 ¼ 4:12 # 0:68 km per 100 pc, is
consistent with that of Burwitz et al. (2001), though the
revised allowed distance range of 111--200 pc (Kaplan et al.
2002), together with the distance upper limit constraint
based on the RCrA cloud, D 100 # 1:70, now implies a radia­
tion radius in the range of R1 ¼ 3:8--8.2 km. The high end
of this range, corresponding to the largest allowed distance,
is still inconsistent with current `` normal '' NS equations of
state (R1e12 km; e.g., Lattimer & Prakash 2000), as well
as with those with extreme softening, such as kaon conden­
sate models.
Pons et al. (2002) find that heavy element­dominated at­
mosphere models provide a plausible match to the low­
resolution ROSAT PSPC spectra and UV and optical fluxes
of RX J1856.5#3754, while Kaplan et al. (2002) alleviate
conflicts with standard EOS through their revised distance.
The heavy element models yield larger radii by virtue of hav­
ing cooler e#ective temperatures than blackbody spectra
with similar energy distributions. However, Burwitz et al.
(2001) argued against such uniform­temperature heavy­ele­
ment--dominated atmosphere solutions based on a lack of
the expected spectral line features. The apparently feature­
less but much higher quality LETGS spectra presented here
strengthen these conclusions.
An alternative favored by Pons et al. (2002), Burwitz et
al. (2001), and Ransom et al. (2002) is a nonuniform­
temperature model---that we are only seeing a localized hot
region on the surface of a cooler star. The latter authors
argue that the gravitational smearing e#ects described by
Psaltis, O
˜ zel, & DeDeo (2000) account for a lack of
observed pulsations. However, the pulse fraction expecta­
tions of Psaltis et al. (2000) indicate X­ray pulse fraction lev­
els below our 2.7% limit would be seen only #10%--15% of
the time.
Pulsation would also be expected for a young (#10 6 yr),
cooling INS with a strong magnetic field. The alternative---
that RX J1856.5#3754 is an older object reheated by ISM
accretion---has been dismissed by Kaplan et al. (2002) and
van Kerkwijk & Kulkarni (2001a), largely based on a
Bondi­Hoyle accretion rate for the space velocity of Walter
(2001) and Kaplan et al. (2002) that would be much too low
to explain the observed luminosity and on a possibly strong,
accretion­inhibiting magnetic field (Pons et al. 2002). Never­
theless, RX J1856.5#3754 appears to be in the outskirts of
the R CrA cloud and, based on its HST­derived velocity
vector, has likely passed through more dense regions than
the one it now resides in. Finding one of only a handful of
INS candidates in such a region by chance is very unlikely,
yet it is just such dense ISM regions that are expected to
power accretion­heated INSs. Based on the large velocity,
however, accretion could only be significant if the
Bondi­Hoyle formalism were to be inapplicable for
RX J1856.5#3754. While we tentatively ascribe the #2%
change in observed zeroth­order count rate in the 19 months
separating the two observation sets to detector e#ects, such
variability could be accommodated by an accretion model
as the star passed through ISM density fluctuations on
#AU scales.
The lack of spectral features is also consistent with a pure
hydrogen atmosphere model that is expected to result from
modest accretion, whereby heavier elements undergo
rapid gravitational settling. Pons et al. (2002) argue that
standard pure H models overpredict the optical flux of
Fig. 3.---The estimated N H (solid curves) and n H (dotted curves) as a func­
tion of distance in the line of sight toward RX J1856.5#3754. The pairs of
curves represent the likely ranges of these quantities and correspond to
deviations of factors of 3 from a smooth locus through the results of the
three­dimensional interpolations. Vertical shaded regions indicate the dis­
tance ranges found by Walter (2001; W) and Kaplan et al. (2002; KvKA).
The horizontal stripe illustrates the allowed range (#1 #) of N H derived in
this study.
1000 DRAKE ET AL. Vol. 572

RX J1856.5#3754 by a factor of 30 and that the magnetic
accreting models of Zane, Turolla, & Treves (2000), while
capable of reproducing the observed X­ray--to--optical flux
ratio, would need to be 2 orders of magnitude brighter and
an order of magnitude hotter than observed. However, dif­
ferent accretion scenarios and atmospheric models might
bear examination in light of the otherwise coincidental loca­
tion of RX J1856.5#3754.
The slightly unfavorable odds of a nonuniform­
temperature model failing to show signs of pulsation leads
us to consider a third possibility. Taken at face value, the
distance, N H , and lack of spectral and temporal features and
pulsations favor a more compact object than current NS
models permit, but one that is allowed for in strange quark
matter solutions (e.g., Lattimer & Prakash 2000; Fraga,
Pisarski, & Scha#ner­Bielich 2001; Hanauske et al. 2001).
There now exists a body of evidence from heavy­ion colli­
sion experiments supporting the viability of a quark­gluon
plasma (Heinz 2001), and the possible existence of `` strange
stars '' (e.g., Itoh 1970; Bodmer 1971; Collins & Perry 1975;
Brecher & Caporaso 1976; Chapline & Nauenberg 1978;
Witten 1984; Alcock, Farhi, & Olinto 1986; Haensel, Zud­
nik, & Schaefer 1986) is perhaps not as speculative as it once
was. As noted by Xu (2002) and Pons et al. (2002), such an
object would be expected to have an approximately thermal
spectrum as we observe (we might then speculate that the
optical flux decrement of the 61 eV blackbody spectrum
could be ameliorated by a small amount of flux redistribu­
tion within a thin crust). Such a suggestion is not unprece­
dented, and there now exists a small handful of objects
whose apparent compactness could be explained if they are
composed of quark matter (e.g., Bombaci 1997; Cheng et al.
1998; Li et al. 1995, 1999; Xu, Qiao, & Zhang 1999). Of the
existing quark star candidates, RX J1856.5#3754 arguably
presents the strongest and most direct case. If this case sur­
vives future scrutiny, then the likelihood of such an object
being the brightest and closest of the current few INS candi­
dates would add some support to speculation that such a
state of matter is a common product of supernovae explo­
sions, or a common phase or endpoint in the evolution of a
neutron star (e.g., Alcock et al. 1986; Kapoor & Shukre
2001; Xu, Zhang, &Qiao 2001).
We extend warm thanks to Pete Ratzla# for invaluable
assistance and `` wonder scripts '' developed at short notice.
We also thank members of the CfA High Energy Astrophy­
sics Division for useful comments and corrections, and Fred
Walter for pointing out an error in the stated optical flux
decrement in an earlier version of the manuscript. The SAO
authors were supported by NASA contract NAS 8­39073 to
the Chandra X­Ray Center during the course of this
research. H. L. M. was supported by NASA contract SAO
SV 1­61010.
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No. 2, 2002 IS RX J1856.5#3754 A QUARK STAR? 1001