A&A 406, 221-232 (2003)
DOI: 10.1051/0004-6361:20030682
R. S. Schnerr1 - T. Reerink1 - M. van der Klis1 - J. Homan1,2 - M. Méndez1,3 - R. P. Fender1 - E. Kuulkers4
1 - Astronomical Institute "Anton Pannekoek'',
University of Amsterdam, and Center for High Energy Astrophysics,
Kruislaan 403,
1098 SJ Amsterdam,
The Netherlands
2 - INAF - Osservatorio Astronomico di Brera,
via Bianchi 46,
23807 Merate (LC),
Italy
3 - SRON,
National Institute for Space Research,
Sorbonnelaan 2,
3584 CA Utrecht,
The Netherlands
4 - ESA-ESTEC,
Science Operations & Data Systems Division,
SCI-SDG,
Keplerlaan 1,
2201 AZ Noordwijk,
The Netherlands
Received 10 December 2002 / Accepted 6 May 2003
Abstract
We present results of an analysis of all 480 ks of Rossi X-ray Timing Explorer Proportional Counter Array data obtained from 17 May 1998 to 11 October 1998 on the luminous low mass X-ray binary GX 13+1. We analysed the spectral properties in colour-colour diagrams (CDs) and hardness-intensity diagrams (HIDs) and fitted the power spectra with a multi-Lorentzian model. GX 13+1 traces out a curved track in the CDs on a time scale of hours, which is very reminiscent of a standard atoll track containing an island, and lower and upper banana branch. However, both count rate and power spectral properties vary along this track in a very unusual way, not seen in any other atoll or Z source. The count rate, which varied by a factor of 1.6, along a given track first decreases and then increases, causing the motion through the HIDs to be in the opposite sense to that in the CD, contrary to all other Z and atoll sources. Along a CD track, the very low frequency noise uniquely decreases in amplitude from
5 to
2% (rms). The high frequency noise amplitude decreases from
4% to less than 1% and its characteristic frequency decreases from
10 to
5 Hz. The 57-69 Hz quasi-periodic oscillation (QPO) found earlier is also detected, and no kHz QPOs are found.
In addition the entire track shows secular motion on a time scale of about a week. The average count rate as well as the amplitude of the very low frequency noise correlate with this secular motion. We discuss a possible explanation for the peculiar properties of GX 13+1 in terms of an unusual orientation or strength of a relativistic jet.
Key words: accretion, accretion disks - stars: individual: GX 13+1 - stars: neutron - binaries: close - X-rays: binaries
Z sources trace out a Z-shaped track in the CD, where the three branches of the Z from top left to bottom right are called the horizontal branch (HB), the normal branch (NB) and the flaring branch (FB). The (sharp) turns in the track separating these branches are called vertices. The power spectral features found in Z sources are very low frequency noise (VLFN), which has an approximately power law shape and is observed up to a few Hz, a band limited noise (BLN) component with a characteristic frequency of 2 to 20 Hz and various different quasi periodic oscillations (QPOs) with a Lorentzian shape and frequencies that range from a few Hz to more than a kHz (van der Klis 2000; van der Klis 1995).
These power spectral features occur correlated to the position of the source in the Z track (see Fig. 1). QPOs with frequencies of 13-55 Hz are found on the HB (HBOs), and with frequencies of 4-7 Hz on the NB (NBOs); in some sources the latter continue onto the FB where the frequency increases up to 20 Hz.
BLN with a fractional root-mean-square amplitude (rms) of up to
8% is found along the HB and disappears as the source moves onto the NB. There is a strong tendency of the amplitude of the VLFN to increase from less then 1% to
10% along the whole track from the HB to the FB. The flux increases from left to right along the HB, stays approximately constant or slightly decreases along the NB, and usually increases again on the FB. The mass accretion rate (
)
is thought to increase along the HB via the NB to the FB on time scales of hours to days (Hasinger & van der Klis 1989), although on longer time scales the relation with
may be more complicated (e.g. Kuulkers et al. 1994; Homan et al. 2002).
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Figure 1: Typical CDs (top) of Z (left) and atoll (right) sources, and power spectra (bottom) corresponding to their different branches in the CD. The power spectra of the HB and the island state (IS) were shifted upwards by a factor of 104 and those of the NB and lower banana (LB) by a factor of 102. The figure was taken from Wijnands (2001). |
Atoll sources trace out a track that generally has a C or U-shape, consisting of an island branch, which often takes the shape of one or more isolated patches because of observational windowing (usually in the upper left region of the CD), and a banana branch (extending from the lower banana (LB) -lower left- to the upper banana (UB) -upper right). VLFN, BLN and QPOs similar to those in Z sources also occur in atoll sources (see Fig. 1; Wijnands 2001). Along an atoll-source track from the island to the lower, middle and then to the upper banana, the rms amplitude of the VLFN increases from 2 to 5%, the characteristic frequency of the BLN, which sometimes consists of several components (van Straaten et al. 2002), first increases and then decreases within a range of
2 to
70 Hz, and its rms decreases from
20% to less than 1%. The count rate increases by a factor of
2-10 (Hasinger & van der Klis 1989) going from the island state to the banana state (except for the transient sources, see below). The frequencies of the various QPOs increase in the same sense along the track in Z and atoll sources.
GX 13+1 is a LMXB containing a neutron star, since X-ray bursts have been found from this source (Fleischman 1985; Matsuba et al. 1995). Together with GX 3+1, GX 9+1 and GX 9+9, GX 13+1 forms the subclass of persistently bright atoll-sources (the "GX atoll sources'') which were observed to be in the banana state by Hasinger & van der Klis (1989). In GX 13+1 and GX 3+1, two branched structures have been observed in the CD and hardness-intensity diagram (HID) (Homan et al. 1998; Lewin et al. 1987; Muno et al. 2002; Stella et al. 1985; Schulz et al. 1989). In luminosity, these sources are intermediate between the very luminous Z sources and the weaker remaining atoll sources (Ford et al. 2000). Contrary to the other atoll sources and the Z sources, these GX atoll sources have so far not shown kHz QPOs. Although GX 13+1 has been classified as an atoll source based on upper-banana branch phenomenology (Hasinger & van der Klis 1989), among the atoll sources it is the source that shows X-ray properties closest to those of the Z sources (VLFN amplitude: Hasinger & van der Klis 1989, CD track: Muno et al. 2002, 65 Hz QPOs: Homan et al. 1998). The radio luminosity of GX 13+1 is similar to that of the Z sources, whereas the other atoll sources are weaker (Garcia et al. 1988; Fender & Hendry 2000). From infra-red spectroscopy Bandyopadhyay et al. (1999) conclude GX 13+1 might be a Z source, based on the nature of its companion suggesting a long orbital period.
High resolution spectroscopic observations of GX 13+1 with ASCA (Ueda et al. 2001) and XMM-Newton (Sidoli et al. 2002), have revealed the presence of narrow absorption lines, similar to those in the strong galactic jet sources GRS 1915+105 and GRO J1655-40 and the dippers MXB 1659-29 and X 1624-49.
For some Z sources it is known that the position of the track they trace out in the CD drifts on a time scale of weeks, with shifts in the colours of up to 10% (Kuulkers et al. 1994,1996); this effect is called secular motion. It is also seen in the atoll source 4U 1636-53 (Prins & van der Klis 1997; Di Salvo et al. 2003), but in the CD it is not as strong as in some Z-sources. In GX 13+1, Muno et al. (2002) also detected secular motion (see also Homan et al. 1998). These shifts are usually larger in the HIDs compared to the CDs due to the shift of the track being strongly correlated with X-ray luminosity (
). The power spectral properties of all LMXBs seem to depend primarily on the position (S, Hasinger et al. 1990) of the source in the atoll or Z track and not on
.
shows a strong correlation (usually positive, sometimes negative) with this S on short time scales (hours to days), but this correlation can be completely absent on long time scales. It has long been assumed that this is because S is a measure of the accretion rate
,
with
not proportional to
due to uncertain causes such as, e.g., beaming, but more recently a scenario has been suggested in which the inner radius of the accretion disc (
)
determines the power spectral properties, while at the same time
is determined by the total mass accretion plus perhaps nuclear burning (van der Klis 2001). The decorrelation of
from the power spectral properties occurs in this scenario through energy release that does not originate from accretion through the inner disc, but does respond to inner disc count rate changes in a time-averaged sense.
Recently, Muno et al. (2002) and Gierlinski & Done (2002) suggested that the distinction between atoll and Z sources might be an artifact of incomplete sampling, because the atoll sources that cover a wide enough range in count rate (the transient sources Aql X-1, 4U 1608-52 and 4U 1705-44, each with a flux range of more than a factor of 10, see also Barret & Olive 2002) also exhibit roughly Z-shaped tracks with at the lowest luminosities a "horizontal branch''. However, the differences in flux ranges and time scales on which these tracks are traced out in the CDs (as also noted by Muno et al. 2002) as well as their topology and power spectral behaviour (van Straaten et al. 2003), argue against any simple unification of these two classes. Being a persistently bright source with a flux range of a factor of 2, GX 13+1 is not among the sources that has been observed to show such an atoll source "horizontal branch''.
In this paper we discuss the X-ray colour, flux and timing properties of GX 13+1 and their mutual dependence. We find highly unusual power spectral - colour-colour diagram behaviour, which does not fit in with either Z or atoll behaviour as well as very unusual secular motion. In Sect. 4.4 we suggest a possible explanation for the unusual properties we find in GX 13+1.
Track | Vertex | Flux at
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Observation ID |
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|||
nr | HC | SC |
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(cnts s-1 PCU-1) | of included observations | month-day hours:min | (ks) | ||
1 | 0.46 | 3.07 | 0.00 |
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30051-01-01-01 | 30051-01-01-00 | 05-17 11:01 | 05-17 20:57 | 28 |
2 | 0.53 | 3.13 | 0.092 |
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30051-01-02-00 | 30051-01-03-00 | 05-21 12:39 | 05-24 19:21 | 29 |
30051-01-03-01 | 30051-01-03-02 | ||||||||
3 | - | - | - | - | 30051-01-04-00 | 30051-01-04-01 | 05-28 08:33 | 05-28 14:30 | 2 |
4 | 0.54 | 3.10 | 0.085 |
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30051-01-05-00 | 06-01 09:60 | 06-01 17:40 | 28 | |
5 | 0.60 | 3.27 | 0.249 |
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30051-01-06-00 | 30051-01-07-00 | 06-05 05:16 | 06-09 12:42 | 25 |
30051-01-07-01 | 30051-01-07-02 | ||||||||
6 | 0.51 | 3.14 | 0.086 |
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30051-01-08-00 | 30051-01-08-01 | 06-13 01:26 | 06-17 15:29 | 110 |
30050-01-01-00 | 30050-01-01-01 | ||||||||
30050-01-01-02 | 30050-01-01-03 | ||||||||
30050-01-01-04 | 30050-01-01-050 | ||||||||
7 | 0.51 | 3.14 | 0.086 |
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30050-01-01-05 | 30050-01-01-06 | 06-17 15:58 | 06-20 10:19 | 69 |
30050-01-01-07 | 30050-01-01-080 | ||||||||
30050-01-01-08 | 30050-01-02-00 | ||||||||
30050-01-02-01 | 30050-01-02-02 | ||||||||
30050-01-02-03 | |||||||||
8 | 0.46 | 3.08 | 0.010 |
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30051-01-09-00 | 30051-01-09-01 | 06-26 09:34 | 06-29 16:09 | 41 |
30051-01-10-00 | |||||||||
9 | 0.66 | 3.30 | 0.305 |
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30051-01-11-00 | 30051-01-11-01 | 07-03 14:22 | 07-09 15:40 | 78 |
30051-01-11-02 | 30051-01-11-03 | ||||||||
30051-01-12-00 | |||||||||
10a | - | - | - | - | 30051-01-12-01 | 07-13 09:36 | 07-13 15:39 | 22 | |
10b | - | - | - | - | 30051-01-12-02 | 07-31 15:11 | 07-31 18:59 | 14 | |
10c | - | - | - | - | 30050-01-03-00 | 08-09 13:31 | 08-09 17:25 | 14 | |
11a | - | - | - | - | 30050-01-03-01 | 08-14 13:18 | 08-14 17:55 | 17 | |
11b | - | - | - | - | 30050-01-03-02 | 08-21 13:02 | 08-21 16:11 | 11 | |
12 | 0.49 | 3.14 | 0.076 |
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30050-01-03-03 | 10-10 01:45 | 10-10 06:44 | 18 |
Standard 2 data of PCUs 0, 1 and 2 (which were always active) were used to create CDs and HIDs with 16 s data points. The data were background corrected using the Bright Source Model for the RXTE PCA. The soft colour (SC) was defined as the count rate ratio between PCA Standard 2 spectral channels 7-14 and 3-6 (3.6-6.5 and 2.2-3.6 keV respectively) and the hard colour (HC) as the ratio between channels 21-40 and 15-20 (8.7-16.0 and 6.5-8.7 keV). The intensity was taken to be the 2-60 keV (channel 1-129) count rate.
We checked the colours for instrumental drifts caused by changes in the response of the RXTE PCA by analysing 4 Crab observations covering the entire observation period of the data we used. We created CDs of the Crab observations using the same bands as we used for the GX 13+1 data and calculated the average colours. The shifts of Crab in the CD are below 1% in SC and below 0.5% in HC. Dead time effects result in shifts of the data points in the CD relative to one another of less than 0.1% in both HC and SC. Both the effect of instrumental drifts and of dead time are negligible compared to the effects reported below.
Tracks 3, 10a,b,c and 11a,b were excluded from further analysis, because they do not show a vertex. Because of this lack of a vertex we cannot with confidence compare the positions of these tracks in the CD with those of the others.
For each of the remaining 9 tracks we defined a parameter
that indicates the position along the track, in a similar way as e.g. Dieters & van der Klis (2000) for the Z source Sco X-1. This was done by approximating the track with a spline onto which the data points are projected along a vector defined by their errors in HC and SC (see Fig. 3). Different from e.g Sco X-1, GX 13+1 only shows one vertex in the CDs instead of two. We chose the value of
at this vertex to be equal to 2, and fixed the arc length along the track by setting
at a point on the top branch at a fixed HC and SC interval
(
,
)
above and to the right of the vertex. The locations of the vertices (the middle of the sharp turn) were estimated by eye. To quantify the shift of each track in the CD we defined a parameter
as the distance of the track's vertex to the point HC=0.46 and
(the vertex of track 1):
.
In Fig. 2 we show the CDs of all 15 tracks in black superimposed on
track 6 (in grey) for reference. As mentioned above GX 13+1 shows a pattern very
reminiscent of a standard atoll track, with, however, a rather sharp vertex. The tracks are shifted relative to each other; the vertex locations are presented in Fig. 4 and Table 1. The shifts are consistent (
)
with being on a diagonal line in the CD.
Representative HIDs of track 1 are shown in Fig. 5, together with the CD. The colour-colour points have different symbols based on their location in the CD to simplify tracking the differences in the shape of the track between the diagrams. Contrary to what is seen in other sources the track is flipped in the HIDs compared to the CD.
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Figure 3:
The CD of track 6 and the spline used to calculate ![]() ![]() ![]() ![]() |
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Figure 7:
The count rate at
![]() ![]() ![]() |
The power spectra were fitted with 2-4 Lorentzians. We chose to
fit the VLFN with two zero-centered Lorentzians instead of a power law,
because it has a "bumpy'' structure, which can not be fitted well with a
single power law (to obtain a good fit one has to add additional components to
fit the bumps in any case) and is too broad a feature to be well fitted by a
single Lorentzian. We will call these two components the low and high VLFN.
This approach has the additional advantage that it avoids
the common problem that the VLFN power law attempts to also fit power at high frequencies, which is probably unrelated to the VLFN, and can bias the power law to ill represent the data at low frequencies. Two more Lorentzians fit the BLN and a QPO, when present. The Lorentzians are reported in the
,
Q, r representation used by Belloni et al. (2002),
where
is the frequency at which the Lorentzian reaches its maximum in a power
frequency vs. frequency power spectral representation, Q the quality factor (for which we use the standard definition of centroid frequency divided by full width at half maximum) and r the fractional rms amplitude. We will regard
to be the characteristic frequency of a Lorentzian component. The rms powers were calculated in the 0-
Hz range, except for the two components fitting the VLFN where we used the 0.125-
Hz range because for these components the unmeasured power below our lowest Fourier frequency provides a significant contribution (which is not the case for the other components). The rms amplitudes were calculated assuming an average background of 140 cnts
(2-60 keV, 5 PCUs); the
20 cnts
deviations from this average are negligible in view of the high source count rates.
In all of the tracks we have found VLFN and BLN and in 5 of the 9 tracks we detected the 57-69 Hz QPO first reported by Homan et al. (1998). For all
intervals we found VLFN; the BLN usually disappeared in the highest
intervals and QPOs were only detected in the lowest
intervals. The power spectral components and their dependence on
and
is discussed in more detail in Sect. 3.4.
The absolute rms of the VLFN as a function of
of all tracks combined is shown in Fig. 11. Up to
the absolute rms decreases from
50 to
30 cnts
,
and for
it stays approximately constant at
30 cnts
.
So for
the decreasing absolute rms is correlated to the decreasing count rate; however, for
the count rate in some tracks increases, but this does not affect the absolute rms, which stays approximately constant.
The characteristic frequency (
)
of the low and high VLFN does not show a clear correlation with
,
but varies between 0.1-0.3 Hz for the low and between 0.5-1.5 Hz for the high VLFN (with a few exceptions up to 2 Hz when no BLN was fitted, and for low
in track 9 where the BLN also shows an extremely high
).
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Figure 9:
VLFN fractional rms amplitude vs. ![]() ![]() ![]() ![]() ![]() |
The characteristics of the BLN do not show a clear correlation with
,
although the BLN in track 9 has a higher rms for a given
than in the other tracks and also shows extremely high frequencies for low
(see above). The Q of the BLN is also not significantly correlated with
.
It varies between 0 and 1, with the exception of the last interval of track 2 (
,
Q=2.6+22.1-1.7) and
interval 7 of track 9 (
,
Q=4+4-2), where the BLN also shows a very high frequency of
12.1+0.7-0.8 Hz; in these two cases (only) this component is sufficiently narrow to be called a QPO.
Track | ![]() |
Sign. |
![]() |
Amplitude | Q |
(![]() |
(Hz) | (% rms) | |||
1 |
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4.3 |
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1 |
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3.8 |
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2 |
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3.7 |
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2 |
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3.5 |
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2 |
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2.7 | ![]() |
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6 |
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7.4 |
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6 |
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6.7 |
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7 |
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6.7 |
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7 |
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11.2 |
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7 |
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2.7 |
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8 |
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10.0 |
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8 |
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4.4 | ![]() |
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We have searched for kHz QPOs up to 2048 Hz, but none were detected above the 3
confidence level. The strongest narrow features (
)
above 300 Hz were between 2 and 3
and resulted in 3
upper limits of up to 3% rms. A broad, 3.1
,
high frequency feature (
% rms,
Q=0.9+0.9-0.6,
Hz) is found in track 7 (
), but when the number of trials is taken into account this is not a significant detection.
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Figure 12:
The characteristic frequency of the BLN vs. ![]() ![]() ![]() ![]() |
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Figure 13:
The fractional rms amplitude of the BLN vs ![]() ![]() ![]() ![]() ![]() |
In a given track the count rate of GX 13+1 decreases along the track up to
-3 after which (in some tracks) it increases again with
,
while in atoll sources on short timescales the count rate generally increases monotonically with
along the whole track (see Sect. 3.2; for references on atoll source behaviour see Hasinger & van der Klis 1989; Di Salvo et al. 2003; van Straaten et al. 2002). This is also different from what is observed in Z sources, where the count rate can slightly decrease along the NB, but increases immediately after the NB/FB vertex so that the minimum in count rate is at the vertex (see Sect. 3.2; for references on Z source behaviour see O'Neill et al. 2001; Jonker et al. 2002; Dieters & van der Klis 2000; Homan et al. 2002; Jonker et al. 2000). The VLFN of GX 13+1 is very strong (usually
4-5.5% rms for
)
compared to atoll sources in the island state or lower banana branch and Z sources in the NB (the VLFN frequency range is similar). The VLFN rms of GX 13+1 generally decreases with
.
This is the opposite of what is normally seen in both atoll and Z sources, where the VLFN increases with
along the whole track (see, however, Wijnands et al. 1999, on the atoll source 4U 1820-30). The BLN is quite weak (
2-5%) compared to that in atoll sources in the island state and lower banana and its rms decreases with
as is usual in atoll and Z sources, but in tracks 4 and 9 it increases again in the highest
intervals. The characteristic frequency of the BLN also decreases with
,
which, in either atoll sources in the island state and lower banana branch or Z sources, has so far only been observed in the atoll source 4U 1820-30, where a BLN-like component that evolves into or is replaced by a
7 Hz QPO (Wijnands et al. 1999) is seen. In the
Z source GX 349+2, a similar noise component is seen of which the centroid frequency decreases with
,
but the
of this component shows a more complicated dependence on
(O'Neill et al. 2002). Generally, in atoll sources the
of the BLN increases with
up to the upper banana, where it starts to decrease again, whereas in Z sources it increases monotonically.
We find the QPO properties of GX 13+1 to be unlike either Z or atoll sources. At the lowest
intervals the 57-69 Hz QPO found by Homan et al. (1998) is detected. In atoll sources a low frequency QPO is usually detected in the island state with a frequency of 10-15 Hz (see, however, Wijnands et al. 1999), increasing up to 40-47 Hz towards the upper banana, where it becomes undetectable (although a second low frequency QPO is sometimes also seen at similar frequencies, but for higher
,
see van Straaten et al. 2002). If the QPO would remain detectable, one would expect to find such a 57-69 Hz QPO at higher
,
in the middle banana branch. The HBO in Z sources has a frequency up to
55 Hz in the upper NB; if the QPO in GX13+1 is interpreted as this QPO one would also expect to find a
6 Hz NBO on this branch; in the
Z source GX 349+2 no NBO is found, but a very broad noise component is seen instead (O'Neill et al. 2001; Kuulkers & van der Klis 1998), and of this GX 13+1 shows no evidence either. The fact that no kHz QPOs are detected anywhere along the branch (see Sect. 3.4.3) is uncharacteristic for both atoll and Z sources.
Clearly, GX 13+1 is a peculiar source in which the properties correlate to
in a very unusual way when compared both to atoll and Z sources. In the final section we explore a possible explanation for this unusual behaviour.
In GX 13+1 the count rate decreases as
increases up to
-3, and only for higher
increases, which is what in a regular atoll source one would expect along the whole track (see Sect. 3.2). One interpretation for this unusual behaviour would be that there is something adding extra flux at low
,
which becomes weaker towards higher
resulting in a count rate that first decreases, and then increases as the standard atoll upper banana behaviour takes over. From the two branched structure in the CDs one can conclude that this extra flux would have to be hard, because for low
the source spectrum gets harder when the count rate increases. This scenario would also explain why the BLN is so weak and shows a decreasing
along the track, and why no kHz QPOs are detected at low
:
these properties are normal for the middle and upper parts of the banana branch, which is, by hypothesis, the true state of the source. In most atoll sources the low frequency QPO disappears at the same
,
or a slightly lower value, as the kHz QPO (Di Salvo et al. 2003; van Straaten et al. 2002); the fact that in GX 13+1 we still see the low frequency QPO while the kHz QPO is absent is unusual, but in line with the relatively high frequency of the low frequency QPO. So the hypothesis we are exploring is that the hard extra flux creates an anomalous second branch in the CD reminiscent of a lower banana and island branch, while the source remains in the "middle'' banana state.
A possible source for this extra, hard flux could be a relativistic jet. In general in atoll and Z sources radio emission, ascribed to the presence of a relativistic jet (Fender 2002), tends to increase towards lower .
If this more intense jet is associated with the extra hard flux, this could cause a second branch in the CD, while the source is still in the upper banana state. We further hypothesize that the unusually strong VLFN would occur in the hard jet flux. Consequently, when the jet becomes weaker towards higher
this causes the VLFN to decrease in amplitude. This is in accordance with the fact that up to
the absolute rms amplitude of the VLFN decreases, from which it can be concluded that it is the flux, that also decreases in this part of the track, that shows relatively strong VLFN that disappears. For
the absolute rms amplitude of the VLFN stays approximately constant, while in some tracks the flux increases for
.
The extra flux in these tracks does lead to a significant increase of the absolute rms amplitude.
The question of course is: why would a jet in GX 13+1 cause this peculiar behaviour and not in other atoll sources? The radio emission of GX 13+1 is unusually strong for an atoll source (Fender & Hendry 2000), so perhaps for a reason unknown the jet is unusually strong in this source. A more attractive possibility is that it is not the jet that is unusual, but the system's orientation. If the jet is pointing directly towards us Doppler boosting could significantly increase its radio flux (e.g. Fender & Hendry 2000); a strongly relativistic jet (which has already been directly imaged for the Z source Sco X-1, Fomalont et al. 2001b,a) could easily Doppler boost the radio emission by more than an order of magnitude, putting the radio emission of GX 13+1 in accordance with measurements and upper limits of other atoll sources (Fender & Hendry 2000). The unusual X-ray phenomena could then for example be the consequence of a better view of the X-ray emitting regions associated with the base of the jet, offered by the unusual orientation of the disk, possibly assisted by Doppler boosting as well. A similar possibility of Doppler-boosted jet X-ray emission causing hard flaring ("microblazar'') was recently explored for Cyg X-1 by Romero et al. (2002).
The large shifts of the track in the CD of GX 13+1 compared to other atoll and Z sources might be related to this better view of the X-ray emitting base of the jet region as well. This would require this region to contribute different baseline levels of hard flux at different epochs in order to produce the secular motion (which, consistent with this, takes place mostly in the HC), VLFN whose amplitude anticorrelates, as well as extra flux at
which correlates, with this baseline flux level. This could for example be related to secular geometry changes (e.g. precession) modulating our view of the base of the jet, or to variations in the jet activity itself.
One can also attempt to explain the peculiar behaviour of GX 13+1 with a similar relativistic jet scenario assuming the source is a Z source. The QPO observed in GX 13+1 would then be interpreted as the HBO, occurring at the top of the NB, leading to the conclusion that the CD shows both a NB and a FB. The strong VLFN that decreases along the whole track might be related to a jet similarly to the atoll scenario. This jet could be related to the high radio over X-ray luminosity ratio compared to other Z sources. However, this model leaves unexplained the decreasing frequency of the BLN with
and the absence of an NBO.
So, when a choice is made between an atoll or a Z classification for GX 13+1 (in either case modified by an unusual jet geometry) the overall phenomenology favours the former option. An alternative would be that GX 13+1 combines some properties of atoll and Z sources (as noted in Sect. 1, GX 13+1 has been described as the atoll source closest to the Z sources). This possibility will be further explored in a forthcoming paper (Homan et al. 2003).
It has been suggested that narrow absorption features, as found in GX 13+1 (Ueda et al. 2001), are related to a high inclination of the source, contrary to the low inclination predicted by the scenario discussed above. However, it is also possible that the absorption features are not related to the inclination of the source, but, as previously suggested by Ueda et al. (1998), to a highly ionized plasma associated with their relativistic jets, causing absorption from the flux from the hot inner accretion region.
Several predictions can be made based on the atoll source+jet hypothesis. Since the unusually strong VLFN occurs in the jet flux, one expects the VLFN, at low ,
to be the strongest in the hard flux. Also, as the jet is the main source of radio emission in LMXBs, we expect the radio emission to be strongest in the upper part of the second branch in the CD (at low
), decreasing as the source moves towards the vertex. This can be tested with simultaneous radio and X-ray observations. One would have to account for a delay factor in the radio emission of perhaps up to half an hour (Klein-Wolt et al. 2002) associated with the expansion of the synchrotron clouds.
Such observations have been done by Garcia et al. (1988), but with a small time range (
5 hours of overlapping X-ray and radio observations) and no CD analysis was performed, and by Berendsen et al. (2000), also with a short period of overlapping X-ray and radio observations. Recent simultaneous radio and X-ray observations also exist (to be reported in Homan et al. 2003), and preliminary results of this work that became available after the current analysis and interpretation were completed, appear to confirm our suggestion.
Acknowledgements
RS and MK acknowledge support from the Netherlands Organisation for Scientific Research (NWO).