A&A 393, 983-989 (2002)
DOI: 10.1051/0004-6361:20021087
F. Verrecchia1,2 - G. L. Israel3, - I. Negueruela4 - S. Covino5 - V. F. Polcaro6 - J. S.
Clark7,8 - I. A.
Steele9 - R. Gualandi10 - R. Speziali3 - L.
Stella3,
1 - Dipartimento di Fisica, Università degli Studi ``La Sapienza'',
piazza
A. Moro 5, 00185 Roma, Italy
2 -
ASI Science Data Center, ESA-Esrin, via Galileo Galilei, 00044
Frascati, Rome, Italy
3 - INAF
- Osservatorio Astronomico di Roma, via Frascati 33, 00040
Monteporzio Catone, Italy
4 -
Observatoire Astronomique de Strasbourg, rue de l'Université 11,
67000 Strasbourg , France
5 -
INAF - Osservatorio Astronomico di Brera, via E. Bianchi 46, 23807
Merate, Italy
6 - Istituto
di Astrofisica Spaziale, CNR, Area della Ricerca di Roma Tor Vergata,
via Fosso del Cavaliere,
00133 Roma, Italy
7 - Astronomy Centre, CPES, University of Sussex,
Brighton BN1 9QH, UK
8 -
Dept. of Physics & Astronomy, University College London, London WC1E
6BT, UK
9 - Astrophysics
Research Institute, Liverpool John Moores University, Liverpool CH41
1LD, UK
10 - INAF -
Osservatorio Astronomico di Bologna, via Ranzani, 1, 40127, Bologna,
Italy
Received 15 May 2002 / Accepted 24 July 2002
Abstract
We report on the discovery of the optical/IR counterpart of
the 15.8 s transient X-ray
pulsar XTE J1946+274. We re-analyzed archival BeppoSAX observations of XTE J1946+274
obtaining a new refined
position (a circle with 22
radius at 90% confidence level).
Based on this new position we
carried out optical and infra-red (IR) follow-up observations. Within
the new error circle we
found a relatively optical faint (B=18.6) IR bright (H=12.1)
early type reddened star
(V-R=1.6). The optical spectra show strong H
and
H
emission lines. The
IR photometric observations of the field confirm the presence of an IR
excess for the
H
-emitting star (K=11.6, J-H=0.6) which is likely
surrounded by a
circumstellar envelope. Spectroscopic and photometric data indicate a
B0-1V-IVe spectral-type
star located at a distance of 8-10 kpc and confirm the
Be-star/X-ray binary nature of XTE J1946+274.
Key words: stars: individual: - XTE J1946+274; GRO J1944+26 - binaries: general - stars: pulsars: general - stars: emission-line, Be - X-rays: stars
High Mass X-ray Binary systems (HMXRBs) hosting a neutron star (NS) are a class of intrinsically variable and bright X-ray sources whose luminosity is powered by accretion from a massive companion, often a B-emission spectral-type star (BeXRBs; White et al. 1995; Coe 2000).
BeXRBs usually show in the X-ray emission three typical behaviours
(sometimes more than one
within the same source): (i) bright X-ray transients characterized by
giant outburst with
luminosity up to
Lx = 1038 erg s-1 (Type II;
Stella et al. 1986),
unrelated to the orbital phase and with high spin-up rates, (ii)
transients characterized by
periodic outbursts of relatively high luminosity
(
-1037
erg s-1; Type I) generally occurring at periastron passage of
the NS, and (iii) sources
which do not display outbursts, but are moderately variable (of a
factor up to 10-100) and show a
low luminosity (
erg s-1) pulsed
persistent emission (Negueruela 1998).
Be stars are early type stars, with mass in the range of
2-20
and luminosity class III to V, which often display Balmer lines in emission in their optical
spectra. These stars are
characterized by high rotational velocity (up to
70% of their
break-up velocity), and by
phenomena of equatorial mass ejection at irregular times that give rise
to a rotational-dominated
quasi-Keplerian decretion disc (Hanuschik 1996; Porter 1999; Okazaki
2001; Okazaki & Negueruela 2001). The widely accepted empirical model
interprets the X-ray emission from these systems, in
the three mentioned behaviours, as due to sporadic mass accretion of
the NS during the expansion
phase of the Be disc, or to periodic close encounters of the NS in
highly eccentric orbit with the
dense circumstellar envelope around the Be star.
At optical wavelengths these stars are difficult to classify owing to
the presence of emissions
lines produced by the circumstellar envelope and to the high absorption
column that affect systems lying in the Galactic plane.
So far only about
30 optical counterparts of BeXRB's have been discovered out of
the >100 known and
104-105 expected (Nelson et al. 1993, 1995; Covino et al.
2000; Covino et al. 2001;
Israel et al. 2000a; Israel et al. 2000b; Israel et al. 2001;
Chakrabarty et al. 2002).
The hard X-ray transient source XTE J1946+274 was discovered with the All Sky
Monitor (ASM) on board the
Rossi XTE satellite during a scan of the Vul-Cyg region on 1998 September 5 (Smith & Takeshima
1998). XTE J1946+274 lies inside the error box of the Ariel V transient
3A 1942+274 discovered in the
1976 (Warwick et al. 1981). Campana et al. (1999) have
estimated a chance probability
of 7% that XTE J1946+274 and 3A 1942+274 are not the same source
(assuming
1000 hard
X-ray transients in the Galaxy). The observed flux raised from
13 mCrab
(2-12 keV) on September 5 to
60 mCrab on
September 15, to
110
mCrab on September 16 (in 2-60 keV; Smith & Takeshima 1998).
Further observations
carried out by BATSE (Wilson et al. 1998) led to the detection of
X-ray pulsations with a period
of
s, later confirmed through pointed
Rossi XTE/PCA observations (Smith & Takeshima 1998). A study of the lightcurve
of RXTE/ASM has revealed an
80 d (or
160 d) X-ray modulation, interpreted as orbital
modulation (Campana et al. 1999).
Telescope & Instrument | Date | Exp. | Seeing | Range/Slit | Candidate | Cand. A H![]() |
(s) | (
![]() |
(Å/
![]() |
(Å) | |||
1.5 m Cassini & BFOSC | 1999 July 11 | 2400 | 2.5 | 4000-9000/2.5 | C,D | - |
- | 1999 July 12 | 150 | 1.9 | V | - | |
- | " | 300 | " | B | - | |
- | " | 100 | " | R | - | |
- | " | 60 | " | I | - | |
- | " | 1800 | " | 4000-9000/2.5 | B,A | -![]() |
- | 1999 July 13 | 2400 | 3.3 | 4000-9000/2.5 | E,F | |
- | " | 1800 | " | " | B,A | -![]() |
- | 1999 July 14 | 2400 | 2.0 | 4000-9000/2.0 | 4 | - |
- | " | 5400 | " | " | A | -40 ![]() |
- | 1999 July 15 | 2400 | 2.6 | 4000-9000/2.5 | A | -40 ![]() |
- | 1999 July 16 | 4500 | 2.0 | 3300-5800/2.5 | A | - |
- | 1999 July 17 | 2700 | 1.6 | " | A | - |
- | 1999 July 19 | 2700 | 1.5 | 5800-8300/2.5 | A | -42 ![]() |
- | " | 2700 | " | 5800-8300/2.0 | A | -40 ![]() |
- | " | 1800 | " | 4000-9000/2.5 | A | -45 ![]() |
3.8 m UKIRT & CGS4 | 1999 July 24 | 300 | 1.5 | 20 400-22 000/1.0 | A | - |
1.1 m AZT-24 & SWIRCAM | 1999 July 27 | 60 | 2.1 | J | - | |
- | 1999 July 27 | 60 | " | H | - | |
- | 1999 July 27 | 60 | " | K | - | |
4.2 m WHT & ISIS | 2000 July 18 | 2400 | 0.9 | 3200-9000/1.0 | A | -41.0 ![]() |
1.5 m Cassini & BFOSC | 2000 July 31 | 3600 | 2.0 | 5000-9500/2.5 | A | -40 ![]() |
- | 2000 August 28 | 1800 | 3.0 | 4000-9000/2.5 | A | -33 ![]() |
- | 2001 June 18 | 3600 | 1.8 | 3600-8000/2.0 | A | -40 ![]() |
The 1998 outburst was also monitored through a BeppoSAX campaign (Campana
et al. 1998) which
revealed a complex pulse profile (in 1-10 keV) and a complex
continuum spectrum, with an
iron line at 6.6 keV and an inferred column density of
1.
cm-2. Using Rossi XTE data,
a cyclotron line has been
detected at
35 keV with a FWHM of
8 keV
(Heindl et al. 2001),
implying a magnetic field strenght of about 3.1(1+
G, where z is the
gravitational redshift of the scattering region.
In this paper we report on the identification of the optical/IR
counterpart of XTE J1946+274 with
a ,
Be spectral-type star. Based on BeppoSAX observations (circular uncertainty
region with a radius of 40
), we detected a reddened
(V-R = 1.6) star, showing in
the optical spectrum a strong H
emission line. The field of
XTE J1946+274 has been subsequently
observed in the IR band. The candidate counterpart resulted as the
second brightest object in the
BeppoSAX error circle (
mag), while the detection of Bracket
H
emission lines
in the IR spectra confirmed the Be spectral classification of the star.
A recently refined
BeppoSAX error circle (22
of radius) has finally confirmed the
identification ruling out the
possible link between the X-ray transient pulsar and the brightest IR
source included in the
previous BeppoSAX uncertainty region.
![]() |
Figure 1:
Optical B, R-filter (upper
panels) and IR K-filter (lower panel) images of the field of XTE J1946+274 together with the X-ray position uncertainty circles derived from
BeppoSAX (inner circles) and Rossi XTE (larger circle in the
R-filter image) observations; RA (h m s) and Dec (![]() ![]() ![]() |
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The positional uncertainty region obtained with the Rossi XTE/PCA is a
circle with a radius of
2.4 (90% confidence level; Takeshima & Chakrabarty
1998); the latter was
reduced with BeppoSAX Narrow Field Instruments pointed observations to
40
(90% confidence
level; Campana et al. 1998). The BeppoSAX X-ray position was
RA = 19
45
38
,
(equinox 2000.0). This position could
be refined thanks to a detailed
study performed on 73 relatively bright sources with known optical
position observed by BeppoSAX during its life (see also Fiore et al. 2001). The position uncertainty
was found to depend on several factors (intensity, off-axis angle,
roll angle, energy-dependent PSF, number of gyros and star-treakers,
etc.) some of which can be taken properly into account ending up with a
MECS error radius of 14
and 22
at 68% and 90%
confidence level, respectively. The first BeppoSAX position was than corrected for
these systematic uncertainties by performing a simple boresight
correction: the final XTE J1946+274 position is RA = 19
45
38
5, Dec =
+27
21
54
3 (radius of 22
at 90% confidence
level, equinox 2000.0).
Object | B | V | R | I | V-R | J | H | K | J-H |
(![]() |
(![]() |
(![]() |
(![]() |
(![]() |
(![]() |
||||
A | 18.62 | 16.90 ![]() |
15.32 | 13.32 | 1.58 | 12.7 | 12.1 | 11.6 | 0.6 |
B | 17.29 | 15.53 ![]() |
15.13 | 13.74 | 0.94 | 13.9 | 13.3 | 13.2 | 0.6 |
C | 17.71 | 16.54 ![]() |
15.68 | 14.37 | 0.86 | 14.7 | 14.2 | 14.1 | 0.5 |
D | 16.71 | 15.71 ![]() |
14.96 | 13.76 | 0.75 | 14.3 | 13.9 | 14.0 | 0.4 |
E | 17.15 | 16.06 ![]() |
15.33 | 14.08 | 0.73 | 14.5 | 13.9 | 13.8 | 0.6 |
F | 18.89 | 17.33 ![]() |
16.50 | 15.16 | 0.83 | 15.7 | 15.2 | 15.1 | 0.5 |
4 | 18.94 | 17.56 ![]() |
16.66 | 15.22 | 0.90 | - | - | - | - |
RA(J2000) = | 19![]() ![]() ![]() |
||||||||
Dec(J2000) = | +27![]() ![]() ![]() |
||||||||
Spectral type: | B0-B1 IV-V e | ||||||||
EB-V: |
![]() |
||||||||
Distance: | ![]() |
We carried out observations in the optical and IR band of the BeppoSAX error circle in several observing periods during 1999-2001 (see Table 1) at the Loiano observing station (Bologna Observatory), at Campo Imperatore (Observatory of Rome), at the United Kingdom Infrared Telescope (UKIRT, Mauna Kea, Hawaii) and at the William Herschel Telescope (WHT, La Palma, Spain).
![]() |
Figure 2: The 3600-s low resolution (18 Å) spectrum of XTE J1946+274 candidate A (see Fig. 1 middle panel) obtained on 2000 July 31 from the 1.5 m Cassini telescope at Loiano (Bologna, Italy). |
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On July 1999 we performed imaging and photometry in the B (300
s), V (150 s),
R (100 s) and I (60 s) filters with the 1.5-m
Cassini telescope equipped
with the Bologna Faint Objects Spectrometer and Camera (BFOSC; Bregoli
et al. 1987; Merighi et al. 1994;
field
of view and 0
58/pixel resolution, and a maximum resolution of
900 in the 3500-9000 Å range with a 1
0 slit). Data
reduction was performed using standard ESO-MIDAS procedures for
bias subtraction and flat-field correction. Aperture and profile-fitting
photometry for each stellar object in the image was
derived with the DAOPHOT II program (Stetson 1987).
We performed astrometry of the field using the USNO
catalog, resulting in a better than 0
4 positional accuracy. The
same field was observed in the IR J (60 s), H (60 s)
and K (60 s) filters on July 1999 with the 1.1-m AZT-24
telescope equipped with the Supernova Watchdogging IR Camera (SWIRCAM;
field of view and 1
04/pixel
resolution). Similar analysis procedures were applied to IR data.
Figure 1 shows the B, R and K-filter images of the field which includes the XTE J1946+274 position: the 90% position uncertainty circles as inferred by Rossi XTE (the largest one in the R-filter image) and BeppoSAX (the inner circles) are also shown.
Within the refined BeppoSAX X-ray positional error circle we detected seven relatively bright (see Fig. 1) stars among which only one (labeled as A) was a reddened (V-R = 1.6, all others had V - R < 1) and relatively bright IR object (H = 12.1). In Table 2 we list results of optical/IR photometry for all seven stars.
During July 1999 we also performed low-resolution (18 Å)
spectroscopy of all relatively bright stars within the refined BeppoSAX
circle (see Table 1). We applied standard corrections to spectra,
i.e. one-dimensional stellar and sky spectra extraction, then removed
cosmic rays and, when weather condition were satisfactory, we performed
absolute calibration both for spectroscopy and photometry.
All the spectra carried out in July 1999 in the 4000-9000 Å
band (with different slit apertures) were rebinned to the largest
resolution, 18.4 Å, and then summed to increase the S/N
ratio. None of the objects showed emission lines except star A.
For the latter source we detected a strong emission line (equivalent
width, EW, of - Å and Full Width Half Maximum,
FWHM, comparable with the spectral resolution) with central wavelength
corresponding to that of H
(see e.g. Fig. 2 for a
5000-9500 Å spectrum of star A obtained during a 3600 s observation
performed on 2000 July 31). Similarly we reduced, rebinned (9 Å of
resolution) and summed two spectra of star A in the band 5800-8300 Å
to better characterize the H
emission line
(
Å).
Spectra for star A were subsequently carried out routinely during
several 2000-2001 observational nights to study the possible presence of
variations of emission line parameters. The H
EWs
are consistent with being constant during our observations (see
Table 1).
![]() |
Figure 3: The 2400-s spectrum (3.3 Å and 1.6 Å spectral resolution in the 4000-7000 Å and 8000-9000 Å range, respectively) of XTE J1946+274 candidate A obtained on 2000 July 18 from the 4.2 m William Herschel Telescope at Roque de los Muchachos (La Palma, Spain). Balmer, Paschen and helium series lines are indicated. |
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Finally, on 2000 July 18 two 1200-s medium-resolution spectra
were taken with the Intermediate Dispersion Spectroscopic and Imaging
System (ISIS) mounted on the 4.2 m William Herschel
Telescope (WHT), located at the Observatorio del Roque de los
Muchachos, (La Palma, Spain). The blue arm was equipped with the R300B
grating and the EEV#10 CCD, which gives a nominal dispersion
of 0.9 Å/pixel. The resolution at H
,
estimated from
the FWHM of arc lines, is
3.3 Å. The red arm was equipped with
the R600R grating and the Tek4 CCD, which gives a nominal dispersion of
0.8 Å/pixel (the resolution is
1.6 Å at
Å). The blue part of the spectrum is dominated
by strong diffuse absorption bands (see Fig. 3). The only feature
clearly recognizable shortwards of H
(which is clearly
in emission with EW of
Å) is the
4430 Å DIB. An exact spectral classification is therefore
impossible. However, based on the usual properties of Be
stars, we can strongly constrain the spectral type. The presence of
He I
5875 Å strongly in emission is only observed in
the earliest Be stars (earlier than B2). Likewise, the presence of
strong emission in the Paschen series (labelled with P in the
lower panel of Fig. 3) confirms that the object is earlier than B2
(Andrillat et al. 1988). As the lack of a detectable
He II
5412 Å absorption line imposes a spectral type
later than O9.5, the object is constrained to lie in the B0-B1 range.
This is within the spectral range occupied by known Galactic and LMC
Be/X-ray binary counterparts, which have spectral types tightly
concentrated around B0 (Negueruela & Coe 2002).
![]() |
Figure 4: The 300-s IR spectrum of XTE J1946+274 candidate A obtained on 1999 July 24 from the UKIRT (Maunea Kea, Hawaii). |
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Spectroscopic IR observations of XTE J1946+274 candidate were made with the United
Kingdom Infrared Telescope (UKIRT; Joint Astronomy Centre, Mauna Kea, Hawaii)
on 1999 July 24, using the Cooled Grating Spectrometer (CGS4). Observations
between 2.04-2.2 microns were made using the long focal length
camera plus the 150 line/ mm grating, giving a velocity
resolution of 50 km s-1.
Initial data reduction was carried out at the telescope using the
CGS4DR software (Puxley et al. 1992).
This removes bad pixels, debiases, flat-fields, linearity corrects and
interleaves oversampled scan positions. The subsequent stages of data
reduction, comprising of sky subtraction, extraction, derippling and
wavelength calibration, were carried out using the Starlink-supported
package FIGARO. Removal of telluric features was accomplished
via the procedure described by Clark & Steele (2000).
The spectrum (see Fig. 4) shows strong emission from He I
2.058 m (
Å) and H I Bracket
(
Å). FWHMs for the two lines are identical
within the errors on the measurements
and yield a mean value of
km s-1. There is
some evidence for weak emission in the Mg II 2.138/2.144
m doublet
(
Å and
Å, respectively),
although further higher S/N observations are required to
confirm this. If the emission features are real the ratio of the EW of
both lines suggests that the lines are optically thin, and are expected
to be excited via Ly
fluorescence. No evidence for emission from
Fe II 2.189
m or He I 2.112/3
m emission or
absorption was found.
Comparison to the K band spectra of classical Be stars presented by
Clark & Steele (2000) suggests that XTE J1946+274 candidate A
can be classified as a Group 1 object consistent with it being of early
(O9-B3) spectral type (given the paucity of photospheric features in
the near IR it is not possible to classify a Be star to greater accuracy
than 2 spectral types or provide a luminosity class) in agreement
with the optical spectroscopic findings. We find no anomalous
emission from higher excitation species to suggest that the star has a
compact companion; in line with the trend displayed by other Be/X-ray
binaries for which K band spectra have been obtained which also
have spectra indistinguishable from those of isolated early spectral
type classical Be stars (e.g. Clark et al. 1999).
We have presented the X-ray, optical and IR observational data of the field containing the error circle of XTE J1946+274; these observations led to the likely identification of the optical counterpart of this 15.8-s transient X-ray pulsar. Optical/IR photometric measurements are consistent with a relatively reddened and distant blue star, similar to known optical/IR counterparts of BeXRBs (see Nelson et al. 1993, 1995; Israel et al. 2000a; Israel et al. 2000b; Covino et al. 2000; Covino et al. 2001; Israel et al. 2001; Chakrabarty et al. 2002).
Owing to the uncertainty in the spectral classification, the
measurement of the distance from optical data becomes difficult;
it is however possible to extract some information from our
optical and IR data. The observed V-R color for XTE J1946+274 is
1.6, while the intrinsic one
should be
(assuming a main sequence or
sub-giant star with spectral class in the B0-B1 III-V range;
Wegner 1994), so the reddening should amount to
.
Assuming a standard reddening law
(Fitzpatrick 1999) this converts to
,
and
(regardless of the distribution of the medium responsible
for the reddening along the line of sight and near the source, i.e.
assuming that the reddening due to circumstellar disk
is negligible).
However from the X-ray spectral data (Campana et al. 1998) a NH of
1.
cm-2 was inferred,
corresponding to an
(Predehl & Schmitt 1995), which is slightly more than what obtained from
optical data. Considering the total Galaxy NH column in the
direction of XTE J1946+274 (Dickey & Lockman 1990), <
cm-2,
a value of
was
inferred, which comfortably overlaps the value inferred from the
optical data. This result suggests that at least part of the inferred X-ray
NH is local to the system and obscures the
neutron star during outbursts. A good agreement with the B to Imeasurements is obtained for a B0-1IV-V star at a distance of about
8-10 kpc and with
(see Table 2). We
can reasonably discard the possibility of a luminosity class III which
would imply an
EB-V > 2.3 and a large IR-deficiency. For
a reference B0-1IV-V star (
)
at a distance of 8-10 kpc
and based on the IR photometry we infer an excess of
0.4, 0.5 and 1.4 mag in J, H and K filters
respectively, suggesting the presence of a circumstellar envelope. Finally
we note that in the direction of XTE J1946+274 there are two spiral arms of our
Galaxy located at
4 kpc and
9 kpc away from the Earth
(Perseus and Cygnus arms; Turner 1980; Taylor & Cordes 1993;
Vallée 2002), so XTE J1946+274 could belong to the second one.
Moreover this distance estimation is very similar to that found for the
nearby source KS 1947+300 (Negueruela et al. 2002),
which could lie on the same arm.
For a distance of 8-10 kpc and a 1-10 keV flux of
erg s-1 cm-2 at the peak of
the 1998 outburst (Takeshima & Chakrabarty 1998) we obtain an X-ray
luminosity of
(1-10 keV
-
erg s-1.Such a luminosity is a typical value shown by X-ray pulsars in binary
systems during Type I outbursts (Stella et al. 1986; Negueruela 1998)
occurring close to the time of periastron passage and with a periodic
recurrence at the orbital period of the system.
Based on both X-ray and optical/IR observations we identify
the likely optical/IR counterpart of XTE J1946+274.
Moreover, photometric and spectroscopic optical/IR data allows us to
conclude that the proposed optical counterpart is most likely a B0-1 V-IVe
star at a distance of about 8-10 kpc. A
more accurate distance and spectral classification would require more
detailed optical/IR spectroscopic observations.
Acknowledgements
This work is partially based on observations carried out at Loiano observing station (Bologna Observatory). The WHT is operated on the island of La Palma by the Isaac Newton Group in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias. We thank the service programme for performing the observations. The United Kingdom Infrared Telescope is operated by the Joint Astronomy Centre on behalf of the U.K. Particle Physics and Astronomy Research Council. JSC acknowledges funding support from PPARC. Moreover authors would like to thank the referee, Dr. L. M. Kuiper, for useful advices.