RX J1856.5-3754

  
2 Spectroscopic observations

We observed RX J1856.5-3754 on the night of 1999 July 15 to 16 at the 8-m Unit Telescope #1 (Antu) of the Very Large Telescope at Paranal, using the Focal Reducer/Low Dispersion Spectrograph FORS1 to obtain spectra covering the optical range and a number of images. A log of the observations is given in Table 1. The conditions were good, with the seeing varying from $0\hbox{$.\!\!^{\prime\prime}$ }8$ to $1\hbox{$.\!\!^{\prime\prime}$ }2$. The night started with patchy high cirrus, which disappeared later. The spectroscopy was done using a $300 \,{\rm lines} \,{\rm mm^{-1}}$ grism, the standard resolution collimator (final f-ratio of 3.13), and a Tektronix CCD detector with $2048\times2048$ pixels of $24 \,{\rm\mu{}m}$. With this setup, the plate scale is $0\hbox{$.\!\!^{\prime\prime}$ }2 \,{\rm pixel^{-1}}$, and the 3600-9000 Å wavelength range is covered at $2.6 \,{\rm\AA} \,{\rm pixel^{-1}}$. With the 1 $^{\prime \prime }$ slit, the wavelength resolution is $\sim$ $13 \,{\rm\AA}$. The detector was read out through all four amplifiers, using the high gain setting ($\sim$ $1.6 \,{\rm e^-} \,{\rm ADU^{-1}}$) for the spectra, and the low gain setting ($\sim$ $3 \,{\rm e^-} \,{\rm ADU^{-1}}$) for the images.

 

 

Table 1: Log of the VLT spectroscopic observations.

Object$^{\rm a}$	UT$^{\rm b}$		Slit or	$t_{\rm int}$	Par.$^{\rm c}$	$\sec z$
	day, time		Filter	(s)	($^\circ$)
EG 274	15,	23 02	5 $^{\prime \prime }$	$3\times10$$^{\rm d}$	-76	1.21
X-F/ $+84\hbox{$.\!\!^\circ$ }6$		23 58	B	300	-94	1.67
	16,	00 08	1 $^{\prime \prime }$	2700	-93	1.60
		00 58	B	$3\times300$	-85	1.33
		01 17	1 $^{\prime \prime }$	$2\times2700$	-82	1.26
		02 54	5 $^{\prime \prime }$	300	-53	1.06
X-L/ $-8\hbox{$.\!\!^\circ$ }5$		03 11	1 $^{\prime \prime }$	$2\times2700$	-44	1.05
		04 45	5 $^{\prime \prime }$	300	+37	1.04
X-F/ $-95\hbox{$.\!\!^\circ$ }4$		05 00	1 $^{\prime \prime }$	$\ldots$$^{\rm e}$
X-F/ $-95\hbox{$.\!\!^\circ$ }4$		06 06	1 $^{\prime \prime }$	$3\times2100$	+72	1.15
		07 56	5 $^{\prime \prime }$	300	+93	1.57
BPM 16274		08 08	5 $^{\prime \prime }$	$3\times20$$^{\rm d}$	-47	1.21
		10 36	5 $^{\prime \prime }$	$3\times20$$^{\rm d}$	+19	1.14

$^{\rm a}$ X-F and X-L are exposures taken at the indicated position angle, for which the slit passes over RX J1856.5-3754 and star F or L (see Fig. 1); standards were taken at position angle $0^\circ$. $^{\rm b}$ UT day in July 1999.
$^{\rm c}$ Parallactic angle at start of observation.
$^{\rm d}$ Third exposure taken with a GG 435 order separation filter.
$^{\rm e}$ Lost because of a glitch in telescope pointing.

 

 

Table 2: Log of the VLT imaging observations.

Object$^{\rm a}$	UT$^{\rm b}$		Sequence$^{\rm c}$	Seeing	$\sec z$
	day, time			( $^{\prime \prime }$)
RX J1856	1,	06:57-07:52	BR$\alpha$R$\alpha$R	0.77	1.14-1.06
		07:54-08:48	BR$\alpha$R$\alpha$R	0.69	1.05-1.02
RX J1856	2,	07:23-08:16	BR$\alpha$R$\alpha$R	0.47	1.09-1.04
		08:19-09:11	BR$\alpha$R$\alpha$R	0.57	1.03-1.02
		09:14-09:45	BR$\alpha$R	$0.77^{\rm d}$	1.03-1.04
PG 0942	2,	23:10-23:20	BRBR	1.2	1.08
RX J1856	3,	06:00-06:54	BR$\alpha$R$\alpha$R	0.57	1.27-1.13
		06:59-07:51	BR$\alpha$R$\alpha$R	0.64	1.12-1.05
		08:00-08:52	BR$\alpha$R$\alpha$R	0.62	1.04-1.02
PG 1657		09:26-09:51	BRBR	1.3	1.60-1.69
		09:54-10:05	BRBR	1.0	1.71-1.81
RX J1856	4,	06:25-07:18	BR$\alpha$R$\alpha$R	0.75	1.19-1.09
		07:23-08:16	BR$\alpha$R$\alpha$R	0.75	1.08-1.03

$^{\rm a}$ In full: RX J1856.5-3754, PG 0942-029 and PG 1657+078.
$^{\rm b}$ UT day in May 2000.
$^{\rm c}$ For RX J1856.5-3754, the integration times were 138 s in B, 135 s in R, and 1019 s in H$\alpha$; for the standards, 3 and 20 s in B, and 1 and 20 s in R.
$^{\rm d}$ The B-band image has $1\hbox{$.\!\!^{\prime\prime}$ }3$ seeing.

Figure 1:  Direct and spectral images of RX J1856.5-3754. The main panel shows the B-band image obtained at the start of our 1999 spectroscopic run (Table 1). The positions of the two settings of the slit for the spectroscopy are indicated by the two sets of parallel lines. The counterpart to RX J1856.5-3754 (star X) is within the intersection of the two slit positions. The position it had in October 1996, when it was observed with HST (Walter & Matthews 1997), is indicated by the plus sign. Previously identified stars for which we obtained photometry (see Table 5) are labeled below their image, except for stars C and 114 (label on the left) and stars D, I, 102, and 106 (label on the right). The panels above and to the right of the main panel show the part around H$\alpha$ and H$\beta$ of the stacked spectral images for the two slit positions, over stars F and L, respectively. Along the spatial axes, the tick marks indicate 5 $^{\prime \prime }$ intervals along the slit. The emission from the nebula around RX J1856.5-3754 can be seen clearly. The flux from RX J1856.5-3754 itself cannot be seen near H$\alpha$.

Since the source is faint, we set up using two brighter stars nearby, stars F and L (here and below we follow the nomenclature of Walter et al. 1996; see also Fig. 1), chosing the position angle such that the counterpart of RX J1856.5-3754 (hereafter star X) should fall in the slit as well. To measure the positions, we reanalysed the images of Walter & Matthews (1997), taken through F606W and F300W filters with the Wide Field Planetary Camera 2 (WFPC2) on board HST. The analysis of the HST images is described in more detail in Sect. 4.6.

In order to minimise slit losses due to differential refraction, we took set-up images through a Bessel B filter, used the differential refraction corrector of FORS1, and chose as reference the star for which the position angle was closest to the parallactic one. We obtained spectra with a total exposure time of 6.5 hour, taking individual exposures at various positions along the slit in order to mitigate the effect of bad pixels and other defects. To calibrate the slit losses, we followed our sequences of long integrations with short exposures through a wide slit (5 $^{\prime \prime }$, formed using the multi-object slitlets). We observed two spectrophotometric flux standards through the wide slit to calibrate the instrumental response.

For verification of our set-up, we took a 5-min B-band image before starting the first spectroscopic observation (which used star F as reference). From the image, star X appeared to have moved to the East. After the first spectrum, therefore, we took three additional 5-min B-band images to measure a more accurate position, and found that star X had, rather fortuitously, moved exactly along the slit; hence, the first spectrum was not lost. For star F, we therefore kept the same set-up, while we changed the position angle for star L appropriately.

RX J1856.5-3754