New hard X-ray sources discovered in the ongoing INTEGRAL Galactic Plane survey after 14 years of observations

TL;DR

This paper presents a catalog of 72 new hard X-ray sources in the Galactic Plane detected by INTEGRAL over 14 years, including potential identifications and many unclassified sources, expanding our understanding of high-energy astrophysical objects.

Contribution

The study provides the first extensive catalog of new hard X-ray sources from INTEGRAL after 14 years, with potential identifications for most sources, enhancing the understanding of Galactic high-energy phenomena.

Findings

72 new hard X-ray sources detected at S/N>4.7 sigma

31 sources also detected by Swift/BAT survey

26 sources have possible astrophysical identifications

Abstract

The International Gamma-Ray Astrophysics Laboratory (INTEGRAL) continues to successfully work in orbit after its launch in 2002. The mission provides the deepest ever survey of hard X-ray sources throughout the Galaxy at energies above 20 keV. We report on a catalogue of new hard X-ray source candidates based on the latest sky maps comprising 14 years of data acquired with the IBIS telescope onboard INTEGRAL in the Galactic Plane (|b|<17.5 deg). The current catalogue includes in total 72 hard X-ray sources detected at S/N>4.7 sigma and not known to previous INTEGRAL surveys. Among them, 31 objects have also been detected in the on-going all-sky survey by the BAT telescope of the Swift observatory. For 26 sources on the list, we suggest possible identifications: 21 active galactic nuclei, two cataclysmic variables, two isolated pulsars or pulsar wind nebulae, and one supernova remnant; 46 sources from the catalogue remain unclassified.

Tables

Table 1. Table 1: The list of known X-ray transients detected in 14-year time averaged map at S / N > 4.7 ​ σ 𝑆 𝑁 4.7 𝜎 S/N>4.7\sigma mainly due to outburst event(s) occured between 2010 and 2016.

No.	Name	RA (J2000)	Dec (J2000)	Flux${}_{17-60\mathrm{keV}}$	$S/N$	Typea	Outburst
		(deg)	(deg)	${10}^{-11}$erg s-1 cm-2			year
1	GS 0834-430	128.979	-43.185	$0.99\pm 0.08$	12.6	HMXB	2012
2	GS 1354$-$64	209.562	-64.733	$3.94\pm 0.09$	45.2	LMXB	2015
3	IGR J17177$-$3656	259.424	-36.880	$0.32\pm 0.06$	4.9	LMXB	2011
4	GRS 1716$-$249	259.903	-25.020	$4.03\pm 0.06$	65.2	LMXB	2016
5	Swift J1734.5$-$3027	263.652	-30.399	$0.87\pm 0.05$	17.2	LMXB	2013
6	GRS 1739$-$278	265.661	-27.748	$2.11\pm 0.05$	42.5	LMXB	2014, 2016
7	GRO J1744$-$28	266.138	-28.741	$4.51\pm 0.05$	91.8	LMXB	2014, 2017
8	Swift J174510.8$-$262411	266.297	-26.401	$30.74\pm 0.05$	607.7	LMXB	2012
9	IGR J17498$-$2921	267.482	-29.323	$0.72\pm 0.05$	14.6	LMXB	2011
10	1RXS J180408.9$-$342058	271.036	-34.356	$5.80\pm 0.06$	99.5	LMXB	2012
11	SAX J1806.5$-$2215	271.634	-22.233	$3.31\pm 0.06$	54.7	LMXB	2011
12	IGR J18179$-$1621	274.477	-16.481	$0.38\pm 0.08$	4.7	HMXB	2012
13	IGR J18245$-$2452	276.106	-24.879	$1.29\pm 0.07$	19.3	LMXB	2013
14	MAXI J1828$-$249	277.238	-25.041	$1.23\pm 0.07$	17.3	BHC	2013
15	MAXI J1836$-$194	278.937	-19.314	$2.17\pm 0.09$	24.5	XRB	2011
16	XTE J1859+083	284.753	8.239	$0.60\pm 0.07$	8.3	HMXB	2015
17	V404 Cyg	306.019	33.867	$9.70\pm 0.08$	119.8	LMXB	2015

No.	Name111INTEGRAL (IGR) name of the source. A dagger symbol $†$ marks that the source is located in the region of high systematic noise, and that its measured flux should be taken with the caution.	RA222Equatorial coordinates (right ascension and declination) are in standard J2000.0 epoch.	Dec222Equatorial coordinates (right ascension and declination) are in standard J2000.0 epoch.	Flux333The measured 17–60 keV flux of the source $\times {10}^{-11}{\text{erg }\text{cm}}^{-2}{\text{s}}^{-1}$.	S/N	Type444General astrophysical type of the object: AGN – active galactic nucleus, SNR – supernova remnant; CV – cataclysmic variable; PSR – isolated pulsar or pulsar wind nebula (PWN). A question mark indicates that the specified type should be confirmed.	Ref555References. – (1) Baumgartner et al. (2013); (2) Bikmaev et al. (2006); (3) Paturel et al. (2003); (4) Massaro et al. (2013); (5) Edelson & Malkan (2012); (6) Evans et al. (2014); (7) Masetti et al. (2010); (8) Cusumano et al. (2010); (9) Skrutskie et al. (2006); Huchra et al. (2012); (10) Krimm et al. (2013); (11) Cieslinski et al. (2004); (12) Reynolds et al. (2013); (13) Rosen et al. (2016); (14) Sguera et al. (2015); Sugizaki et al. (2001);	Hard X-ray/$\gamma$	Soft X-ray	Optical/IR
		(deg)	(deg)	(17–60 keV)				counterpart	counterpart	counterpart
1	J00555+4610	13.876	46.173	$1.42\pm 0.28$	5.1	CV	1,2	Swift J0055.4+4612	XSS J00564+4548
2	J01017+6519	15.440	65.330	$0.61\pm 0.08$	8.0
3	J02145+5142	33.625	51.710	$1.10\pm 0.19$	5.7	AGN	1	Swift J0213.7+5147
4	J03117+5028	47.934	50.472	$0.70\pm 0.15$	4.7	AGN	1	Swift J0311.9+5032	1RXS J031202.7+502922	2MASX J03120291+5029147
5	J06402$-$2552	100.050	-25.868	$2.24\pm 0.32$	7.0	AGN	1	Swift J0640.4$-$2554		ESO 490$-$26
6	J07141+0146	108.547	1.744	$1.44\pm 0.24$	6.0
7	J07258+0054	111.471	-0.907	$1.39\pm 0.22$	6.3	AGN	1	Swift J0725.7$-$0055		QSO B0723$-$007
8	J07396$-$3143	114.908	-31.732	$1.73\pm 0.21$	8.1	AGN?	1	Swift J0739.7$-$3142		2MASX J07394469$-$3143024
9	J07433$-$2544	115.844	-25.736	$1.22\pm 0.15$	7.9	AGN?	1	Swift J0743.3$-$2546	1RXS J074315.6$-$254545	2MASX J07431472$-$2545501
10	J08004$-$4309	120.121	-43.160	$0.58\pm 0.10$	5.7		1	Swift J0800.7$-$4309
11	J08215$-$1320	125.387	-13.339	$0.98\pm 0.14$	6.9	AGN?	3			NGC 2578, PGC023449
12	J08321$-$1808	128.022	-18.141	$1.11\pm 0.15$	7.3	AGN?	4,5,6		$\left\{\frac{1RXSJ083158.1-180828}{1SXPS\mathrm{J083158}.6-180840}\right\}$
13	J08398$-$1214	129.961	-12.243	$1.80\pm 0.15$	11.7	AGN?	5,1	Swift J0839.6$-$1213	1RXS J083950.7$-$121424	2E 2028
14	J08453$-$3529	131.329	-35.493	$0.72\pm 0.12$	6.0	AGN	1,7	Swift J0845.0$-$3531	1RXS J084521.7$-$353048	WISE J084521.37$-$353024.2
15	J09278$-$3935	141.965	-39.590	$0.66\pm 0.12$	5.7
16	J09331$-$4725	143.302	-47.441	$0.57\pm 0.09$	6.2
17	J11275$-$5319	171.898	-53.369	$0.71\pm 0.11$	6.5		8	PBC J1127.7$-$5320
18	J11299$-$6557	172.490	-65.960	$0.58\pm 0.10$	5.8	AGN?	5		1RXS J112955.1$-$655542	2MASS J11295643$-$6555218
19	J12086$-$6327	182.157	-63.452	$0.54\pm 0.09$	6.0
20	J12489-5930	192.161	-59.507	$0.49\pm 0.09$	5.5
21	J12529$-$6351	193.241	-63.868	$0.49\pm 0.09$	5.5
22	J14044$-$6146	211.029	-61.700	$0.69\pm 0.08$	8.1		1	Swift J1403.6$-$6146
23	J14417$-$5533	220.427	-55.550	$0.69\pm 0.09$	7.4	AGN?	5		1RXS J144116.4$-$553329	2MASS J14411645$-$5533306
24	J14192$-$6048	214.819	-60.810	$0.43\pm 0.09$	5.1	PSR/PWN	1	Swift J1418.8$-$6055	AX J1418.7$-$6058
25	J15550$-$4306	238.769	-43.090	$0.50\pm 0.09$	5.7
26	J16459$-$2325	251.477	-23.428	$1.39\pm 0.13$	11.0
27	J16494$-$1740	252.355	-17.676	$0.80\pm 0.14$	5.6	AGN?	4,9	4PBC J1649.3$-$1738	1SXPS J164920.9$-$173840	ESO 586$-$4
28	J17040$-$4305	256.010	-43.080	$0.44\pm 0.07$	6.2				1RXS J170406.3$-$430637
29	J17098$-$2344	257.455	-23.747	$0.63\pm 0.08$	8.4	AGN?	5,6	4PBC J1709.7$-$2348	1RXS J170944.9$-$234658	2MASS J17094469$-$2346531
30	J17158$-$2124	258.959	-21.411	$0.59\pm 0.08$	7.5
31	J17255$-$4509	261.380	-45.170	$0.58\pm 0.08$	7.1	AGN?	8,9	4PBC J1725.8$-$4510		2MASX J17253053$-$4510279
32	J17326$-$3445	263.169	-34.754	$0.29\pm 0.06$	5.2				1RXS J173251.1$-$344728
33	J17327$-$4405	263.183	-44.103	$0.62\pm 0.09$	6.9
34	J17422$-$2108	265.560	-21.106	$0.25\pm 0.05$	5.0		6		1SXPS J174211.7$-$210354
35	J17528$-$2022	268.217	-20.367	$0.43\pm 0.06$	6.7		8	4PBC J1752.6$-$2020
36	J17538$-$2544	268.463	-25.749	$0.54\pm 0.05$	10.5		10	Swift J1753.7$-$2544
37	J17570$-$2500	269.266	-25.015	$0.34\pm 0.05$	6.4
38	J17596$-$2315	269.907	-23.266	$0.39\pm 0.06$	6.9
39	J18010$-$3045	270.271	-30.764	$0.37\pm 0.05$	7.2
40	J18013$-$3222	270.326	-32.371	$0.34\pm 0.05$	6.4
41	J18017$-$3542	270.371	-35.638	$0.42\pm 0.06$	7.0		8	4PBC J1801.7$-$3540
42	J18044$-$1829	271.107	-18.487	$0.45\pm 0.07$	6.3
43	J18070$-$3507	271.750	-35.132	$0.30\pm 0.06$	4.9
44	J18102$-$1751	272.555	-17.853	$0.55\pm 0.07$	7.4
45	J18112$-$2641	272.854	-26.707	$0.48\pm 0.06$	8.7
46	J18141$-$0606	273.525	-6.114	$0.45\pm 0.09$	5.1
47	J18141$-$1823	273.538	-18.395	$0.68\pm 0.07$	9.0		8	4PBC J1814.1$-$1822
48	J18147$-$3400	273.690	-34.010	$0.54\pm 0.06$	8.6
49	J18165$-$3912	274.136	-39.202	$0.61\pm 0.09$	7.0
50	J18172$-$1944	274.307	-19.740	$0.55\pm 0.07$	7.6
51	J18184$-$2352	274.610	-23.880	$0.96\pm 0.06$	15.2	CV?	11		MACHO 311.37389.3983
52	J18263$-$1345	276.576	-13.753	$0.55\pm 0.08$	6.6	PWN?		HESS J1825$-$137?
53	J13545$-$5958	208.621	-59.982	$0.54\pm 0.09$	6.4
54	J18434$-$0508	280.855	-5.138	$0.52\pm 0.08$	6.2		8,12	4PBC J1842.8$-$0506	Swift J184311.0$-$050539
55	J18544+0839$†$	283.605	8.661	$0.50\pm 0.08$	6.7
56	J19039+3348	285.948	33.825	$1.05\pm 0.17$	6.2	AGN	1	Swift J1903.9+3349
57	J19071+0716	286.783	7.274	$0.40\pm 0.07$	5.7		5,13		$\left\{\frac{1SXPS\mathrm{J190706}.3+072004}{3XMM\mathrm{J190706}.3+072003}\right\}$
58	J19260+4136	291.517	41.608	$0.66\pm 0.14$	4.7	AGN	1	Swift J1926.9+4140	1RXS J192630.6+413314	2MASX J19263018+4133053
59	J19305+1851	292.632	18.857	$0.78\pm 0.10$	7.7	SNR?			1RXS J193029.9+185205
60	J19421+3613	295.530	36.219	$0.61\pm 0.09$	6.6
61	J19498+2534	297.470	25.557	$0.52\pm 0.10$	5.0		8,14	4PBC J1950.0+2532	AX J1949.8+2534
62	J19577+3339	299.429	33.658	$0.46\pm 0.08$	5.6
63	J19504+3318$†$	297.615	33.311	$0.63\pm 0.09$	7.4				1RXS J195020.5+331419
64	J20063+3641	301.601	36.684	$0.72\pm 0.08$	9.1		1	Swift J2006.4+3645
65	J20084+3221	302.124	32.350	$0.68\pm 0.08$	8.4		5,8	4PBC J2008.7+3221	1SXPS J200843.8+321824
66	J20596+4303	314.914	43.054	$0.57\pm 0.08$	6.8		1	Swift J2059.6+4301A/B
67	J21099+3533	317.490	35.560	$0.71\pm 0.11$	6.6
68	J21133+3154	318.328	31.923	$0.75\pm 0.15$	4.8				1RXS J211319.3+315211
69	J21382+3204	324.563	32.072	$1.27\pm 0.24$	5.4	AGN	1	Swift J2138.8-3207	1RXS J213833.0+320507	WISE J213833.43+320505.8
70	J21397+5949	324.945	59.832	$0.78\pm 0.11$	7.1	AGN	1	Swift J2139.7+5951	1RXS J213944.3+595016	WISE J213944.96+595015.1
71	J22018+5049	330.474	50.830	$0.68\pm 0.10$	6.7	AGN	1	Swift J2201.9+5057		87GB 215950.2+503417
72	J22455+3940	341.383	39.683	$2.43\pm 0.45$	5.4	AGN	1	Swift J2246.0+3941		3C452

Full text

††pagerange: New hard X-ray sources discovered in the ongoing INTEGRAL Galactic Plane survey after 14 years of observations–New hard X-ray sources discovered in the ongoing INTEGRAL Galactic Plane survey after 14 years of observations

1 Introduction

X-ray surveys play a key role in our understanding of energetic phenomena in the Universe. Detailed investigations of the physics and evolution of X-ray selected sources are usually based on systematic studies of their properties. Observations in recent decades have revealed a variety of X-ray point sources beyond the solar system in the Milky Way and Magellanic Clouds. Although the bright X-ray sources in the Milky Way can be effectively studied, many of them are not observable due to the heavy obscuration by the Galactic disk. Studies of nearby galaxies with modern sensitive soft X-ray telescopes are relatively free from the obscuration problem and can provide us uniform samples of X-ray binaries in different environments (see Fabbiano & White, 2006; Fabbiano, 2006, for a review). As a result, we may know better the properties of X-ray source populations and structure of the nearby galaxies, than of our own Milky Way.

X-ray observations of our Galaxy at energies above 10 keV are free from the obscuration bias. However, due to the large extent of the Milky Way across the sky, a systematic survey of the Galactic X-ray source population and discovery of new X-ray emitters require wide-angle instruments. This makes the IBIS coded-mask telescope (Ubertini et al., 2003) onboard the INTEGRAL observatory (Winkler et al., 2003) unique and most suitable for surveying the Galaxy in the hard X-ray domain.

The INTEGRAL observatory has been successfully operating in orbit since its launch in October 2002. Over the past years, INTEGRAL acquired a huge data set, which allowed us to construct high quality X-ray catalogs in the Galactic Plane (GP), starting from our early papers by Revnivtsev et al. (2004, 2006); Molkov et al. (2004), to more recent surveys (see Krivonos et al., 2012, 2015; Bird et al., 2016, and references therein). These works were subsequently used for many relevant studies, including systematic discoveries of strongly absorbed high-mass X-ray binaries (HMXBs) and the study of their luminosity function and distribution in the Galaxy (Lutovinov et al., 2005; Lutovinov et al., 2013b; Bodaghee et al., 2007; Bodaghee et al., 2012; Chaty et al., 2008; Coleiro et al., 2013), the statistics of low mass X-ray binaries (Revnivtsev et al., 2008b) and cataclysmic variables (CVs) (Revnivtsev et al., 2008a; Scaringi et al., 2010).

In the previous paper (Krivonos et al., 2012), we presented a GP survey ($|b|<17.5$∘) based on nine years of INTEGRAL operations. The survey catalogue lists 402 sources detected in the $17-60$ keV energy band and time-average sky maps at more than $4.7\sigma$ significance, including 253 Galactic sources of known or tentatively identified nature, and 34 unidentified sources. The upper panel of Fig. 1 illustrates the limiting flux of the INTEGRAL 9-year survey over the Galactic longitude, demonstrating that the survey’s completeness in the inner part of the Galaxy raises to 100% above the flux limit of 0.7 mCrab (shown in the figure with the dashed line). The sample of 26 persistent unidentified sources is shown by red circles (see Lutovinov et al., 2013b, for details). A number of multi-wavelength follow-up observations were initiated to unveil the nature of these unclassified objects (Karasev et al., 2012; Masetti et al., 2013; Revnivtsev et al., 2013; Zolotukhin & Revnivtsev, 2015; Lutovinov et al., 2013a; Lutovinov et al., 2015; Tomsick et al., 2015; Tomsick et al., 2016a; Tomsick et al., 2016b; Clavel et al., 2016; Burenin et al., 2016; Rahoui et al., 2017) which led to the classification of 11 sources (shown by solid red circles in Fig. 1), rising the total survey identification completeness from $\mathord{\sim}92\%$ to $\mathord{\sim}94\%$.

The 9-year INTEGRAL Galactic survey by Krivonos et al. (2012) and the all-sky survey by Bird et al. (2016) were based on similar data sets, available by January 2011 and by the end of 2010, respectively. Over about 6 years that have passed since then, INTEGRAL accumulated an additional $\mathord{\sim}80$ Ms and $\mathord{\sim}50$ Ms of exposure (dead-time corrected) over the whole sky and in the GP ($|b|<17.5$°), respectively. The increased sensitivity of the currently available INTEGRAL data set allows us to make a next iteration in the process of finding previously unknown hard X-ray sources. Mereminskiy et al. (2016) recently released a $17-60$ keV deep survey of three extragalactic fields (M81, Large Magellanic Cloud and 3C 273/Coma), based on 12 years of observations (2003-2015) with the detection of 147 sources at $S/N>4\sigma$, including 37 sources observed in hard X-rays for the first time.

In this short report we present a catalogue of newly discovered hard X-ray sources detected in the latest maps of the GP comprising 14 years of data acquired with INTEGRAL/IBIS.

2 Data analysis

For this work, we selected all publicly available INTEGRAL data from December 2002 to March 2017 (spacecraft revolutions 26-1790). Prior to actual data analysis we applied the latest energy calibration (Caballero et al., 2013) for the registered IBIS/ISGRI detector events with the INTEGRAL Offline Scientific Analysis version 10.2 provided by ISDC Data Centre for Astrophysics up to the COR level. Then events were processed with a proprietary analysis package developed at IKI111Space Research Institute of the Russian Academy of Sciences, Moscow, Russia (details availiable in Krivonos et al., 2010a, 2012; Churazov et al., 2014) to produce a $17-60$ keV sky image of every individual INTEGRAL observation with a typical exposure time of 2 ks (usually referred as Science Window, or ScW). The flux scale in each ScW sky image was renormalized using the flux of the Crab nebula measured in the nearest observation. This procedure was used to account for the loss of sensitivity at low energies caused by ongoing detector degradation.

In total, we obtained 124727 ScW images covering the whole sky, comprising $\mathord{\sim}220$ Ms of the effective (dead time-corrected) exposure. For the purposes of this work we selected 79234 ScWs ($\mathord{\sim}130$ Ms) within the GP ($|b|<17.5$°). Following Krivonos et al. (2012) we constructed six overlapping 70∘$\times$35∘cartesian projections centered at the GP ($|b|=0$∘) and Galactic longitudes $l=0^{\circ},\pm 50^{\circ},\pm 115^{\circ}$, and $l=180^{\circ}$.

The peak sensitivity of the survey is $2.2\times 10^{-12}$ erg s*-1* cm*-2* ($\mathord{\sim}0.15$ mCrab222 1 mCrab corresponds to $1.43\times 10^{-11}$ erg s*-1* cm*-2* assuming a spectral shape $10(E/{\rm 1~{}keV})^{-2.1}$ photons cm*-2* s*-1* keV*-1*. in the 17-60 keV energy band) at a $4.7\sigma$ detection level. The survey covers $90\%$ of the geometrical area ($12680$ degrees) down to the flux limit of $1.3\times 10^{-11}$ erg s*-1* cm*-2* ($\mathord{\sim}0.93$ mCrab) and $10\%$ of the total area down to the flux limit of $3.8\times 10^{-12}$ erg s*-1* cm*-2* ($\mathord{\sim}0.26$ mCrab). Given the added exposure in the GP, the achieved improvement in sensitivity with respect to the 9-year survey is in the range of $10-30\%$. The updated sensitivity of the current survey over the Galactic longitude is shown in the bottom panel of Fig. 1. Note that the overall improvement in sensitivity makes it possible to probe deeper into the Galaxy. Fig. 2 shows a face-on schematic view of the Galaxy and the distances at which we can detect a hard X-ray source of a given luminosity $L_{\rm HX}$ in the $17-60$ keV band. One can see that (i) we can now detect all sources with the luminosity $L_{\rm HX}>2\times 10^{35}$ erg s*-1at the far end of the Galaxy in the direction towards the Galactic Centre (GC), (ii) the distance range for the luminosity $L_{\rm HX}>2\times 10^{35}$ erg s-1covers most of the Galactic stellar mass, and (iii) the Galactic central bar is fully reachable at luminosities $L_{\rm HX}>5\times 10^{34}$ erg s-1*.

Following Krivonos et al. (2012), we adopted a conservative detection threshold of $(S/N)_{\rm lim}>4.7\sigma$ to ensure that the final catalogue contains no more than one spurious source assuming Poisson statistics. The regions around bright sources, such as Crab, Sco X-1, Cyg X-1, Cyg X-3, Vela X-1, GX 301-2, and GRS 1915+105 were excluded from the automated excess selection to prevent false detections triggered by high systematic noise. However, manual inspection of these regions was performed to select possible source candidates (properly marked as being detected in noisy environment).

A special care was taken for the source detection in the region of $\mathord{\sim}17$ degrees around the GC due to enchanced systematics (see e.g. Krivonos et al., 2010a). False detections were revealed by a distorted excess shape that differs significantly from the instrumental point-spread function, which is a symmetric two-dimensional Gaussian ($\sigma=5$′). One can reduce IBIS/ISGRI false detections by using additional information from the BAT coded-mask telescope (Barthelmy et al., 2005) onboard the Swift observatory (Gehrels et al., 2004) working at hard X-ray energies. Since BAT has a different coded-mask design compared to IBIS, it suffers different systematics, which allows one to supress the non-statistical uncertainties known to IBIS (the idea on which the combined Swift-INTEGRAL survey by Bottacini et al., 2012, is based). We assume that finding a hard X-ray counterpart in the ongoing Swift/BAT surveys (Cusumano et al., 2010; Baumgartner et al., 2013) of a suspected IBIS/ISGRI systematic excess adds more evidence that the excess is a real source.

3 Results

Our analysis of 14-year averaged sky images of the GP ($|b|<17.5$°) led to the detection of 522 hard X-ray sources at significance $S/N>4.7\sigma$, which is $\mathord{\sim}$30% more compared to 402 sources detected in the 9-year survey (Krivonos et al., 2012) with the same detection threshold. Note that 14 weak sources333IGR J17315$-$3221, IGR J17331$-$2406, Swift J2113.5+5422, IGR J18175$-$1530, IGR J17448$-$3231, XTE J1543$-$568, IGR J16293$-$4603, IGR J17197$-$3010, IGR J16358$-$4726, IGR J18497$-$0248, XTE J1751$-$305, AX J1753.5$-$2745, IGR J09189$-$4418 and IGR J20107+4534. listed in the 9-year survey with fluxes of $0.2-0.5$ mCrab are not detected in the current study, probably due to an intrinsic variability. Among 134 newly added sources (522-402+14), we identified 62 previously known X-ray emitters, including 17 known sources that experienced transient events after 2010 (Table 1). A detailed analysis of the survey’s catalogue will be presented elsewhere. For the current report we selected those 72 (out of 134) newly detected hard X-ray sources that have not been listed in the INTEGRAL surveys based on the data acquired before 2010 (Krivonos et al., 2007, 2010b, 2012; Bird et al., 2004, 2006, 2007, 2010, 2016), i.e. those sources whose detection is mainly determined by the $\mathord{\sim}$6-year increased INTEGRAL survey sensitivity.

Table 2444. Table 2 is only available in the online version of the paper. lists new INTEGRAL sources detected in the current work with significances between $4.7\sigma$ and $15\sigma$ and fluxes between 0.17 and 1.7 mCrab ($2.5\times 10^{-12}-2.4\times 10^{-11}$erg s*-1* cm*-2*). We searched for source counterparts within a 3.6*′* error circle (90% confidence), as typical for the INTEGRAL sources detected at $S/N=5-6\sigma$ (Krivonos et al., 2007). As seen from Table 2, 31 source candidates are also detected in the ongoing Swift/BAT all-sky hard X-ray survey (Cusumano et al., 2010; Baumgartner et al., 2013). No any hard X-ray counterpart were found for 41 sources, thus they have been detected in hard X-rays for the first time.

We utilized also the SIMBAD555http://simbad.u-strasbg.fr/simbad and NED666http://ned.ipac.caltech.edu data bases to perform a preliminary identification of the detected source candidates within 3.6*′* of the INTEGRAL position. However, usually unique optical/IR counterparts and hence firm astronomical classification can only be obtained based on arcsecond positions provided by soft X-ray focusing telescopes. Therefore, we paid a special attention for finding soft X-ray counterparts in the HEASARC777https://heasarc.gsfc.nasa.gov data base, Swift/XRT point source catalogue (1SXPS; Evans et al., 2014) and the third XMM-Newton serendipitous source catalogue (3XMM-DR5; Rosen et al., 2016). As a result, we suggest classification for 26 sources from the list, with two (IGR J00555+4610 and IGR J18184$-$2352) being most likely CVs and 21 probably being active galactic nuclei (Table 2 and filled squares in the bottom panel of Fig. 1). The remaining 46 unclassified sources are shown by open squares in the bottom panel of Fig. 1. Note, that most of them are detected close to the limiting flux of the survey below $\mathord{\sim}0.5$ mCrab (except for IGR J16459$-$2325 with a measured flux of $1.0\pm 0.1$ mCrab). Twenty out of the 46 nonidentified sources are located in the Galactic bulge at $|l|<15$°.

4 Concluding remark

Regular observations of the GP with INTEGRAL are consistently improving the sensitivity of the hard X-ray survey and allowing us to extend our knowledge of the Galactic X-ray source population, both for weak and nearby sources (mostly CVs, see e.g. Lutovinov et al., 2010; Clavel et al., 2016; Tomsick et al., 2016a), and more distant objects located at far end of the Galaxy (Lutovinov et al., 2016; Rahoui et al., 2017). The presented catalogue opens the path to a large program of follow-up observations, dedicated both to unveil new classes of objects and to increase the overall completeness of the source sample, needed for many Galactic population studies.

Acknowledgments

This work is based on observations with INTEGRAL, an ESA project with instruments and the science data centre funded by ESA member states (especially the PI countries: Denmark, France, Germany, Italy, Switzerland, Spain), and Poland, and with the participation of Russia and the USA. This research has made use of: data obtained from the High Energy Astrophysics Science Archive Research Center (HEASARC) provided by NASA’s Goddard Space Flight Center; the SIMBAD database operated at CDS, Strasbourg, France; the NASA/IPAC Extragalactic Database (NED) operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration; the Palermo BAT Catalogue and database operated at INAF – IASF Palermo. The data were obtained from the European888http://isdc.unige.ch and Russian999http://hea.iki.rssi.ru/rsdc INTEGRAL Science Data Centers. The authors are grateful to E.M. Churazov, who developed the INTEGRAL/IBIS data analysis methods and provided the software, and thank the Max Planck Institute for Astrophysics for computational support. This work was financially supported by Russian Science Foundation grant 14-22-00271.